Re: [PATCH net] tcp: check if skb is true to avoid crash

[Eric Dumazet]

On Thu, Sep 26, 2024 at 9:55 AM Lena Wang <lena.wang@xxxxxxxxxxxx> wrote:
>
> A kernel NULL pointer dereference reported.
> Backtrace:
> vmlinux tcp_can_coalesce_send_queue_head(sk=0xFFFFFF80316D9400, len=755)
> + 28 </alps/OfficialRelease/Of/alps/kernel-6.6/net/ipv4/tcp_output.c:2315>
> vmlinux  tcp_mtu_probe(sk=0xFFFFFF80316D9400) + 3196
> </alps/OfficialRelease/Of/alps/kernel-6.6/net/ipv4/tcp_output.c:2452>
> vmlinux  tcp_write_xmit(sk=0xFFFFFF80316D9400, mss_now=128,
> nonagle=-2145862684, push_one=0, gfp=2080) + 3296
> </alps/OfficialRelease/Of/alps/kernel-6.6/net/ipv4/tcp_output.c:2689>
> vmlinux  tcp_tsq_write() + 172
> </alps/OfficialRelease/Of/alps/kernel-6.6/net/ipv4/tcp_output.c:1033>
> vmlinux  tcp_tsq_handler() + 104
> </alps/OfficialRelease/Of/alps/kernel-6.6/net/ipv4/tcp_output.c:1042>
> vmlinux  tcp_tasklet_func() + 208
>
> When there is no pending skb in sk->sk_write_queue, tcp_send_head
> returns NULL. Directly dereference of skb->len will result crash.
> So it is necessary to evaluate the skb to be true here.
>
> Fixes: 808cf9e38cd7 ("tcp: Honor the eor bit in tcp_mtu_probe")
> Signed-off-by: Lena Wang <lena.wang@xxxxxxxxxxxx>
> ---

I am not sure why tcp_send_head() can return NULL.

Before tcp_can_coalesce_send_queue_head() is called, we have this code :

size_needed = probe_size + (tp->reordering + 1) * tp->mss_cache;

/* Have enough data in the send queue to probe? */
if (tp->write_seq - tp->snd_nxt < size_needed)
    return -1;



Do you have a repro ?