Re: [PATCH net] tcp: remove empty skb from write queue in error cases

Hello,

On Mon, Aug 26, 2019 at 11:04 AM Eric Dumazet [off-list ref] wrote:

quoted

Vladimir Rutsky reported stuck TCP sessions after memory pressure
events. Edge Trigger epoll() user would never receive an EPOLLOUT
notification allowing them to retry a sendmsg().

Jason tested the case of sk_stream_alloc_skb() returning NULL,
but there are other paths that could lead both sendmsg() and sendpage()
to return -1 (EAGAIN), with an empty skb queued on the write queue.

This patch makes sure we remove this empty skb so that
Jason code can detect that the queue is empty, and
call sk->sk_write_space(sk) accordingly.

Fixes: ce5ec440994b ("tcp: ensure epoll edge trigger wakeup when write queue is empty")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Jason Baron <jbaron@akamai.com>
Reported-by: Vladimir Rutsky <redacted>
Cc: Soheil Hassas Yeganeh <redacted>
Cc: Neal Cardwell <ncardwell@google.com>
---
 net/ipv4/tcp.c | 30 ++++++++++++++++++++----------
 1 file changed, 20 insertions(+), 10 deletions(-)

I got syzkaller complaining now on 4.14.143 with the following reproducer:

# {Threaded:true Collide:true Repeat:true RepeatTimes:0 Procs:1
Sandbox: Fault:false FaultCall:-1 FaultNth:0 EnableTun:false
UseTmpDir:false EnableCgroups:false EnableNetdev:false ResetNet:false
HandleSegv:false Repro:false Trace:false}
r0 = socket$inet_tcp(0x2, 0x1, 0x0)
setsockopt$inet_tcp_TCP_REPAIR(r0, 0x6, 0x13, &(0x7f0000000040)=0x1, 0x4)
setsockopt$inet_tcp_TCP_REPAIR_QUEUE(r0, 0x6, 0x14, &(0x7f00000012c0)=0x2, 0x4)
setsockopt$inet_tcp_int(r0, 0x6, 0x19, &(0x7f0000000000)=0x9, 0x4)
setsockopt$inet_tcp_TCP_MD5SIG(r0, 0x6, 0xe,
&(0x7f00000001c0)={@in={{0x2, 0x0, @empty}}, 0x0, 0x2, 0x0,
"c157cf4809151e5e89cfd6d934fbe981ec8ff6afc252ccf486c325c7ff3d35f3a89412a5cb6430e169092617df2ba65bf0ab844572e4e7dd4ece8ec1de5ac1ccd870067b018cb3b1f05f2391d872b67d"},
0xd8)
connect$inet(r0, &(0x7f0000000080)={0x2, 0x0, @dev={0xac, 0x14, 0x14,
0x1d}}, 0x10)
sendto(r0, 0x0, 0x87, 0x0, 0x0, 0x391)

kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] SMP KASAN PTI
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 1 PID: 2529 Comm: syz-executor709 Not tainted 4.14.143 #5
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 0.5.1 01/01/2011
task: ffff8880677fdc00 task.stack: ffff8880642b0000
RIP: 0010:tcp_sendmsg_locked+0x6b4/0x4390 net/ipv4/tcp.c:1350
RSP: 0018:ffff8880642bf718 EFLAGS: 00010206
RAX: 0000000000000014 RBX: 0000000000000087 RCX: ffff88806a794f50
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 00000000000000a0
RBP: ffff8880642bfaa8 R08: 0000000000000006 R09: ffff8880677fe3a0
R10: 0000000000000000 R11: 0000000000000000 R12: dffffc0000000000
R13: ffff88806a794f50 R14: ffff88806a794d00 R15: 0000000000000087
FS:  00007f644b697700(0000) GS:ffff88806cf00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ffcd37370b0 CR3: 00000000679f2006 CR4: 00000000003606e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 tcp_sendmsg+0x2a/0x40 net/ipv4/tcp.c:1533
 inet_sendmsg+0x173/0x4e0 net/ipv4/af_inet.c:784
 sock_sendmsg_nosec net/socket.c:646 [inline]
 sock_sendmsg+0xc3/0x100 net/socket.c:656
 SYSC_sendto+0x35d/0x5e0 net/socket.c:1766
 do_syscall_64+0x241/0x680 arch/x86/entry/common.c:292
 entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x7f644afc6469
RSP: 002b:00007f644b696f28 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
RAX: ffffffffffffffda RBX: 0000000000602130 RCX: 00007f644afc6469
RDX: 0000000000000087 RSI: 0000000000000000 RDI: 0000000000000003
RBP: 0000000000602138 R08: 0000000000000000 R09: 0000000000000391
R10: 0000000000000000 R11: 0000000000000246 R12: 000000000060213c
R13: 00007ffcd373700f R14: 00007f644b677000 R15: 0000000000000003
Code: 74 08 3c 03 0f 8e f1 32 00 00 8b 85 98 fd ff ff 89 85 60 fd ff
ff 48 8b 85 70 fd ff ff 48 8d b8 a0 00 00 00 48 89 f8 48 c1 e8 03 <42>
0f b6 04 20 84 c0 74 06 0f 8e d2 32 00 00 4c 8b bd 70 fd ff
RIP: tcp_sendmsg_locked+0x6b4/0x4390 net/ipv4/tcp.c:1350 RSP: ffff8880642bf718
---[ end trace 70f07f242cd3b9d8 ]---


It's because skb is NULL in tcp_sendmsg_locked at:
                  skb = tcp_write_queue_tail(sk);
                  if (tcp_send_head(sk)) {
                          if (skb->ip_summed == CHECKSUM_NONE)


I think we need this here on pre-rb-tree kernels :

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 5ce069ce2a97..efe767e20d01 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c

@@ -924,8 +924,7 @@ static void tcp_remove_empty_skb(struct sock *sk,

struct sk_buff *skb)
 {
  if (skb && !skb->len) {
  tcp_unlink_write_queue(skb, sk);
- if (tcp_write_queue_empty(sk))
- tcp_chrono_stop(sk, TCP_CHRONO_BUSY);
+ tcp_check_send_head(sk, skb);
  sk_wmem_free_skb(sk, skb);
  }
 }

Does that look good?