crypto: hmac - require that the underlying hash algorithm is unkeyed
authorEric Biggers <>
Wed, 29 Nov 2017 02:01:38 +0000 (18:01 -0800)
committerBen Hutchings <>
Mon, 1 Jan 2018 20:51:05 +0000 (20:51 +0000)
commit af3ff8045bbf3e32f1a448542e73abb4c8ceb6f1 upstream.

Because the HMAC template didn't check that its underlying hash
algorithm is unkeyed, trying to use "hmac(hmac(sha3-512-generic))"
through AF_ALG or through KEYCTL_DH_COMPUTE resulted in the inner HMAC
being used without having been keyed, resulting in sha3_update() being
called without sha3_init(), causing a stack buffer overflow.

This is a very old bug, but it seems to have only started causing real
problems when SHA-3 support was added (requires CONFIG_CRYPTO_SHA3)
because the innermost hash's state is ->import()ed from a zeroed buffer,
and it just so happens that other hash algorithms are fine with that,
but SHA-3 is not.  However, there could be arch or hardware-dependent
hash algorithms also affected; I couldn't test everything.

Fix the bug by introducing a function crypto_shash_alg_has_setkey()
which tests whether a shash algorithm is keyed.  Then update the HMAC
template to require that its underlying hash algorithm is unkeyed.

Here is a reproducer:

    #include <linux/if_alg.h>
    #include <sys/socket.h>

    int main()
        int algfd;
        struct sockaddr_alg addr = {
            .salg_type = "hash",
            .salg_name = "hmac(hmac(sha3-512-generic))",
        char key[4096] = { 0 };

        algfd = socket(AF_ALG, SOCK_SEQPACKET, 0);
        bind(algfd, (const struct sockaddr *)&addr, sizeof(addr));
        setsockopt(algfd, SOL_ALG, ALG_SET_KEY, key, sizeof(key));

Here was the KASAN report from syzbot:

    BUG: KASAN: stack-out-of-bounds in memcpy include/linux/string.h:341  [inline]
    BUG: KASAN: stack-out-of-bounds in sha3_update+0xdf/0x2e0  crypto/sha3_generic.c:161
    Write of size 4096 at addr ffff8801cca07c40 by task syzkaller076574/3044

    CPU: 1 PID: 3044 Comm: syzkaller076574 Not tainted 4.14.0-mm1+ #25
    Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  Google 01/01/2011
    Call Trace:
      __dump_stack lib/dump_stack.c:17 [inline]
      dump_stack+0x194/0x257 lib/dump_stack.c:53
      print_address_description+0x73/0x250 mm/kasan/report.c:252
      kasan_report_error mm/kasan/report.c:351 [inline]
      kasan_report+0x25b/0x340 mm/kasan/report.c:409
      check_memory_region_inline mm/kasan/kasan.c:260 [inline]
      check_memory_region+0x137/0x190 mm/kasan/kasan.c:267
      memcpy+0x37/0x50 mm/kasan/kasan.c:303
      memcpy include/linux/string.h:341 [inline]
      sha3_update+0xdf/0x2e0 crypto/sha3_generic.c:161
      crypto_shash_update+0xcb/0x220 crypto/shash.c:109
      shash_finup_unaligned+0x2a/0x60 crypto/shash.c:151
      crypto_shash_finup+0xc4/0x120 crypto/shash.c:165
      hmac_finup+0x182/0x330 crypto/hmac.c:152
      crypto_shash_finup+0xc4/0x120 crypto/shash.c:165
      shash_digest_unaligned+0x9e/0xd0 crypto/shash.c:172
      crypto_shash_digest+0xc4/0x120 crypto/shash.c:186
      hmac_setkey+0x36a/0x690 crypto/hmac.c:66
      crypto_shash_setkey+0xad/0x190 crypto/shash.c:64
      shash_async_setkey+0x47/0x60 crypto/shash.c:207
      crypto_ahash_setkey+0xaf/0x180 crypto/ahash.c:200
      hash_setkey+0x40/0x90 crypto/algif_hash.c:446
      alg_setkey crypto/af_alg.c:221 [inline]
      alg_setsockopt+0x2a1/0x350 crypto/af_alg.c:254
      SYSC_setsockopt net/socket.c:1851 [inline]
      SyS_setsockopt+0x189/0x360 net/socket.c:1830

Reported-by: syzbot <>
Signed-off-by: Eric Biggers <>
Signed-off-by: Herbert Xu <>
Signed-off-by: Ben Hutchings <>

index ade790b..758688b 100644 (file)
@@ -197,11 +197,15 @@ static int hmac_create(struct crypto_template *tmpl, struct rtattr **tb)
        salg = shash_attr_alg(tb[1], 0, 0);
        if (IS_ERR(salg))
                return PTR_ERR(salg);
+       alg = &salg->base;
+       /* The underlying hash algorithm must be unkeyed */
        err = -EINVAL;
+       if (crypto_shash_alg_has_setkey(salg))
+               goto out_put_alg;
        ds = salg->digestsize;
        ss = salg->statesize;
-       alg = &salg->base;
        if (ds > alg->cra_blocksize ||
            ss < alg->cra_blocksize)
                goto out_put_alg;
index 3e2c385..0d66c2e 100644 (file)
 static const struct crypto_type crypto_shash_type;
-static int shash_no_setkey(struct crypto_shash *tfm, const u8 *key,
-                          unsigned int keylen)
+int shash_no_setkey(struct crypto_shash *tfm, const u8 *key,
+                   unsigned int keylen)
        return -ENOSYS;
 static int shash_setkey_unaligned(struct crypto_shash *tfm, const u8 *key,
                                  unsigned int keylen)
index 7bdb812..b813ae9 100644 (file)
@@ -70,6 +70,14 @@ int ahash_register_instance(struct crypto_template *tmpl,
                            struct ahash_instance *inst);
 void ahash_free_instance(struct crypto_instance *inst);
+int shash_no_setkey(struct crypto_shash *tfm, const u8 *key,
+                   unsigned int keylen);
+static inline bool crypto_shash_alg_has_setkey(struct shash_alg *alg)
+       return alg->setkey != shash_no_setkey;
 int crypto_init_ahash_spawn(struct crypto_ahash_spawn *spawn,
                            struct hash_alg_common *alg,
                            struct crypto_instance *inst);