[PATCH][SRU][X] tcp: refine memory limit test in tcp_fragment()

[Jay Vosburgh]

Tyler Hicks <tyhicks at canonical.com> wrote:

>From: Eric Dumazet <edumazet at google.com>
>
>tcp_fragment() might be called for skbs in the write queue.
>
>Memory limits might have been exceeded because tcp_sendmsg() only
>checks limits at full skb (64KB) boundaries.
>
>Therefore, we need to make sure tcp_fragment() wont punish applications
>that might have setup very low SO_SNDBUF values.
>
>Fixes: f070ef2ac667 ("tcp: tcp_fragment() should apply sane memory limits")
>Signed-off-by: Eric Dumazet <edumazet at google.com>
>Reported-by: Christoph Paasch <cpaasch at apple.com>
>Tested-by: Christoph Paasch <cpaasch at apple.com>
>Signed-off-by: David S. Miller <davem at davemloft.net>
>
>CVE-2019-11478
>
>(backported from commit b6653b3629e5b88202be3c9abc44713973f5c4b4)
>[tyhicks: Don't enforce the limit on the skb that tcp_send_head points
> as that skb has never been sent out. In newer kernels containing commit
> 75c119afe14f ("tcp: implement rb-tree based retransmit queue"), where
> there the retransmission queue is separate from the write queue, this
> skb would be in the write queue.
> With the modified check in this backported patch, we run the risk of
> enforcing the memory limit on an skb that is after tcp_send_head in the
> queue yet has never been sent out. However, an inspection of all
> tcp_fragment() call sites finds that this shouldn't occur and the limit
> will only be enforced on skbs that are up for retransmission.]
>Signed-off-by: Tyler Hicks <tyhicks at canonical.com>

Reviewed-by: Jay Vosburgh <jay.vosburgh at canonical.com>


>---
>
>I've successfully tested this patch using a slightly modified version of
>a packetdrill test that was sent to the netdev list. Without this kernel
>change, the test hangs. The test successfully completes with this kernel
>change.
>
> net/ipv4/tcp_output.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
>diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
>index ede265fbf7ba..719d2cc8770c 100644
>--- a/net/ipv4/tcp_output.c
>+++ b/net/ipv4/tcp_output.c
>@@ -1163,7 +1163,8 @@ int tcp_fragment(struct sock *sk, struct sk_buff *skb, u32 len,
> 	if (nsize < 0)
> 		nsize = 0;
> 
>-	if (unlikely((sk->sk_wmem_queued >> 1) > sk->sk_sndbuf)) {
>+	if (unlikely((sk->sk_wmem_queued >> 1) > sk->sk_sndbuf &&
>+		     skb != tcp_send_head(sk))) {
> 		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPWQUEUETOOBIG);
> 		return -ENOMEM;
> 	}
>-- 
>2.7.4
>