Re: [PATCH] bpf, sockmap: Fix cork use-after-free in tcp_bpf_sendmsg()

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Sun Jul 19, 2026 at 12:16 PM EDT, Chengfeng Ye wrote:
> tcp_bpf_sendmsg() keeps msg_tx across sk_stream_wait_memory(), which
> drops and reacquires the socket lock.  Its error path tries to decide
> whether msg_tx names the local temporary message by comparing it with
> the current value of psock->cork.
>
> This comparison is unsafe when two threads send on the same socket:
>
>   Thread A                         Thread B
>   msg_tx = psock->cork
>   sk_msg_alloc() fails
>   sk_stream_wait_memory()
>     releases the socket lock      acquires the socket lock
>                                   completes the cork
>                                   psock->cork = NULL
>                                   frees the cork
>     reacquires the socket lock
>   msg_tx != psock->cork
>   sk_msg_free(msg_tx)
>
> The stale cork is therefore mistaken for the local temporary message
> and freed again.  KASAN reported:
>
>   BUG: KASAN: slab-use-after-free in sk_msg_free+0x49/0x50
>   Read of size 4 at addr ffff88810c908800 by task poc/90
>   Call Trace:
>    sk_msg_free+0x49/0x50
>    tcp_bpf_sendmsg+0x14f5/0x1cc0
>    __sys_sendto+0x32c/0x3a0
>    __x64_sys_sendto+0xdb/0x1b0
>   Allocated by task 89:
>    __kasan_kmalloc+0x8f/0xa0
>    tcp_bpf_sendmsg+0x16b3/0x1cc0
>   Freed by task 91:
>    __kasan_slab_free+0x43/0x70
>    kfree+0x131/0x3c0
>    tcp_bpf_sendmsg+0xec3/0x1cc0
>
> msg_tx can only name the stack-local tmp or the shared cork.  Test for
> tmp directly so a changed psock->cork cannot turn a shared message into
> an apparent local one.
>
> Fixes: 604326b41a6f ("bpf, sockmap: convert to generic sk_msg interface")
> Cc: stable@vger.kernel.org
> Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
> ---

Hi Chengfeng,

The patch looks good:

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>

There is one caveat: Normally we ignore pre-existing issues Sashiko
finds while reviewing the patch that are unrelated to the change itself.
For this function, however, I think we should make an exception because
it has multiple glaring issues we can fix more cleanly if we do it all
at once. E.g., tmp never gets cleaned up even if there are allocations
hanging off of it.

Would you be willing to expand the patch that addresses the Sashiko
comments, even if unrelated to your fix? That would save us the time
to review the inevitable followups and provide more coherent
refactoring.

>  net/ipv4/tcp_bpf.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c
> index 8e905b50dead..a30475afb6f8 100644
> --- a/net/ipv4/tcp_bpf.c
> +++ b/net/ipv4/tcp_bpf.c
> @@ -604,7 +604,7 @@ static int tcp_bpf_sendmsg(struct sock *sk, struct msghdr *msg, size_t size)
>  wait_for_memory:
>  		err = sk_stream_wait_memory(sk, &timeo);
>  		if (err) {
> -			if (msg_tx && msg_tx != psock->cork)
> +			if (msg_tx == &tmp)
>  				sk_msg_free(sk, msg_tx);
>  			goto out_err;
>  		}