Re: [PATCH] crypto: af_alg - avoid undefined behavior accessing salg_name

On Mon, Oct 26, 2020 at 9:08 PM Eric Biggers ebiggers@kernel.org 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().

[...]

@@ -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)`.