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![David Howells](/assets/img/avatar_default.png)
A PKCS#7 or CMS message can have per-signature authenticated attributes that are digested as a lump and signed by the authorising key for that signature. If such attributes exist, the content digest isn't itself signed, but rather it is included in a special authattr which then contributes to the signature. Further, we already require the master message content type to be pkcs7_signedData - but there's also a separate content type for the data itself within the SignedData object and this must be repeated inside the authattrs for each signer [RFC2315 9.2, RFC5652 11.1]. We should really validate the authattrs if they exist or forbid them entirely as appropriate. To this end: (1) Alter the PKCS#7 parser to reject any message that has more than one signature where at least one signature has authattrs and at least one that does not. (2) Validate authattrs if they are present and strongly restrict them. Only the following authattrs are permitted and all others are rejected: (a) contentType. This is checked to be an OID that matches the content type in the SignedData object. (b) messageDigest. This must match the crypto digest of the data. (c) signingTime. If present, we check that this is a valid, parseable UTCTime or GeneralTime and that the date it encodes fits within the validity window of the matching X.509 cert. (d) S/MIME capabilities. We don't check the contents. (e) Authenticode SP Opus Info. We don't check the contents. (f) Authenticode Statement Type. We don't check the contents. The message is rejected if (a) or (b) are missing. If the message is an Authenticode type, the message is rejected if (e) is missing; if not Authenticode, the message is rejected if (d) - (f) are present. The S/MIME capabilities authattr (d) unfortunately has to be allowed to support kernels already signed by the pesign program. This only affects kexec. sign-file suppresses them (CMS_NOSMIMECAP). The message is also rejected if an authattr is given more than once or if it contains more than one element in its set of values. (3) Add a parameter to pkcs7_verify() to select one of the following restrictions and pass in the appropriate option from the callers: (*) VERIFYING_MODULE_SIGNATURE This requires that the SignedData content type be pkcs7-data and forbids authattrs. sign-file sets CMS_NOATTR. We could be more flexible and permit authattrs optionally, but only permit minimal content. (*) VERIFYING_FIRMWARE_SIGNATURE This requires that the SignedData content type be pkcs7-data and requires authattrs. In future, this will require an attribute holding the target firmware name in addition to the minimal set. (*) VERIFYING_UNSPECIFIED_SIGNATURE This requires that the SignedData content type be pkcs7-data but allows either no authattrs or only permits the minimal set. (*) VERIFYING_KEXEC_PE_SIGNATURE This only supports the Authenticode SPC_INDIRECT_DATA content type and requires at least an SpcSpOpusInfo authattr in addition to the minimal set. It also permits an SPC_STATEMENT_TYPE authattr (and an S/MIME capabilities authattr because the pesign program doesn't remove these). (*) VERIFYING_KEY_SIGNATURE (*) VERIFYING_KEY_SELF_SIGNATURE These are invalid in this context but are included for later use when limiting the use of X.509 certs. (4) The pkcs7_test key type is given a module parameter to select between the above options for testing purposes. For example: echo 1 >/sys/module/pkcs7_test_key/parameters/usage keyctl padd pkcs7_test foo @s </tmp/stuff.pkcs7 will attempt to check the signature on stuff.pkcs7 as if it contains a firmware blob (1 being VERIFYING_FIRMWARE_SIGNATURE). Suggested-by: Andy Lutomirski <luto@kernel.org> Signed-off-by: David Howells <dhowells@redhat.com> Reviewed-by: Marcel Holtmann <marcel@holtmann.org> Reviewed-by: David Woodhouse <David.Woodhouse@intel.com>
477 lines
12 KiB
C
477 lines
12 KiB
C
/* Parse a signed PE binary
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*
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* Copyright (C) 2014 Red Hat, Inc. All Rights Reserved.
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* Written by David Howells (dhowells@redhat.com)
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public Licence
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* as published by the Free Software Foundation; either version
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* 2 of the Licence, or (at your option) any later version.
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*/
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#define pr_fmt(fmt) "PEFILE: "fmt
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#include <linux/module.h>
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#include <linux/kernel.h>
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#include <linux/slab.h>
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#include <linux/err.h>
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#include <linux/pe.h>
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#include <linux/asn1.h>
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#include <crypto/pkcs7.h>
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#include <crypto/hash.h>
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#include "verify_pefile.h"
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/*
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* Parse a PE binary.
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*/
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static int pefile_parse_binary(const void *pebuf, unsigned int pelen,
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struct pefile_context *ctx)
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{
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const struct mz_hdr *mz = pebuf;
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const struct pe_hdr *pe;
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const struct pe32_opt_hdr *pe32;
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const struct pe32plus_opt_hdr *pe64;
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const struct data_directory *ddir;
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const struct data_dirent *dde;
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const struct section_header *secs, *sec;
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size_t cursor, datalen = pelen;
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kenter("");
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#define chkaddr(base, x, s) \
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do { \
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if ((x) < base || (s) >= datalen || (x) > datalen - (s)) \
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return -ELIBBAD; \
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} while (0)
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chkaddr(0, 0, sizeof(*mz));
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if (mz->magic != MZ_MAGIC)
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return -ELIBBAD;
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cursor = sizeof(*mz);
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chkaddr(cursor, mz->peaddr, sizeof(*pe));
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pe = pebuf + mz->peaddr;
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if (pe->magic != PE_MAGIC)
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return -ELIBBAD;
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cursor = mz->peaddr + sizeof(*pe);
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chkaddr(0, cursor, sizeof(pe32->magic));
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pe32 = pebuf + cursor;
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pe64 = pebuf + cursor;
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switch (pe32->magic) {
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case PE_OPT_MAGIC_PE32:
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chkaddr(0, cursor, sizeof(*pe32));
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ctx->image_checksum_offset =
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(unsigned long)&pe32->csum - (unsigned long)pebuf;
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ctx->header_size = pe32->header_size;
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cursor += sizeof(*pe32);
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ctx->n_data_dirents = pe32->data_dirs;
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break;
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case PE_OPT_MAGIC_PE32PLUS:
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chkaddr(0, cursor, sizeof(*pe64));
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ctx->image_checksum_offset =
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(unsigned long)&pe64->csum - (unsigned long)pebuf;
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ctx->header_size = pe64->header_size;
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cursor += sizeof(*pe64);
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ctx->n_data_dirents = pe64->data_dirs;
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break;
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default:
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pr_debug("Unknown PEOPT magic = %04hx\n", pe32->magic);
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return -ELIBBAD;
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}
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pr_debug("checksum @ %x\n", ctx->image_checksum_offset);
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pr_debug("header size = %x\n", ctx->header_size);
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if (cursor >= ctx->header_size || ctx->header_size >= datalen)
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return -ELIBBAD;
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if (ctx->n_data_dirents > (ctx->header_size - cursor) / sizeof(*dde))
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return -ELIBBAD;
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ddir = pebuf + cursor;
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cursor += sizeof(*dde) * ctx->n_data_dirents;
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ctx->cert_dirent_offset =
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(unsigned long)&ddir->certs - (unsigned long)pebuf;
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ctx->certs_size = ddir->certs.size;
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if (!ddir->certs.virtual_address || !ddir->certs.size) {
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pr_debug("Unsigned PE binary\n");
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return -EKEYREJECTED;
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}
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chkaddr(ctx->header_size, ddir->certs.virtual_address,
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ddir->certs.size);
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ctx->sig_offset = ddir->certs.virtual_address;
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ctx->sig_len = ddir->certs.size;
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pr_debug("cert = %x @%x [%*ph]\n",
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ctx->sig_len, ctx->sig_offset,
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ctx->sig_len, pebuf + ctx->sig_offset);
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ctx->n_sections = pe->sections;
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if (ctx->n_sections > (ctx->header_size - cursor) / sizeof(*sec))
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return -ELIBBAD;
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ctx->secs = secs = pebuf + cursor;
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return 0;
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}
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/*
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* Check and strip the PE wrapper from around the signature and check that the
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* remnant looks something like PKCS#7.
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*/
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static int pefile_strip_sig_wrapper(const void *pebuf,
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struct pefile_context *ctx)
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{
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struct win_certificate wrapper;
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const u8 *pkcs7;
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unsigned len;
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if (ctx->sig_len < sizeof(wrapper)) {
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pr_debug("Signature wrapper too short\n");
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return -ELIBBAD;
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}
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memcpy(&wrapper, pebuf + ctx->sig_offset, sizeof(wrapper));
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pr_debug("sig wrapper = { %x, %x, %x }\n",
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wrapper.length, wrapper.revision, wrapper.cert_type);
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/* Both pesign and sbsign round up the length of certificate table
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* (in optional header data directories) to 8 byte alignment.
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*/
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if (round_up(wrapper.length, 8) != ctx->sig_len) {
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pr_debug("Signature wrapper len wrong\n");
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return -ELIBBAD;
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}
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if (wrapper.revision != WIN_CERT_REVISION_2_0) {
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pr_debug("Signature is not revision 2.0\n");
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return -ENOTSUPP;
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}
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if (wrapper.cert_type != WIN_CERT_TYPE_PKCS_SIGNED_DATA) {
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pr_debug("Signature certificate type is not PKCS\n");
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return -ENOTSUPP;
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}
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/* It looks like the pkcs signature length in wrapper->length and the
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* size obtained from the data dir entries, which lists the total size
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* of certificate table, are both aligned to an octaword boundary, so
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* we may have to deal with some padding.
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*/
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ctx->sig_len = wrapper.length;
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ctx->sig_offset += sizeof(wrapper);
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ctx->sig_len -= sizeof(wrapper);
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if (ctx->sig_len < 4) {
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pr_debug("Signature data missing\n");
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return -EKEYREJECTED;
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}
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/* What's left should be a PKCS#7 cert */
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pkcs7 = pebuf + ctx->sig_offset;
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if (pkcs7[0] != (ASN1_CONS_BIT | ASN1_SEQ))
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goto not_pkcs7;
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switch (pkcs7[1]) {
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case 0 ... 0x7f:
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len = pkcs7[1] + 2;
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goto check_len;
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case ASN1_INDEFINITE_LENGTH:
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return 0;
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case 0x81:
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len = pkcs7[2] + 3;
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goto check_len;
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case 0x82:
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len = ((pkcs7[2] << 8) | pkcs7[3]) + 4;
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goto check_len;
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case 0x83 ... 0xff:
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return -EMSGSIZE;
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default:
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goto not_pkcs7;
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}
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check_len:
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if (len <= ctx->sig_len) {
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/* There may be padding */
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ctx->sig_len = len;
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return 0;
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}
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not_pkcs7:
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pr_debug("Signature data not PKCS#7\n");
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return -ELIBBAD;
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}
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/*
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* Compare two sections for canonicalisation.
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*/
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static int pefile_compare_shdrs(const void *a, const void *b)
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{
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const struct section_header *shdra = a;
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const struct section_header *shdrb = b;
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int rc;
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if (shdra->data_addr > shdrb->data_addr)
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return 1;
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if (shdrb->data_addr > shdra->data_addr)
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return -1;
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if (shdra->virtual_address > shdrb->virtual_address)
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return 1;
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if (shdrb->virtual_address > shdra->virtual_address)
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return -1;
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rc = strcmp(shdra->name, shdrb->name);
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if (rc != 0)
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return rc;
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if (shdra->virtual_size > shdrb->virtual_size)
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return 1;
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if (shdrb->virtual_size > shdra->virtual_size)
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return -1;
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if (shdra->raw_data_size > shdrb->raw_data_size)
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return 1;
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if (shdrb->raw_data_size > shdra->raw_data_size)
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return -1;
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return 0;
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}
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/*
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* Load the contents of the PE binary into the digest, leaving out the image
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* checksum and the certificate data block.
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*/
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static int pefile_digest_pe_contents(const void *pebuf, unsigned int pelen,
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struct pefile_context *ctx,
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struct shash_desc *desc)
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{
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unsigned *canon, tmp, loop, i, hashed_bytes;
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int ret;
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/* Digest the header and data directory, but leave out the image
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* checksum and the data dirent for the signature.
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*/
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ret = crypto_shash_update(desc, pebuf, ctx->image_checksum_offset);
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if (ret < 0)
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return ret;
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tmp = ctx->image_checksum_offset + sizeof(uint32_t);
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ret = crypto_shash_update(desc, pebuf + tmp,
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ctx->cert_dirent_offset - tmp);
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if (ret < 0)
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return ret;
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tmp = ctx->cert_dirent_offset + sizeof(struct data_dirent);
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ret = crypto_shash_update(desc, pebuf + tmp, ctx->header_size - tmp);
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if (ret < 0)
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return ret;
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canon = kcalloc(ctx->n_sections, sizeof(unsigned), GFP_KERNEL);
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if (!canon)
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return -ENOMEM;
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/* We have to canonicalise the section table, so we perform an
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* insertion sort.
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*/
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canon[0] = 0;
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for (loop = 1; loop < ctx->n_sections; loop++) {
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for (i = 0; i < loop; i++) {
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if (pefile_compare_shdrs(&ctx->secs[canon[i]],
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&ctx->secs[loop]) > 0) {
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memmove(&canon[i + 1], &canon[i],
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(loop - i) * sizeof(canon[0]));
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break;
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}
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}
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canon[i] = loop;
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}
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hashed_bytes = ctx->header_size;
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for (loop = 0; loop < ctx->n_sections; loop++) {
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i = canon[loop];
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if (ctx->secs[i].raw_data_size == 0)
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continue;
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ret = crypto_shash_update(desc,
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pebuf + ctx->secs[i].data_addr,
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ctx->secs[i].raw_data_size);
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if (ret < 0) {
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kfree(canon);
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return ret;
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}
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hashed_bytes += ctx->secs[i].raw_data_size;
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}
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kfree(canon);
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if (pelen > hashed_bytes) {
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tmp = hashed_bytes + ctx->certs_size;
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ret = crypto_shash_update(desc,
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pebuf + hashed_bytes,
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pelen - tmp);
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if (ret < 0)
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return ret;
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}
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return 0;
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}
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/*
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* Digest the contents of the PE binary, leaving out the image checksum and the
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* certificate data block.
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*/
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static int pefile_digest_pe(const void *pebuf, unsigned int pelen,
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struct pefile_context *ctx)
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{
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struct crypto_shash *tfm;
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struct shash_desc *desc;
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size_t digest_size, desc_size;
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void *digest;
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int ret;
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kenter(",%u", ctx->digest_algo);
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/* Allocate the hashing algorithm we're going to need and find out how
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* big the hash operational data will be.
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*/
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tfm = crypto_alloc_shash(hash_algo_name[ctx->digest_algo], 0, 0);
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if (IS_ERR(tfm))
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return (PTR_ERR(tfm) == -ENOENT) ? -ENOPKG : PTR_ERR(tfm);
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desc_size = crypto_shash_descsize(tfm) + sizeof(*desc);
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digest_size = crypto_shash_digestsize(tfm);
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if (digest_size != ctx->digest_len) {
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pr_debug("Digest size mismatch (%zx != %x)\n",
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digest_size, ctx->digest_len);
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ret = -EBADMSG;
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goto error_no_desc;
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}
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pr_debug("Digest: desc=%zu size=%zu\n", desc_size, digest_size);
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ret = -ENOMEM;
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desc = kzalloc(desc_size + digest_size, GFP_KERNEL);
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if (!desc)
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goto error_no_desc;
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desc->tfm = tfm;
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desc->flags = CRYPTO_TFM_REQ_MAY_SLEEP;
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ret = crypto_shash_init(desc);
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if (ret < 0)
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goto error;
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ret = pefile_digest_pe_contents(pebuf, pelen, ctx, desc);
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if (ret < 0)
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goto error;
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digest = (void *)desc + desc_size;
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ret = crypto_shash_final(desc, digest);
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if (ret < 0)
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goto error;
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pr_debug("Digest calc = [%*ph]\n", ctx->digest_len, digest);
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/* Check that the PE file digest matches that in the MSCODE part of the
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* PKCS#7 certificate.
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*/
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if (memcmp(digest, ctx->digest, ctx->digest_len) != 0) {
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pr_debug("Digest mismatch\n");
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ret = -EKEYREJECTED;
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} else {
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pr_debug("The digests match!\n");
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}
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error:
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kfree(desc);
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error_no_desc:
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crypto_free_shash(tfm);
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kleave(" = %d", ret);
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return ret;
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}
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/**
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* verify_pefile_signature - Verify the signature on a PE binary image
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* @pebuf: Buffer containing the PE binary image
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* @pelen: Length of the binary image
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* @trust_keyring: Signing certificates to use as starting points
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* @usage: The use to which the key is being put.
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* @_trusted: Set to true if trustworth, false otherwise
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*
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* Validate that the certificate chain inside the PKCS#7 message inside the PE
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* binary image intersects keys we already know and trust.
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*
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* Returns, in order of descending priority:
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*
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* (*) -ELIBBAD if the image cannot be parsed, or:
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*
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* (*) -EKEYREJECTED if a signature failed to match for which we have a valid
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* key, or:
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*
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* (*) 0 if at least one signature chain intersects with the keys in the trust
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* keyring, or:
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*
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* (*) -ENOPKG if a suitable crypto module couldn't be found for a check on a
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* chain.
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*
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* (*) -ENOKEY if we couldn't find a match for any of the signature chains in
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* the message.
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*
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* May also return -ENOMEM.
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*/
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int verify_pefile_signature(const void *pebuf, unsigned pelen,
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struct key *trusted_keyring,
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enum key_being_used_for usage,
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bool *_trusted)
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{
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struct pkcs7_message *pkcs7;
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struct pefile_context ctx;
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const void *data;
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size_t datalen;
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int ret;
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kenter("");
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memset(&ctx, 0, sizeof(ctx));
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ret = pefile_parse_binary(pebuf, pelen, &ctx);
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if (ret < 0)
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return ret;
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ret = pefile_strip_sig_wrapper(pebuf, &ctx);
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if (ret < 0)
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return ret;
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pkcs7 = pkcs7_parse_message(pebuf + ctx.sig_offset, ctx.sig_len);
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if (IS_ERR(pkcs7))
|
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return PTR_ERR(pkcs7);
|
|
ctx.pkcs7 = pkcs7;
|
|
|
|
ret = pkcs7_get_content_data(ctx.pkcs7, &data, &datalen, false);
|
|
if (ret < 0 || datalen == 0) {
|
|
pr_devel("PKCS#7 message does not contain data\n");
|
|
ret = -EBADMSG;
|
|
goto error;
|
|
}
|
|
|
|
ret = mscode_parse(&ctx);
|
|
if (ret < 0)
|
|
goto error;
|
|
|
|
pr_debug("Digest: %u [%*ph]\n",
|
|
ctx.digest_len, ctx.digest_len, ctx.digest);
|
|
|
|
/* Generate the digest and check against the PKCS7 certificate
|
|
* contents.
|
|
*/
|
|
ret = pefile_digest_pe(pebuf, pelen, &ctx);
|
|
if (ret < 0)
|
|
goto error;
|
|
|
|
ret = pkcs7_verify(pkcs7, usage);
|
|
if (ret < 0)
|
|
goto error;
|
|
|
|
ret = pkcs7_validate_trust(pkcs7, trusted_keyring, _trusted);
|
|
|
|
error:
|
|
pkcs7_free_message(ctx.pkcs7);
|
|
return ret;
|
|
}
|