crypto: authenc - fix parsing key with misaligned rta_len

Keys for "authenc" AEADs are formatted as an rtattr containing a 4-byte
'enckeylen', followed by an authentication key and an encryption key.
crypto_authenc_extractkeys() parses the key to find the inner keys.

However, it fails to consider the case where the rtattr's payload is
longer than 4 bytes but not 4-byte aligned, and where the key ends
before the next 4-byte aligned boundary.  In this case, 'keylen -=
RTA_ALIGN(rta->rta_len);' underflows to a value near UINT_MAX.  This
causes a buffer overread and crash during crypto_ahash_setkey().

Fix it by restricting the rtattr payload to the expected size.

Reproducer using AF_ALG:

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

	int main()
	{
		int fd;
		struct sockaddr_alg addr = {
			.salg_type = "aead",
			.salg_name = "authenc(hmac(sha256),cbc(aes))",
		};
		struct {
			struct rtattr attr;
			__be32 enckeylen;
			char keys[1];
		} __attribute__((packed)) key = {
			.attr.rta_len = sizeof(key),
			.attr.rta_type = 1 /* CRYPTO_AUTHENC_KEYA_PARAM */,
		};

		fd = socket(AF_ALG, SOCK_SEQPACKET, 0);
		bind(fd, (void *)&addr, sizeof(addr));
		setsockopt(fd, SOL_ALG, ALG_SET_KEY, &key, sizeof(key));
	}

It caused:

	BUG: unable to handle kernel paging request at ffff88007ffdc000
	PGD 2e01067 P4D 2e01067 PUD 2e04067 PMD 2e05067 PTE 0
	Oops: 0000 [#1] SMP
	CPU: 0 PID: 883 Comm: authenc Not tainted 4.20.0-rc1-00108-g00c9fe37a7f27 #13
	Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-20181126_142135-anatol 04/01/2014
	RIP: 0010:sha256_ni_transform+0xb3/0x330 arch/x86/crypto/sha256_ni_asm.S:155
	[...]
	Call Trace:
	 sha256_ni_finup+0x10/0x20 arch/x86/crypto/sha256_ssse3_glue.c:321
	 crypto_shash_finup+0x1a/0x30 crypto/shash.c:178
	 shash_digest_unaligned+0x45/0x60 crypto/shash.c:186
	 crypto_shash_digest+0x24/0x40 crypto/shash.c:202
	 hmac_setkey+0x135/0x1e0 crypto/hmac.c:66
	 crypto_shash_setkey+0x2b/0xb0 crypto/shash.c:66
	 shash_async_setkey+0x10/0x20 crypto/shash.c:223
	 crypto_ahash_setkey+0x2d/0xa0 crypto/ahash.c:202
	 crypto_authenc_setkey+0x68/0x100 crypto/authenc.c:96
	 crypto_aead_setkey+0x2a/0xc0 crypto/aead.c:62
	 aead_setkey+0xc/0x10 crypto/algif_aead.c:526
	 alg_setkey crypto/af_alg.c:223 [inline]
	 alg_setsockopt+0xfe/0x130 crypto/af_alg.c:256
	 __sys_setsockopt+0x6d/0xd0 net/socket.c:1902
	 __do_sys_setsockopt net/socket.c:1913 [inline]
	 __se_sys_setsockopt net/socket.c:1910 [inline]
	 __x64_sys_setsockopt+0x1f/0x30 net/socket.c:1910
	 do_syscall_64+0x4a/0x180 arch/x86/entry/common.c:290
	 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Fixes: e236d4a89a2f ("[CRYPTO] authenc: Move enckeylen into key itself")
Cc: <stable@vger.kernel.org> # v2.6.25+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
