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![Loganaden Velvindron](/assets/img/avatar_default.png)
Signed-off-by: Loganaden Velvindron <logan@hackers.mu> Signed-off-by: Yasir Auleear <yasirmx@hackers.mu> Signed-off-by: David Howells <dhowells@redhat.com> Signed-off-by: James Morris <james.l.morris@oracle.com>
455 lines
11 KiB
C
455 lines
11 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 <linux/verification.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(",%s", 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(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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kzfree(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_keys: Signing certificate(s) to use as starting points
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* @usage: The use to which the key is being put.
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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_keys,
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enum key_being_used_for usage)
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{
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struct pefile_context ctx;
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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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ret = verify_pkcs7_signature(NULL, 0,
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pebuf + ctx.sig_offset, ctx.sig_len,
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trusted_keys, usage,
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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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error:
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kzfree(ctx.digest);
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return ret;
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}
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