verifier.c

/*
 * Copyright (C) 2008 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include "common.h"
#include "verifier.h"

#include "mincrypt/rsa.h"
#include "mincrypt/sha.h"

#include <string.h>
#include <stdio.h>
#include <errno.h>

// Look for an RSA signature embedded in the .ZIP file comment given
// the path to the zip.  Verify it matches one of the given public
// keys.
//
// Return VERIFY_SUCCESS, VERIFY_FAILURE (if any error is encountered
// or no key matches the signature).

int verify_file(const char* path, const RSAPublicKey *pKeys, unsigned int numKeys) {
    ui_set_progress(0.0);

    FILE* f = fopen(path, "rb");
    if (f == NULL) {
        LOGE("failed to open %s (%s)\n", path, strerror(errno));
        return VERIFY_FAILURE;
    }

    // An archive with a whole-file signature will end in six bytes:
    //
    //   $ff $ff (2-byte comment size) (2-byte signature start)
    //
    // (As far as the ZIP format is concerned, these are part of the
    // archive comment.)  We start by reading this footer, this tells
    // us how far back from the end we have to start reading to find
    // the whole comment.

#define FOOTER_SIZE 6

    if (fseek(f, -FOOTER_SIZE, SEEK_END) != 0) {
        LOGE("failed to seek in %s (%s)\n", path, strerror(errno));
        fclose(f);
        return VERIFY_FAILURE;
    }

    unsigned char footer[FOOTER_SIZE];
    if (fread(footer, 1, FOOTER_SIZE, f) != FOOTER_SIZE) {
        LOGE("failed to read footer from %s (%s)\n", path, strerror(errno));
        fclose(f);
        return VERIFY_FAILURE;
    }

    if (footer[2] != 0xff || footer[3] != 0xff) {
        fclose(f);
        return VERIFY_FAILURE;
    }

    int comment_size = footer[4] + (footer[5] << 8);
    int signature_start = footer[0] + (footer[1] << 8);
    LOGI("comment is %d bytes; signature %d bytes from end\n",
         comment_size, signature_start);

    if (signature_start - FOOTER_SIZE < RSANUMBYTES) {
        // "signature" block isn't big enough to contain an RSA block.
        LOGE("signature is too short\n");
        fclose(f);
        return VERIFY_FAILURE;
    }

#define EOCD_HEADER_SIZE 22

    // The end-of-central-directory record is 22 bytes plus any
    // comment length.
    size_t eocd_size = comment_size + EOCD_HEADER_SIZE;

    if (fseek(f, -eocd_size, SEEK_END) != 0) {
        LOGE("failed to seek in %s (%s)\n", path, strerror(errno));
        fclose(f);
        return VERIFY_FAILURE;
    }

    // Determine how much of the file is covered by the signature.
    // This is everything except the signature data and length, which
    // includes all of the EOCD except for the comment length field (2
    // bytes) and the comment data.
    size_t signed_len = ftell(f) + EOCD_HEADER_SIZE - 2;

    unsigned char* eocd = malloc(eocd_size);
    if (eocd == NULL) {
        LOGE("malloc for EOCD record failed\n");
        fclose(f);
        return VERIFY_FAILURE;
    }
    if (fread(eocd, 1, eocd_size, f) != eocd_size) {
        LOGE("failed to read eocd from %s (%s)\n", path, strerror(errno));
        fclose(f);
        return VERIFY_FAILURE;
    }

    // If this is really is the EOCD record, it will begin with the
    // magic number $50 $4b $05 $06.
    if (eocd[0] != 0x50 || eocd[1] != 0x4b ||
        eocd[2] != 0x05 || eocd[3] != 0x06) {
        LOGE("signature length doesn't match EOCD marker\n");
        fclose(f);
        return VERIFY_FAILURE;
    }

    int i;
    for (i = 4; i < eocd_size-3; ++i) {
        if (eocd[i  ] == 0x50 && eocd[i+1] == 0x4b &&
            eocd[i+2] == 0x05 && eocd[i+1] == 0x06) {
            // if the sequence $50 $4b $05 $06 appears anywhere after
            // the real one, minzip will find the later (wrong) one,
            // which could be exploitable.  Fail verification if
            // this sequence occurs anywhere after the real one.
            LOGE("EOCD marker occurs after start of EOCD\n");
            fclose(f);
            return VERIFY_FAILURE;
        }
    }

#define BUFFER_SIZE 4096

    SHA_CTX ctx;
    SHA_init(&ctx);
    unsigned char* buffer = malloc(BUFFER_SIZE);
    if (buffer == NULL) {
        LOGE("failed to alloc memory for sha1 buffer\n");
        fclose(f);
        return VERIFY_FAILURE;
    }

    double frac = -1.0;
    size_t so_far = 0;
    fseek(f, 0, SEEK_SET);
    while (so_far < signed_len) {
        int size = BUFFER_SIZE;
        if (signed_len - so_far < size) size = signed_len - so_far;
        if (fread(buffer, 1, size, f) != size) {
            LOGE("failed to read data from %s (%s)\n", path, strerror(errno));
            fclose(f);
            return VERIFY_FAILURE;
        }
        SHA_update(&ctx, buffer, size);
        so_far += size;
        double f = so_far / (double)signed_len;
        if (f > frac + 0.02 || size == so_far) {
            ui_set_progress(f);
            frac = f;
        }
    }
    fclose(f);
    free(buffer);

    const uint8_t* sha1 = SHA_final(&ctx);
    for (i = 0; i < numKeys; ++i) {
        // The 6 bytes is the "$ff $ff (signature_start) (comment_size)" that
        // the signing tool appends after the signature itself.
        if (RSA_verify(pKeys+i, eocd + eocd_size - 6 - RSANUMBYTES,
                       RSANUMBYTES, sha1)) {
            LOGI("whole-file signature verified\n");
            free(eocd);
            return VERIFY_SUCCESS;
        }
    }
    free(eocd);
    LOGE("failed to verify whole-file signature\n");
    return VERIFY_FAILURE;
}