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

[Lena Wang (王娜)]

On Thu, 2024-09-26 at 11:07 +0200, Eric Dumazet wrote:
>  	 
> External email : Please do not click links or open attachments until
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>  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 ?
Hi Eric,
It just happens once in monkey test. I can't reproduce it.

from the dump log, it can see these values:
(gdb) p tp->reordering
$16 = 4
(gdb) p tp->mss_cache
$17 = 128
probe_size = 755
size_needed = 755 + (4+1)*128 = 1395

(gdb) p tp->write_seq
$18 = 1571343838
(gdb) p tp->snd_nxt
$19 = 1571336917 
tp->write_seq - tp->snd_nxt = 1571343838 - 1571336917 = 6921 > 1395