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![David Howells](/assets/img/avatar_default.png)
Relax the check on the length of the PKCS#7 cert as it appears that the PE file wrapper size gets rounded up to the nearest 8. The debugging output looks like this: PEFILE: ==> verify_pefile_signature() PEFILE: ==> pefile_parse_binary() PEFILE: checksum @ 110 PEFILE: header size = 200 PEFILE: cert = 968 @547be0 [68 09 00 00 00 02 02 00 30 82 09 56 ] PEFILE: sig wrapper = { 968, 200, 2 } PEFILE: Signature data not PKCS#7 The wrapper is the first 8 bytes of the hex dump inside []. This indicates a length of 0x968 bytes, including the wrapper header - so 0x960 bytes of payload. The ASN.1 wrapper begins [ ... 30 82 09 56 ]. That indicates an object of size 0x956 - a four byte discrepency, presumably just padding for alignment purposes. So we just check that the ASN.1 container is no bigger than the payload and reduce the recorded size appropriately. Whilst we're at it, allow shorter PKCS#7 objects that manage to squeeze within 127 or 255 bytes. It's just about conceivable if no X.509 certs are included in the PKCS#7 message. Reported-by: Vivek Goyal <vgoyal@redhat.com> Signed-off-by: David Howells <dhowells@redhat.com> Acked-by: Vivek Goyal <vgoyal@redhat.com> Acked-by: Peter Jones <pjones@redhat.com> Signed-off-by: James Morris <james.l.morris@oracle.com>
474 lines
12 KiB
C
474 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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* @_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, 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);
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ctx.pkcs7 = pkcs7;
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ret = pkcs7_get_content_data(ctx.pkcs7, &data, &datalen, false);
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if (ret < 0 || datalen == 0) {
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pr_devel("PKCS#7 message does not contain data\n");
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ret = -EBADMSG;
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goto error;
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}
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ret = mscode_parse(&ctx);
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if (ret < 0)
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goto error;
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pr_debug("Digest: %u [%*ph]\n",
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ctx.digest_len, ctx.digest_len, ctx.digest);
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/* Generate the digest and check against the PKCS7 certificate
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* contents.
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*/
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ret = pefile_digest_pe(pebuf, pelen, &ctx);
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if (ret < 0)
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goto error;
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ret = pkcs7_verify(pkcs7);
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if (ret < 0)
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goto error;
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ret = pkcs7_validate_trust(pkcs7, trusted_keyring, _trusted);
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error:
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pkcs7_free_message(ctx.pkcs7);
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return ret;
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}
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