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

[Jann Horn <jannh@google.com>]

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)`.