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: syzbot+92ead4eb8e26a26d465e@syzkaller.appspotmail.com
Fixes: 3f69cc60768b ("crypto: af_alg - Allow arbitrarily long algorithm names")
Cc: <stable@vger.kernel.org> # v4.12+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --git a/crypto/af_alg.c b/crypto/af_alg.c
index d11db80d24cd14a46b424885e9e6bcfd1299c58c..9acb9d2c4bcf9368142cbbfc556ebfdc72aa051d 100644 (file)
--- a/crypto/af_alg.c
+++ b/crypto/af_alg.c
@@ -147,7 +147,7 @@ static int alg_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
        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;
@@ -155,7 +155,11 @@ static int alg_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
        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. */
@@ -163,7 +167,7 @@ static int alg_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
                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) {

diff --git a/include/uapi/linux/if_alg.h b/include/uapi/linux/if_alg.h
index 60b7c2efd921c1144e5aba9d9acec137aced2a29..dc52a11ba6d1585d44cb2351ff7268299885ba81 100644 (file)
--- a/include/uapi/linux/if_alg.h
+++ b/include/uapi/linux/if_alg.h
@@ -24,6 +24,22 @@ struct sockaddr_alg {
        __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];