crypto: af_alg - avoid undefined behavior accessing salg_name

Commit 3f69cc60768b ("crypto: af_alg - Allow arbitrarily long algorithm
names") made the kernel start accepting arbitrarily long algorithm names
in sockaddr_alg. However, the actual length of the salg_name field
stayed at the original 64 bytes.

This is broken because the kernel can access indices >= 64 in salg_name,
which is undefined behavior -- even though the memory that is accessed
is still located within the sockaddr structure. It would only be
defined behavior if the array were properly marked as arbitrary-length
(either by making it a flexible array, which is the recommended way
these days, or by making it an array of length 0 or 1).

We can't simply change salg_name into a flexible array, since that would
break source compatibility with userspace programs that embed
sockaddr_alg into another struct, or (more commonly) declare a
sockaddr_alg like 'struct sockaddr_alg sa = { .salg_name = "foo" };'.

One solution would be to change salg_name into a flexible array only
when '#ifdef __KERNEL__'. However, that would keep userspace without an
easy way to actually use the longer algorithm names.

Instead, add a new structure 'sockaddr_alg_new' that has the flexible
array field, and expose it to both userspace and the kernel.
Make the kernel use it correctly in alg_bind().

This addresses the syzbot report
"UBSAN: array-index-out-of-bounds in alg_bind"
(https://syzkaller.appspot.com/bug?extid=92ead4eb8e26a26d465e).

Reported-by: [e-mail]
Fixes: 3f69cc60768b ("crypto: af_alg - Allow arbitrarily long algorithm names")
Cc: <[e-mail]> # v4.12+
Signed-off-by: Eric Biggers <[e-mail]>
Signed-off-by: Herbert Xu <[e-mail]>

crypto/af_alg.c

···
 	const u32 allowed = CRYPTO_ALG_KERN_DRIVER_ONLY;
 	struct sock *sk = sock->sk;
 	struct alg_sock *ask = alg_sk(sk);
-	struct sockaddr_alg *sa = (void *)uaddr;
+	struct sockaddr_alg_new *sa = (void *)uaddr;
 	const struct af_alg_type *type;
 	void *private;
 	int err;
···
 	if (sock->state == SS_CONNECTED)
 		return -EINVAL;
 
-	if (addr_len < sizeof(*sa))
+	BUILD_BUG_ON(offsetof(struct sockaddr_alg_new, salg_name) !=
+		     offsetof(struct sockaddr_alg, salg_name));
+	BUILD_BUG_ON(offsetof(struct sockaddr_alg, salg_name) != sizeof(*sa));
+
+	if (addr_len < sizeof(*sa) + 1)
 		return -EINVAL;
 
 	/* If caller uses non-allowed flag, return error. */
···
 		return -EINVAL;
 
 	sa->salg_type[sizeof(sa->salg_type) - 1] = 0;
-	sa->salg_name[sizeof(sa->salg_name) + addr_len - sizeof(*sa) - 1] = 0;
+	sa->salg_name[addr_len - sizeof(*sa) - 1] = 0;
 
 	type = alg_get_type(sa->salg_type);
 	if (PTR_ERR(type) == -ENOENT) {

include/uapi/linux/if_alg.h

···
 	__u8	salg_name[64];
 };
 
+/*
+ * Linux v4.12 and later removed the 64-byte limit on salg_name[]; it's now an
+ * arbitrary-length field.  We had to keep the original struct above for source
+ * compatibility with existing userspace programs, though.  Use the new struct
+ * below if support for very long algorithm names is needed.  To do this,
+ * allocate 'sizeof(struct sockaddr_alg_new) + strlen(algname) + 1' bytes, and
+ * copy algname (including the null terminator) into salg_name.
+ */
+struct sockaddr_alg_new {
+	__u16	salg_family;
+	__u8	salg_type[14];
+	__u32	salg_feat;
+	__u32	salg_mask;
+	__u8	salg_name[];
+};
+
 struct af_alg_iv {
 	__u32	ivlen;
 	__u8	iv[0];