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

[Vadim Fedorenko <vadim.fedorenko@linux.dev>]

On 26/09/2024 08:56, Lena Wang 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@mediatek.com>
> ---
>   net/ipv4/tcp_output.c | 20 +++++++++++---------
>   1 file changed, 11 insertions(+), 9 deletions(-)
> 
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 4fd746bd4d54..12cde5d879c5 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -2338,17 +2338,19 @@ static bool tcp_can_coalesce_send_queue_head(struct sock *sk, int len)
>   	struct sk_buff *skb, *next;
>   
>   	skb = tcp_send_head(sk);
> -	tcp_for_write_queue_from_safe(skb, next, sk) {
> -		if (len <= skb->len)
> -			break;
> +	if (skb) {

Thinking more of this, I don't really understand how is it possible to
reach tcp_can_coalesce_send_queue_head() with empty send queue, but
anyway, this patch will move NULL dereference further to tcp_mtu_probe()
where new skb is build with the same tcp_send_head() call.

I believe the proper way is to return false here in case of missing skb:

if (!skb)
	return false;

This will prevent tcp_mtu_probe() to continue execution and avoid
possible NULL dereference.

> +		tcp_for_write_queue_from_safe(skb, next, sk) {
> +			if (len <= skb->len)
> +				break;
>   
> -		if (unlikely(TCP_SKB_CB(skb)->eor) ||
> -		    tcp_has_tx_tstamp(skb) ||
> -		    !skb_pure_zcopy_same(skb, next) ||
> -		    skb_frags_readable(skb) != skb_frags_readable(next))
> -			return false;
> +			if (unlikely(TCP_SKB_CB(skb)->eor) ||
> +			    tcp_has_tx_tstamp(skb) ||
> +			    !skb_pure_zcopy_same(skb, next) ||
> +			    skb_frags_readable(skb) != skb_frags_readable(next))
> +				return false;
>   
> -		len -= skb->len;
> +			len -= skb->len;
> +		}
>   	}
>   
>   	return true;