/* Parse a signed PE binary
 *
 * Copyright (C) 2014 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells (dhowells@redhat.com)
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public Licence
 * as published by the Free Software Foundation; either version
 * 2 of the Licence, or (at your option) any later version.
 */

#define pr_fmt(fmt) "PEFILE: "fmt
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/slab.h>
#include <linux/err.h>
#include <linux/pe.h>
#include <linux/asn1.h>
#include <linux/verification.h>
#include <crypto/hash.h>
#include "verify_pefile.h"

/*
 * Parse a PE binary.
 */
static int pefile_parse_binary(const void *pebuf, unsigned int pelen,
                               struct pefile_context *ctx)
{
        const struct mz_hdr *mz = pebuf;
        const struct pe_hdr *pe;
        const struct pe32_opt_hdr *pe32;
        const struct pe32plus_opt_hdr *pe64;
        const struct data_directory *ddir;
        const struct data_dirent *dde;
        const struct section_header *secs, *sec;
        size_t cursor, datalen = pelen;

        kenter("");

#define chkaddr(base, x, s)                                             \
        do {                                                            \
                if ((x) < base || (s) >= datalen || (x) > datalen - (s)) \
                        return -ELIBBAD;                                \
        } while (0)

        chkaddr(0, 0, sizeof(*mz));
        if (mz->magic != MZ_MAGIC)
                return -ELIBBAD;
        cursor = sizeof(*mz);

        chkaddr(cursor, mz->peaddr, sizeof(*pe));
        pe = pebuf + mz->peaddr;
        if (pe->magic != PE_MAGIC)
                return -ELIBBAD;
        cursor = mz->peaddr + sizeof(*pe);

        chkaddr(0, cursor, sizeof(pe32->magic));
        pe32 = pebuf + cursor;
        pe64 = pebuf + cursor;

        switch (pe32->magic) {
        case PE_OPT_MAGIC_PE32:
                chkaddr(0, cursor, sizeof(*pe32));
                ctx->image_checksum_offset =
                        (unsigned long)&pe32->csum - (unsigned long)pebuf;
                ctx->header_size = pe32->header_size;
                cursor += sizeof(*pe32);
                ctx->n_data_dirents = pe32->data_dirs;
                break;

        case PE_OPT_MAGIC_PE32PLUS:
                chkaddr(0, cursor, sizeof(*pe64));
                ctx->image_checksum_offset =
                        (unsigned long)&pe64->csum - (unsigned long)pebuf;
                ctx->header_size = pe64->header_size;
                cursor += sizeof(*pe64);
                ctx->n_data_dirents = pe64->data_dirs;
                break;

        default:
                pr_debug("Unknown PEOPT magic = %04hx\n", pe32->magic);
                return -ELIBBAD;
        }

        pr_debug("checksum @ %x\n", ctx->image_checksum_offset);
        pr_debug("header size = %x\n", ctx->header_size);

        if (cursor >= ctx->header_size || ctx->header_size >= datalen)
                return -ELIBBAD;

        if (ctx->n_data_dirents > (ctx->header_size - cursor) / sizeof(*dde))
                return -ELIBBAD;

        ddir = pebuf + cursor;
        cursor += sizeof(*dde) * ctx->n_data_dirents;

        ctx->cert_dirent_offset =
                (unsigned long)&ddir->certs - (unsigned long)pebuf;
        ctx->certs_size = ddir->certs.size;

        if (!ddir->certs.virtual_address || !ddir->certs.size) {
                pr_debug("Unsigned PE binary\n");
                return -EKEYREJECTED;
        }

        chkaddr(ctx->header_size, ddir->certs.virtual_address,
                ddir->certs.size);
        ctx->sig_offset = ddir->certs.virtual_address;
        ctx->sig_len = ddir->certs.size;
        pr_debug("cert = %x @%x [%*ph]\n",
                 ctx->sig_len, ctx->sig_offset,
                 ctx->sig_len, pebuf + ctx->sig_offset);

        ctx->n_sections = pe->sections;
        if (ctx->n_sections > (ctx->header_size - cursor) / sizeof(*sec))
                return -ELIBBAD;
        ctx->secs = secs = pebuf + cursor;

        return 0;
}

/*
 * Check and strip the PE wrapper from around the signature and check that the
 * remnant looks something like PKCS#7.
 */
static int pefile_strip_sig_wrapper(const void *pebuf,
                                    struct pefile_context *ctx)
{
        struct win_certificate wrapper;
        const u8 *pkcs7;
        unsigned len;

        if (ctx->sig_len < sizeof(wrapper)) {
                pr_debug("Signature wrapper too short\n");
                return -ELIBBAD;
        }

        memcpy(&wrapper, pebuf + ctx->sig_offset, sizeof(wrapper));
        pr_debug("sig wrapper = { %x, %x, %x }\n",
                 wrapper.length, wrapper.revision, wrapper.cert_type);

        /* Both pesign and sbsign round up the length of certificate table
         * (in optional header data directories) to 8 byte alignment.
         */
        if (round_up(wrapper.length, 8) != ctx->sig_len) {
                pr_debug("Signature wrapper len wrong\n");
                return -ELIBBAD;
        }
        if (wrapper.revision != WIN_CERT_REVISION_2_0) {
                pr_debug("Signature is not revision 2.0\n");
                return -ENOTSUPP;
        }
        if (wrapper.cert_type != WIN_CERT_TYPE_PKCS_SIGNED_DATA) {
                pr_debug("Signature certificate type is not PKCS\n");
                return -ENOTSUPP;
        }

        /* It looks like the pkcs signature length in wrapper->length and the
         * size obtained from the data dir entries, which lists the total size
         * of certificate table, are both aligned to an octaword boundary, so
         * we may have to deal with some padding.
         */
        ctx->sig_len = wrapper.length;
        ctx->sig_offset += sizeof(wrapper);
        ctx->sig_len -= sizeof(wrapper);
        if (ctx->sig_len < 4) {
                pr_debug("Signature data missing\n");
                return -EKEYREJECTED;
        }

        /* What's left should be a PKCS#7 cert */
        pkcs7 = pebuf + ctx->sig_offset;
        if (pkcs7[0] != (ASN1_CONS_BIT | ASN1_SEQ))
                goto not_pkcs7;

        switch (pkcs7[1]) {
        case 0 ... 0x7f:
                len = pkcs7[1] + 2;
                goto check_len;
        case ASN1_INDEFINITE_LENGTH:
                return 0;
        case 0x81:
                len = pkcs7[2] + 3;
                goto check_len;
        case 0x82:
                len = ((pkcs7[2] << 8) | pkcs7[3]) + 4;
                goto check_len;
        case 0x83 ... 0xff:
                return -EMSGSIZE;
        default:
                goto not_pkcs7;
        }

check_len:
        if (len <= ctx->sig_len) {
                /* There may be padding */
                ctx->sig_len = len;
                return 0;
        }
not_pkcs7:
        pr_debug("Signature data not PKCS#7\n");
        return -ELIBBAD;
}

/*
 * Compare two sections for canonicalisation.
 */
static int pefile_compare_shdrs(const void *a, const void *b)
{
        const struct section_header *shdra = a;
        const struct section_header *shdrb = b;
        int rc;

        if (shdra->data_addr > shdrb->data_addr)
                return 1;
        if (shdrb->data_addr > shdra->data_addr)
                return -1;

        if (shdra->virtual_address > shdrb->virtual_address)
                return 1;
        if (shdrb->virtual_address > shdra->virtual_address)
                return -1;

        rc = strcmp(shdra->name, shdrb->name);
        if (rc != 0)
                return rc;

        if (shdra->virtual_size > shdrb->virtual_size)
                return 1;
        if (shdrb->virtual_size > shdra->virtual_size)
                return -1;

        if (shdra->raw_data_size > shdrb->raw_data_size)
                return 1;
        if (shdrb->raw_data_size > shdra->raw_data_size)
                return -1;

        return 0;
}

/*
 * Load the contents of the PE binary into the digest, leaving out the image
 * checksum and the certificate data block.
 */
static int pefile_digest_pe_contents(const void *pebuf, unsigned int pelen,
                                     struct pefile_context *ctx,
                                     struct shash_desc *desc)
{
        unsigned *canon, tmp, loop, i, hashed_bytes;
        int ret;

        /* Digest the header and data directory, but leave out the image
         * checksum and the data dirent for the signature.
         */
        ret = crypto_shash_update(desc, pebuf, ctx->image_checksum_offset);
        if (ret < 0)
                return ret;

        tmp = ctx->image_checksum_offset + sizeof(uint32_t);
        ret = crypto_shash_update(desc, pebuf + tmp,
                                  ctx->cert_dirent_offset - tmp);
        if (ret < 0)
                return ret;

        tmp = ctx->cert_dirent_offset + sizeof(struct data_dirent);
        ret = crypto_shash_update(desc, pebuf + tmp, ctx->header_size - tmp);
        if (ret < 0)
                return ret;

        canon = kcalloc(ctx->n_sections, sizeof(unsigned), GFP_KERNEL);
        if (!canon)
                return -ENOMEM;

        /* We have to canonicalise the section table, so we perform an
         * insertion sort.
         */
        canon[0] = 0;
        for (loop = 1; loop < ctx->n_sections; loop++) {
                for (i = 0; i < loop; i++) {
                        if (pefile_compare_shdrs(&ctx->secs[canon[i]],
                                                 &ctx->secs[loop]) > 0) {
                                memmove(&canon[i + 1], &canon[i],
                                        (loop - i) * sizeof(canon[0]));
                                break;
                        }
                }
                canon[i] = loop;
        }

        hashed_bytes = ctx->header_size;
        for (loop = 0; loop < ctx->n_sections; loop++) {
                i = canon[loop];
                if (ctx->secs[i].raw_data_size == 0)
                        continue;
                ret = crypto_shash_update(desc,
                                          pebuf + ctx->secs[i].data_addr,
                                          ctx->secs[i].raw_data_size);
                if (ret < 0) {
                        kfree(canon);
                        return ret;
                }
                hashed_bytes += ctx->secs[i].raw_data_size;
        }
        kfree(canon);

        if (pelen > hashed_bytes) {
                tmp = hashed_bytes + ctx->certs_size;
                ret = crypto_shash_update(desc,
                                          pebuf + hashed_bytes,
                                          pelen - tmp);
                if (ret < 0)
                        return ret;
        }

        return 0;
}

/*
 * Digest the contents of the PE binary, leaving out the image checksum and the
 * certificate data block.
 */
static int pefile_digest_pe(const void *pebuf, unsigned int pelen,
                            struct pefile_context *ctx)
{
        struct crypto_shash *tfm;
        struct shash_desc *desc;
        size_t digest_size, desc_size;
        void *digest;
        int ret;

        kenter(",%s", ctx->digest_algo);

        /* Allocate the hashing algorithm we're going to need and find out how
         * big the hash operational data will be.
         */
        tfm = crypto_alloc_shash(ctx->digest_algo, 0, 0);
        if (IS_ERR(tfm))
                return (PTR_ERR(tfm) == -ENOENT) ? -ENOPKG : PTR_ERR(tfm);

        desc_size = crypto_shash_descsize(tfm) + sizeof(*desc);
        digest_size = crypto_shash_digestsize(tfm);

        if (digest_size != ctx->digest_len) {
                pr_debug("Digest size mismatch (%zx != %x)\n",
                         digest_size, ctx->digest_len);
                ret = -EBADMSG;
                goto error_no_desc;
        }
        pr_debug("Digest: desc=%zu size=%zu\n", desc_size, digest_size);

        ret = -ENOMEM;
        desc = kzalloc(desc_size + digest_size, GFP_KERNEL);
        if (!desc)
                goto error_no_desc;

        desc->tfm   = tfm;
        desc->flags = CRYPTO_TFM_REQ_MAY_SLEEP;
        ret = crypto_shash_init(desc);
        if (ret < 0)
                goto error;

        ret = pefile_digest_pe_contents(pebuf, pelen, ctx, desc);
        if (ret < 0)
                goto error;

        digest = (void *)desc + desc_size;
        ret = crypto_shash_final(desc, digest);
        if (ret < 0)
                goto error;

        pr_debug("Digest calc = [%*ph]\n", ctx->digest_len, digest);

        /* Check that the PE file digest matches that in the MSCODE part of the
         * PKCS#7 certificate.
         */
        if (memcmp(digest, ctx->digest, ctx->digest_len) != 0) {
                pr_debug("Digest mismatch\n");
                ret = -EKEYREJECTED;
        } else {
                pr_debug("The digests match!\n");
        }

error:
        kzfree(desc);
error_no_desc:
        crypto_free_shash(tfm);
        kleave(" = %d", ret);
        return ret;
}

/**
 * verify_pefile_signature - Verify the signature on a PE binary image
 * @pebuf: Buffer containing the PE binary image
 * @pelen: Length of the binary image
 * @trust_keys: Signing certificate(s) to use as starting points
 * @usage: The use to which the key is being put.
 *
 * Validate that the certificate chain inside the PKCS#7 message inside the PE
 * binary image intersects keys we already know and trust.
 *
 * Returns, in order of descending priority:
 *
 *  (*) -ELIBBAD if the image cannot be parsed, or:
 *
 *  (*) -EKEYREJECTED if a signature failed to match for which we have a valid
 *      key, or:
 *
 *  (*) 0 if at least one signature chain intersects with the keys in the trust
 *      keyring, or:
 *
 *  (*) -ENOPKG if a suitable crypto module couldn't be found for a check on a
 *      chain.
 *
 *  (*) -ENOKEY if we couldn't find a match for any of the signature chains in
 *      the message.
 *
 * May also return -ENOMEM.
 */
int verify_pefile_signature(const void *pebuf, unsigned pelen,
                            struct key *trusted_keys,
                            enum key_being_used_for usage)
{
        struct pefile_context ctx;
        int ret;

        kenter("");

        memset(&ctx, 0, sizeof(ctx));
        ret = pefile_parse_binary(pebuf, pelen, &ctx);
        if (ret < 0)
                return ret;

        ret = pefile_strip_sig_wrapper(pebuf, &ctx);
        if (ret < 0)
                return ret;

        ret = verify_pkcs7_signature(NULL, 0,
                                     pebuf + ctx.sig_offset, ctx.sig_len,
                                     trusted_keys, usage,
                                     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);

error:
        kzfree(ctx.digest);
        return ret;
}