Re: [PATCH] crypto: af_alg - avoid undefined behavior accessing salg_name
flat view
From: Jann Horn <jannh@google.com>
Date: 2020-10-26 21:24:07
Also in:
linux-crypto, linux-hardening, lkml, stable
On Mon, Oct 26, 2020 at 9:08 PM Eric Biggers [off-list ref] wrote:
Commit 3f69cc60768b ("crypto: af_alg - Allow arbitrarily long algorithm
names") made the kernel start accepting arbitrarily long algorithm names
in sockaddr_alg.That's not true; it's still limited by the size of struct sockaddr_storage (128 bytes total for the entire address). If you make it longer, __copy_msghdr_from_user() will silently truncate the size.
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().[...]
quoted hunk ↗ jump to hunk
@@ -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;
This looks like an out-of-bounds write in the case `addr_len == sizeof(struct sockaddr_storage)`.