[PATCH v2 mptcp-net] mptcp: fix possible stall on recvmsg()

[PATCH v2 mptcp-net] mptcp: fix possible stall on recvmsg()

[Paolo Abeni]

recvmsg() can enter an infinite loop if the caller provides the
MSG_WAITALL, the data present in the receive queue is not
sufficient to fulfill the request and no more data is received by
the peer.

When the above happens, mptcp_wait_data() will always return with
no wait, as the MPTCP_DATA_READY flag checked by such function is
set and never cleared in such code path.

Leveraging the above syzbot was able to trigger an RCU stall:

rcu: INFO: rcu_preempt self-detected stall on CPU
rcu:    0-...!: (10499 ticks this GP) idle=0af/1/0x4000000000000000 softirq=10678/10678 fqs=1
        (t=10500 jiffies g=13089 q=109)
rcu: rcu_preempt kthread starved for 10497 jiffies! g13089 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=1
rcu:    Unless rcu_preempt kthread gets sufficient CPU time, OOM is now expected behavior.
rcu: RCU grace-period kthread stack dump:
task:rcu_preempt     state:R  running task     stack:28696 pid:   14 ppid:     2 flags:0x00004000
Call Trace:
 context_switch kernel/sched/core.c:4955 [inline]
 __schedule+0x940/0x26f0 kernel/sched/core.c:6236
 schedule+0xd3/0x270 kernel/sched/core.c:6315
 schedule_timeout+0x14a/0x2a0 kernel/time/timer.c:1881
 rcu_gp_fqs_loop+0x186/0x810 kernel/rcu/tree.c:1955
 rcu_gp_kthread+0x1de/0x320 kernel/rcu/tree.c:2128
 kthread+0x405/0x4f0 kernel/kthread.c:327
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
rcu: Stack dump where RCU GP kthread last ran:
Sending NMI from CPU 0 to CPUs 1:
NMI backtrace for cpu 1
CPU: 1 PID: 8510 Comm: syz-executor827 Not tainted 5.15.0-rc2-next-20210920-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:bytes_is_nonzero mm/kasan/generic.c:84 [inline]
RIP: 0010:memory_is_nonzero mm/kasan/generic.c:102 [inline]
RIP: 0010:memory_is_poisoned_n mm/kasan/generic.c:128 [inline]
RIP: 0010:memory_is_poisoned mm/kasan/generic.c:159 [inline]
RIP: 0010:check_region_inline mm/kasan/generic.c:180 [inline]
RIP: 0010:kasan_check_range+0xc8/0x180 mm/kasan/generic.c:189
Code: 38 00 74 ed 48 8d 50 08 eb 09 48 83 c0 01 48 39 d0 74 7a 80 38 00 74 f2 48 89 c2 b8 01 00 00 00 48 85 d2 75 56 5b 5d 41 5c c3 <48> 85 d2 74 5e 48 01 ea eb 09 48 83 c0 01 48 39 d0 74 50 80 38 00
RSP: 0018:ffffc9000cd676c8 EFLAGS: 00000283
RAX: ffffed100e9a110e RBX: ffffed100e9a110f RCX: ffffffff88ea062a
RDX: 0000000000000001 RSI: 0000000000000008 RDI: ffff888074d08870
RBP: ffffed100e9a110e R08: 0000000000000001 R09: ffff888074d08877
R10: ffffed100e9a110e R11: 0000000000000000 R12: ffff888074d08000
R13: ffff888074d08000 R14: ffff888074d08088 R15: ffff888074d08000
FS:  0000555556d8e300(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
S:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020000180 CR3: 0000000068909000 CR4: 00000000001506e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 instrument_atomic_read_write include/linux/instrumented.h:101 [inline]
 test_and_clear_bit include/asm-generic/bitops/instrumented-atomic.h:83 [inline]
 mptcp_release_cb+0x14a/0x210 net/mptcp/protocol.c:3016
 release_sock+0xb4/0x1b0 net/core/sock.c:3204
 mptcp_wait_data net/mptcp/protocol.c:1770 [inline]
 mptcp_recvmsg+0xfd1/0x27b0 net/mptcp/protocol.c:2080
 inet6_recvmsg+0x11b/0x5e0 net/ipv6/af_inet6.c:659
 sock_recvmsg_nosec net/socket.c:944 [inline]
 ____sys_recvmsg+0x527/0x600 net/socket.c:2626
 ___sys_recvmsg+0x127/0x200 net/socket.c:2670
 do_recvmmsg+0x24d/0x6d0 net/socket.c:2764
 __sys_recvmmsg net/socket.c:2843 [inline]
 __do_sys_recvmmsg net/socket.c:2866 [inline]
 __se_sys_recvmmsg net/socket.c:2859 [inline]
 __x64_sys_recvmmsg+0x20b/0x260 net/socket.c:2859
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x7fc200d2dc39
Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 41 15 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 c0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffc5758e5a8 EFLAGS: 00000246 ORIG_RAX: 000000000000012b
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007fc200d2dc39
RDX: 0000000000000002 RSI: 00000000200017c0 RDI: 0000000000000003
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000f0b5ff
R10: 0000000000000100 R11: 0000000000000246 R12: 0000000000000003
R13: 00007ffc5758e5d0 R14: 00007ffc5758e5c0 R15: 0000000000000003

Fix the issue replacing the MPTCP_DATA_READY bit with direct
inspection of the msk receive queue.

Reported-and-tested-by: syzbot+3360da629681aa0d22fe@syzkaller.appspotmail.com
Fixes: 7a6a6cbc3e59 ("mptcp: recvmsg() can drain data from multiple subflow")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
v1 -> v2:
 - instead of do more fiddling with the DATA_READY bit, just
   check sk_receive_queue
---
 net/mptcp/protocol.c | 52 ++++++++++----------------------------------
 1 file changed, 12 insertions(+), 40 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index e5df0b5971c8..cc996e8973b3 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -528,7 +528,6 @@ static bool mptcp_check_data_fin(struct sock *sk)
 
 		sk->sk_shutdown |= RCV_SHUTDOWN;
 		smp_mb__before_atomic(); /* SHUTDOWN must be visible first */
-		set_bit(MPTCP_DATA_READY, &msk->flags);
 
 		switch (sk->sk_state) {
 		case TCP_ESTABLISHED:
@@ -742,10 +741,9 @@ void mptcp_data_ready(struct sock *sk, struct sock *ssk)
 
 	/* Wake-up the reader only for in-sequence data */
 	mptcp_data_lock(sk);
-	if (move_skbs_to_msk(msk, ssk)) {
-		set_bit(MPTCP_DATA_READY, &msk->flags);
+	if (move_skbs_to_msk(msk, ssk))
 		sk->sk_data_ready(sk);
-	}
+
 	mptcp_data_unlock(sk);
 }
 
@@ -847,7 +845,6 @@ static void mptcp_check_for_eof(struct mptcp_sock *msk)
 		sk->sk_shutdown |= RCV_SHUTDOWN;
 
 		smp_mb__before_atomic(); /* SHUTDOWN must be visible first */
-		set_bit(MPTCP_DATA_READY, &msk->flags);
 		sk->sk_data_ready(sk);
 	}
 
@@ -1759,21 +1756,6 @@ static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	return copied ? : ret;
 }
 
-static void mptcp_wait_data(struct sock *sk, long *timeo)
-{
-	DEFINE_WAIT_FUNC(wait, woken_wake_function);
-	struct mptcp_sock *msk = mptcp_sk(sk);
-
-	add_wait_queue(sk_sleep(sk), &wait);
-	sk_set_bit(SOCKWQ_ASYNC_WAITDATA, sk);
-
-	sk_wait_event(sk, timeo,
-		      test_bit(MPTCP_DATA_READY, &msk->flags), &wait);
-
-	sk_clear_bit(SOCKWQ_ASYNC_WAITDATA, sk);
-	remove_wait_queue(sk_sleep(sk), &wait);
-}
-
 static int __mptcp_recvmsg_mskq(struct mptcp_sock *msk,
 				struct msghdr *msg,
 				size_t len, int flags,
@@ -2077,19 +2059,7 @@ static int mptcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
 		}
 
 		pr_debug("block timeout %ld", timeo);
-		mptcp_wait_data(sk, &timeo);
-	}
-
-	if (skb_queue_empty_lockless(&sk->sk_receive_queue) &&
-	    skb_queue_empty(&msk->receive_queue)) {
-		/* entire backlog drained, clear DATA_READY. */
-		clear_bit(MPTCP_DATA_READY, &msk->flags);
-
-		/* .. race-breaker: ssk might have gotten new data
-		 * after last __mptcp_move_skbs() returned false.
-		 */
-		if (unlikely(__mptcp_move_skbs(msk)))
-			set_bit(MPTCP_DATA_READY, &msk->flags);
+		sk_wait_data(sk, &timeo, NULL);
 	}
 
 out_err:
@@ -2098,9 +2068,9 @@ static int mptcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
 			tcp_recv_timestamp(msg, sk, &tss);
 	}
 
-	pr_debug("msk=%p data_ready=%d rx queue empty=%d copied=%d",
-		 msk, test_bit(MPTCP_DATA_READY, &msk->flags),
-		 skb_queue_empty_lockless(&sk->sk_receive_queue), copied);
+	pr_debug("msk=%p rx queue empty=%d:%d copied=%d",
+		 msk, skb_queue_empty_lockless(&sk->sk_receive_queue),
+		 skb_queue_empty(&msk->receive_queue), copied);
 	if (!(flags & MSG_PEEK))
 		mptcp_rcv_space_adjust(msk, copied);
 
@@ -2368,7 +2338,6 @@ static void mptcp_check_fastclose(struct mptcp_sock *msk)
 	inet_sk_state_store(sk, TCP_CLOSE);
 	sk->sk_shutdown = SHUTDOWN_MASK;
 	smp_mb__before_atomic(); /* SHUTDOWN must be visible first */
-	set_bit(MPTCP_DATA_READY, &msk->flags);
 	set_bit(MPTCP_WORK_CLOSE_SUBFLOW, &msk->flags);
 
 	mptcp_close_wake_up(sk);
@@ -3385,8 +3354,11 @@ static int mptcp_stream_accept(struct socket *sock, struct socket *newsock,
 
 static __poll_t mptcp_check_readable(struct mptcp_sock *msk)
 {
-	return test_bit(MPTCP_DATA_READY, &msk->flags) ? EPOLLIN | EPOLLRDNORM :
-	       0;
+	if (skb_queue_empty_lockless(&((struct sock *)msk)->sk_receive_queue) &&
+            skb_queue_empty_lockless(&msk->receive_queue))
+		return 0;
+
+	return EPOLLIN | EPOLLRDNORM;
 }
 
 static __poll_t mptcp_check_writeable(struct mptcp_sock *msk)
@@ -3421,7 +3393,7 @@ static __poll_t mptcp_poll(struct file *file, struct socket *sock,
 	state = inet_sk_state_load(sk);
 	pr_debug("msk=%p state=%d flags=%lx", msk, state, msk->flags);
 	if (state == TCP_LISTEN)
-		return mptcp_check_readable(msk);
+		return test_bit(MPTCP_DATA_READY, &msk->flags) ? EPOLLIN | EPOLLRDNORM : 0;
 
 	if (state != TCP_SYN_SENT && state != TCP_SYN_RECV) {
 		mask |= mptcp_check_readable(msk);
-- 
2.26.3

Re: [PATCH v2 mptcp-net] mptcp: fix possible stall on recvmsg()

[Paolo Abeni]

On Fri, 2021-09-24 at 15:38 +0200, Paolo Abeni wrote:
> recvmsg() can enter an infinite loop if the caller provides the
> MSG_WAITALL, the data present in the receive queue is not
> sufficient to fulfill the request and no more data is received by
> the peer.
> 
> When the above happens, mptcp_wait_data() will always return with
> no wait, as the MPTCP_DATA_READY flag checked by such function is
> set and never cleared in such code path.
> 
> Leveraging the above syzbot was able to trigger an RCU stall:
> 
> rcu: INFO: rcu_preempt self-detected stall on CPU
> rcu:    0-...!: (10499 ticks this GP) idle=0af/1/0x4000000000000000 softirq=10678/10678 fqs=1
>         (t=10500 jiffies g=13089 q=109)
> rcu: rcu_preempt kthread starved for 10497 jiffies! g13089 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=1
> rcu:    Unless rcu_preempt kthread gets sufficient CPU time, OOM is now expected behavior.
> rcu: RCU grace-period kthread stack dump:
> task:rcu_preempt     state:R  running task     stack:28696 pid:   14 ppid:     2 flags:0x00004000
> Call Trace:
>  context_switch kernel/sched/core.c:4955 [inline]
>  __schedule+0x940/0x26f0 kernel/sched/core.c:6236
>  schedule+0xd3/0x270 kernel/sched/core.c:6315
>  schedule_timeout+0x14a/0x2a0 kernel/time/timer.c:1881
>  rcu_gp_fqs_loop+0x186/0x810 kernel/rcu/tree.c:1955
>  rcu_gp_kthread+0x1de/0x320 kernel/rcu/tree.c:2128
>  kthread+0x405/0x4f0 kernel/kthread.c:327
>  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
> rcu: Stack dump where RCU GP kthread last ran:
> Sending NMI from CPU 0 to CPUs 1:
> NMI backtrace for cpu 1
> CPU: 1 PID: 8510 Comm: syz-executor827 Not tainted 5.15.0-rc2-next-20210920-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> RIP: 0010:bytes_is_nonzero mm/kasan/generic.c:84 [inline]
> RIP: 0010:memory_is_nonzero mm/kasan/generic.c:102 [inline]
> RIP: 0010:memory_is_poisoned_n mm/kasan/generic.c:128 [inline]
> RIP: 0010:memory_is_poisoned mm/kasan/generic.c:159 [inline]
> RIP: 0010:check_region_inline mm/kasan/generic.c:180 [inline]
> RIP: 0010:kasan_check_range+0xc8/0x180 mm/kasan/generic.c:189
> Code: 38 00 74 ed 48 8d 50 08 eb 09 48 83 c0 01 48 39 d0 74 7a 80 38 00 74 f2 48 89 c2 b8 01 00 00 00 48 85 d2 75 56 5b 5d 41 5c c3 <48> 85 d2 74 5e 48 01 ea eb 09 48 83 c0 01 48 39 d0 74 50 80 38 00
> RSP: 0018:ffffc9000cd676c8 EFLAGS: 00000283
> RAX: ffffed100e9a110e RBX: ffffed100e9a110f RCX: ffffffff88ea062a
> RDX: 0000000000000001 RSI: 0000000000000008 RDI: ffff888074d08870
> RBP: ffffed100e9a110e R08: 0000000000000001 R09: ffff888074d08877
> R10: ffffed100e9a110e R11: 0000000000000000 R12: ffff888074d08000
> R13: ffff888074d08000 R14: ffff888074d08088 R15: ffff888074d08000
> FS:  0000555556d8e300(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
> S:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000020000180 CR3: 0000000068909000 CR4: 00000000001506e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  instrument_atomic_read_write include/linux/instrumented.h:101 [inline]
>  test_and_clear_bit include/asm-generic/bitops/instrumented-atomic.h:83 [inline]
>  mptcp_release_cb+0x14a/0x210 net/mptcp/protocol.c:3016
>  release_sock+0xb4/0x1b0 net/core/sock.c:3204
>  mptcp_wait_data net/mptcp/protocol.c:1770 [inline]
>  mptcp_recvmsg+0xfd1/0x27b0 net/mptcp/protocol.c:2080
>  inet6_recvmsg+0x11b/0x5e0 net/ipv6/af_inet6.c:659
>  sock_recvmsg_nosec net/socket.c:944 [inline]
>  ____sys_recvmsg+0x527/0x600 net/socket.c:2626
>  ___sys_recvmsg+0x127/0x200 net/socket.c:2670
>  do_recvmmsg+0x24d/0x6d0 net/socket.c:2764
>  __sys_recvmmsg net/socket.c:2843 [inline]
>  __do_sys_recvmmsg net/socket.c:2866 [inline]
>  __se_sys_recvmmsg net/socket.c:2859 [inline]
>  __x64_sys_recvmmsg+0x20b/0x260 net/socket.c:2859
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> RIP: 0033:0x7fc200d2dc39
> Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 41 15 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 c0 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007ffc5758e5a8 EFLAGS: 00000246 ORIG_RAX: 000000000000012b
> RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007fc200d2dc39
> RDX: 0000000000000002 RSI: 00000000200017c0 RDI: 0000000000000003
> RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000f0b5ff
> R10: 0000000000000100 R11: 0000000000000246 R12: 0000000000000003
> R13: 00007ffc5758e5d0 R14: 00007ffc5758e5c0 R15: 0000000000000003
> 
> Fix the issue replacing the MPTCP_DATA_READY bit with direct
> inspection of the msk receive queue.
> 
> Reported-and-tested-by: syzbot+3360da629681aa0d22fe@syzkaller.appspotmail.com
> Fixes: 7a6a6cbc3e59 ("mptcp: recvmsg() can drain data from multiple subflow")
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>
> ---
> v1 -> v2:
>  - instead of do more fiddling with the DATA_READY bit, just
>    check sk_receive_queue
> ---
>  net/mptcp/protocol.c | 52 ++++++++++----------------------------------
>  1 file changed, 12 insertions(+), 40 deletions(-)
> 
> diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
> index e5df0b5971c8..cc996e8973b3 100644
> --- a/net/mptcp/protocol.c
> +++ b/net/mptcp/protocol.c
> @@ -528,7 +528,6 @@ static bool mptcp_check_data_fin(struct sock *sk)
>  
>  		sk->sk_shutdown |= RCV_SHUTDOWN;
>  		smp_mb__before_atomic(); /* SHUTDOWN must be visible first */
> -		set_bit(MPTCP_DATA_READY, &msk->flags);
>  
>  		switch (sk->sk_state) {
>  		case TCP_ESTABLISHED:
> @@ -742,10 +741,9 @@ void mptcp_data_ready(struct sock *sk, struct sock *ssk)
>  
>  	/* Wake-up the reader only for in-sequence data */
>  	mptcp_data_lock(sk);
> -	if (move_skbs_to_msk(msk, ssk)) {
> -		set_bit(MPTCP_DATA_READY, &msk->flags);
> +	if (move_skbs_to_msk(msk, ssk))
>  		sk->sk_data_ready(sk);
> -	}
> +
>  	mptcp_data_unlock(sk);
>  }
>  
> @@ -847,7 +845,6 @@ static void mptcp_check_for_eof(struct mptcp_sock *msk)
>  		sk->sk_shutdown |= RCV_SHUTDOWN;
>  
>  		smp_mb__before_atomic(); /* SHUTDOWN must be visible first */
> -		set_bit(MPTCP_DATA_READY, &msk->flags);
>  		sk->sk_data_ready(sk);
>  	}
>  
> @@ -1759,21 +1756,6 @@ static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
>  	return copied ? : ret;
>  }
>  
> -static void mptcp_wait_data(struct sock *sk, long *timeo)
> -{
> -	DEFINE_WAIT_FUNC(wait, woken_wake_function);
> -	struct mptcp_sock *msk = mptcp_sk(sk);
> -
> -	add_wait_queue(sk_sleep(sk), &wait);
> -	sk_set_bit(SOCKWQ_ASYNC_WAITDATA, sk);
> -
> -	sk_wait_event(sk, timeo,
> -		      test_bit(MPTCP_DATA_READY, &msk->flags), &wait);
> -
> -	sk_clear_bit(SOCKWQ_ASYNC_WAITDATA, sk);
> -	remove_wait_queue(sk_sleep(sk), &wait);
> -}
> -
>  static int __mptcp_recvmsg_mskq(struct mptcp_sock *msk,
>  				struct msghdr *msg,
>  				size_t len, int flags,
> @@ -2077,19 +2059,7 @@ static int mptcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
>  		}
>  
>  		pr_debug("block timeout %ld", timeo);
> -		mptcp_wait_data(sk, &timeo);
> -	}
> -
> -	if (skb_queue_empty_lockless(&sk->sk_receive_queue) &&
> -	    skb_queue_empty(&msk->receive_queue)) {
> -		/* entire backlog drained, clear DATA_READY. */
> -		clear_bit(MPTCP_DATA_READY, &msk->flags);
> -
> -		/* .. race-breaker: ssk might have gotten new data
> -		 * after last __mptcp_move_skbs() returned false.
> -		 */
> -		if (unlikely(__mptcp_move_skbs(msk)))
> -			set_bit(MPTCP_DATA_READY, &msk->flags);
> +		sk_wait_data(sk, &timeo, NULL);
>  	}
>  
>  out_err:
> @@ -2098,9 +2068,9 @@ static int mptcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
>  			tcp_recv_timestamp(msg, sk, &tss);
>  	}
>  
> -	pr_debug("msk=%p data_ready=%d rx queue empty=%d copied=%d",
> -		 msk, test_bit(MPTCP_DATA_READY, &msk->flags),
> -		 skb_queue_empty_lockless(&sk->sk_receive_queue), copied);
> +	pr_debug("msk=%p rx queue empty=%d:%d copied=%d",
> +		 msk, skb_queue_empty_lockless(&sk->sk_receive_queue),
> +		 skb_queue_empty(&msk->receive_queue), copied);
>  	if (!(flags & MSG_PEEK))
>  		mptcp_rcv_space_adjust(msk, copied);
>  
> @@ -2368,7 +2338,6 @@ static void mptcp_check_fastclose(struct mptcp_sock *msk)
>  	inet_sk_state_store(sk, TCP_CLOSE);
>  	sk->sk_shutdown = SHUTDOWN_MASK;
>  	smp_mb__before_atomic(); /* SHUTDOWN must be visible first */
> -	set_bit(MPTCP_DATA_READY, &msk->flags);
>  	set_bit(MPTCP_WORK_CLOSE_SUBFLOW, &msk->flags);
>  
>  	mptcp_close_wake_up(sk);
> @@ -3385,8 +3354,11 @@ static int mptcp_stream_accept(struct socket *sock, struct socket *newsock,
>  
>  static __poll_t mptcp_check_readable(struct mptcp_sock *msk)
>  {
> -	return test_bit(MPTCP_DATA_READY, &msk->flags) ? EPOLLIN | EPOLLRDNORM :
> -	       0;
> +	if (skb_queue_empty_lockless(&((struct sock *)msk)->sk_receive_queue) &&
> +            skb_queue_empty_lockless(&msk->receive_queue))
> +		return 0;
> +
> +	return EPOLLIN | EPOLLRDNORM;
>  }
>  
>  static __poll_t mptcp_check_writeable(struct mptcp_sock *msk)
> @@ -3421,7 +3393,7 @@ static __poll_t mptcp_poll(struct file *file, struct socket *sock,
>  	state = inet_sk_state_load(sk);
>  	pr_debug("msk=%p state=%d flags=%lx", msk, state, msk->flags);
>  	if (state == TCP_LISTEN)
> -		return mptcp_check_readable(msk);
> +		return test_bit(MPTCP_DATA_READY, &msk->flags) ? EPOLLIN | EPOLLRDNORM : 0;
>  
>  	if (state != TCP_SYN_SENT && state != TCP_SYN_RECV) {
>  		mask |= mptcp_check_readable(msk);

@netdev maintainer: this was intended for mptcp@lists.linux.dev only
consumption, it landed here due to PEBKAC, I'm sorry for the noise.
I'll drop this one from the netdev patchwork.

Cheers,

Paolo

Re: [PATCH v2 mptcp-net] mptcp: fix possible stall on recvmsg()

[Mat Martineau]

On Fri, 24 Sep 2021, Paolo Abeni wrote:

> recvmsg() can enter an infinite loop if the caller provides the
> MSG_WAITALL, the data present in the receive queue is not
> sufficient to fulfill the request and no more data is received by
> the peer.
>
> When the above happens, mptcp_wait_data() will always return with
> no wait, as the MPTCP_DATA_READY flag checked by such function is
> set and never cleared in such code path.
>
> Leveraging the above syzbot was able to trigger an RCU stall:
>
> rcu: INFO: rcu_preempt self-detected stall on CPU
> rcu:    0-...!: (10499 ticks this GP) idle=0af/1/0x4000000000000000 softirq=10678/10678 fqs=1
>        (t=10500 jiffies g=13089 q=109)
> rcu: rcu_preempt kthread starved for 10497 jiffies! g13089 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=1
> rcu:    Unless rcu_preempt kthread gets sufficient CPU time, OOM is now expected behavior.
> rcu: RCU grace-period kthread stack dump:
> task:rcu_preempt     state:R  running task     stack:28696 pid:   14 ppid:     2 flags:0x00004000
> Call Trace:
> context_switch kernel/sched/core.c:4955 [inline]
> __schedule+0x940/0x26f0 kernel/sched/core.c:6236
> schedule+0xd3/0x270 kernel/sched/core.c:6315
> schedule_timeout+0x14a/0x2a0 kernel/time/timer.c:1881
> rcu_gp_fqs_loop+0x186/0x810 kernel/rcu/tree.c:1955
> rcu_gp_kthread+0x1de/0x320 kernel/rcu/tree.c:2128
> kthread+0x405/0x4f0 kernel/kthread.c:327
> ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
> rcu: Stack dump where RCU GP kthread last ran:
> Sending NMI from CPU 0 to CPUs 1:
> NMI backtrace for cpu 1
> CPU: 1 PID: 8510 Comm: syz-executor827 Not tainted 5.15.0-rc2-next-20210920-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> RIP: 0010:bytes_is_nonzero mm/kasan/generic.c:84 [inline]
> RIP: 0010:memory_is_nonzero mm/kasan/generic.c:102 [inline]
> RIP: 0010:memory_is_poisoned_n mm/kasan/generic.c:128 [inline]
> RIP: 0010:memory_is_poisoned mm/kasan/generic.c:159 [inline]
> RIP: 0010:check_region_inline mm/kasan/generic.c:180 [inline]
> RIP: 0010:kasan_check_range+0xc8/0x180 mm/kasan/generic.c:189
> Code: 38 00 74 ed 48 8d 50 08 eb 09 48 83 c0 01 48 39 d0 74 7a 80 38 00 74 f2 48 89 c2 b8 01 00 00 00 48 85 d2 75 56 5b 5d 41 5c c3 <48> 85 d2 74 5e 48 01 ea eb 09 48 83 c0 01 48 39 d0 74 50 80 38 00
> RSP: 0018:ffffc9000cd676c8 EFLAGS: 00000283
> RAX: ffffed100e9a110e RBX: ffffed100e9a110f RCX: ffffffff88ea062a
> RDX: 0000000000000001 RSI: 0000000000000008 RDI: ffff888074d08870
> RBP: ffffed100e9a110e R08: 0000000000000001 R09: ffff888074d08877
> R10: ffffed100e9a110e R11: 0000000000000000 R12: ffff888074d08000
> R13: ffff888074d08000 R14: ffff888074d08088 R15: ffff888074d08000
> FS:  0000555556d8e300(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
> S:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000020000180 CR3: 0000000068909000 CR4: 00000000001506e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
> instrument_atomic_read_write include/linux/instrumented.h:101 [inline]
> test_and_clear_bit include/asm-generic/bitops/instrumented-atomic.h:83 [inline]
> mptcp_release_cb+0x14a/0x210 net/mptcp/protocol.c:3016
> release_sock+0xb4/0x1b0 net/core/sock.c:3204
> mptcp_wait_data net/mptcp/protocol.c:1770 [inline]
> mptcp_recvmsg+0xfd1/0x27b0 net/mptcp/protocol.c:2080
> inet6_recvmsg+0x11b/0x5e0 net/ipv6/af_inet6.c:659
> sock_recvmsg_nosec net/socket.c:944 [inline]
> ____sys_recvmsg+0x527/0x600 net/socket.c:2626
> ___sys_recvmsg+0x127/0x200 net/socket.c:2670
> do_recvmmsg+0x24d/0x6d0 net/socket.c:2764
> __sys_recvmmsg net/socket.c:2843 [inline]
> __do_sys_recvmmsg net/socket.c:2866 [inline]
> __se_sys_recvmmsg net/socket.c:2859 [inline]
> __x64_sys_recvmmsg+0x20b/0x260 net/socket.c:2859
> do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
> entry_SYSCALL_64_after_hwframe+0x44/0xae
> RIP: 0033:0x7fc200d2dc39
> Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 41 15 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 c0 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007ffc5758e5a8 EFLAGS: 00000246 ORIG_RAX: 000000000000012b
> RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007fc200d2dc39
> RDX: 0000000000000002 RSI: 00000000200017c0 RDI: 0000000000000003
> RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000f0b5ff
> R10: 0000000000000100 R11: 0000000000000246 R12: 0000000000000003
> R13: 00007ffc5758e5d0 R14: 00007ffc5758e5c0 R15: 0000000000000003
>
> Fix the issue replacing the MPTCP_DATA_READY bit

for incoming data

> with direct
> inspection of the msk receive queue.

(since MPTCP_DATA_READY is still used for incoming connections)

>
> Reported-and-tested-by: syzbot+3360da629681aa0d22fe@syzkaller.appspotmail.com
> Fixes: 7a6a6cbc3e59 ("mptcp: recvmsg() can drain data from multiple subflow")
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>
> ---
> v1 -> v2:
> - instead of do more fiddling with the DATA_READY bit, just
>   check sk_receive_queue
> ---
> net/mptcp/protocol.c | 52 ++++++++++----------------------------------
> 1 file changed, 12 insertions(+), 40 deletions(-)
>
> diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
> index e5df0b5971c8..cc996e8973b3 100644
> --- a/net/mptcp/protocol.c
> +++ b/net/mptcp/protocol.c
> @@ -528,7 +528,6 @@ static bool mptcp_check_data_fin(struct sock *sk)
>
> 		sk->sk_shutdown |= RCV_SHUTDOWN;
> 		smp_mb__before_atomic(); /* SHUTDOWN must be visible first */
> -		set_bit(MPTCP_DATA_READY, &msk->flags);
>
> 		switch (sk->sk_state) {
> 		case TCP_ESTABLISHED:
> @@ -742,10 +741,9 @@ void mptcp_data_ready(struct sock *sk, struct sock *ssk)
>
> 	/* Wake-up the reader only for in-sequence data */
> 	mptcp_data_lock(sk);
> -	if (move_skbs_to_msk(msk, ssk)) {
> -		set_bit(MPTCP_DATA_READY, &msk->flags);
> +	if (move_skbs_to_msk(msk, ssk))
> 		sk->sk_data_ready(sk);
> -	}
> +
> 	mptcp_data_unlock(sk);
> }
>
> @@ -847,7 +845,6 @@ static void mptcp_check_for_eof(struct mptcp_sock *msk)
> 		sk->sk_shutdown |= RCV_SHUTDOWN;
>
> 		smp_mb__before_atomic(); /* SHUTDOWN must be visible first */
> -		set_bit(MPTCP_DATA_READY, &msk->flags);
> 		sk->sk_data_ready(sk);
> 	}
>
> @@ -1759,21 +1756,6 @@ static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
> 	return copied ? : ret;
> }
>
> -static void mptcp_wait_data(struct sock *sk, long *timeo)
> -{
> -	DEFINE_WAIT_FUNC(wait, woken_wake_function);
> -	struct mptcp_sock *msk = mptcp_sk(sk);
> -
> -	add_wait_queue(sk_sleep(sk), &wait);
> -	sk_set_bit(SOCKWQ_ASYNC_WAITDATA, sk);
> -
> -	sk_wait_event(sk, timeo,
> -		      test_bit(MPTCP_DATA_READY, &msk->flags), &wait);
> -
> -	sk_clear_bit(SOCKWQ_ASYNC_WAITDATA, sk);
> -	remove_wait_queue(sk_sleep(sk), &wait);
> -}
> -
> static int __mptcp_recvmsg_mskq(struct mptcp_sock *msk,
> 				struct msghdr *msg,
> 				size_t len, int flags,
> @@ -2077,19 +2059,7 @@ static int mptcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
> 		}
>
> 		pr_debug("block timeout %ld", timeo);
> -		mptcp_wait_data(sk, &timeo);
> -	}
> -
> -	if (skb_queue_empty_lockless(&sk->sk_receive_queue) &&
> -	    skb_queue_empty(&msk->receive_queue)) {
> -		/* entire backlog drained, clear DATA_READY. */
> -		clear_bit(MPTCP_DATA_READY, &msk->flags);
> -
> -		/* .. race-breaker: ssk might have gotten new data
> -		 * after last __mptcp_move_skbs() returned false.
> -		 */
> -		if (unlikely(__mptcp_move_skbs(msk)))
> -			set_bit(MPTCP_DATA_READY, &msk->flags);
> +		sk_wait_data(sk, &timeo, NULL);
> 	}
>
> out_err:
> @@ -2098,9 +2068,9 @@ static int mptcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
> 			tcp_recv_timestamp(msg, sk, &tss);
> 	}
>
> -	pr_debug("msk=%p data_ready=%d rx queue empty=%d copied=%d",
> -		 msk, test_bit(MPTCP_DATA_READY, &msk->flags),
> -		 skb_queue_empty_lockless(&sk->sk_receive_queue), copied);
> +	pr_debug("msk=%p rx queue empty=%d:%d copied=%d",
> +		 msk, skb_queue_empty_lockless(&sk->sk_receive_queue),
> +		 skb_queue_empty(&msk->receive_queue), copied);
> 	if (!(flags & MSG_PEEK))
> 		mptcp_rcv_space_adjust(msk, copied);
>
> @@ -2368,7 +2338,6 @@ static void mptcp_check_fastclose(struct mptcp_sock *msk)
> 	inet_sk_state_store(sk, TCP_CLOSE);
> 	sk->sk_shutdown = SHUTDOWN_MASK;
> 	smp_mb__before_atomic(); /* SHUTDOWN must be visible first */
> -	set_bit(MPTCP_DATA_READY, &msk->flags);
> 	set_bit(MPTCP_WORK_CLOSE_SUBFLOW, &msk->flags);
>
> 	mptcp_close_wake_up(sk);
> @@ -3385,8 +3354,11 @@ static int mptcp_stream_accept(struct socket *sock, struct socket *newsock,
>
> static __poll_t mptcp_check_readable(struct mptcp_sock *msk)
> {
> -	return test_bit(MPTCP_DATA_READY, &msk->flags) ? EPOLLIN | EPOLLRDNORM :
> -	       0;
> +	if (skb_queue_empty_lockless(&((struct sock *)msk)->sk_receive_queue) &&
> +            skb_queue_empty_lockless(&msk->receive_queue))

This is the tricky bit, as it has potential to be racy. I _think_ it works 
out ok because skbs are moved from the msk->sk_receive_queue to 
msk->receive_queue by skb_queue_splice_tail_init().

skb_queue_splice_tail_init() will reinitialize sk_receive_queue only after 
the splice is complete (although __skb_queue_head_init() unfortunately 
doesn't use WRITE_ONCE()). If both sk_receive_queue and receive_queue 
appear to be non-empty while a splice is in progress, that's not a problem 
since the code above will bypass the second check and still correctly 
return EPOLLIN | EPOLLRDNORM.

Do you agree? Seems like this should be explained in a comment in 
mptcp_check_readable() too.

> +		return 0;
> +
> +	return EPOLLIN | EPOLLRDNORM;
> }
>
> static __poll_t mptcp_check_writeable(struct mptcp_sock *msk)
> @@ -3421,7 +3393,7 @@ static __poll_t mptcp_poll(struct file *file, struct socket *sock,
> 	state = inet_sk_state_load(sk);
> 	pr_debug("msk=%p state=%d flags=%lx", msk, state, msk->flags);
> 	if (state == TCP_LISTEN)
> -		return mptcp_check_readable(msk);
> +		return test_bit(MPTCP_DATA_READY, &msk->flags) ? EPOLLIN | EPOLLRDNORM : 0;
>
> 	if (state != TCP_SYN_SENT && state != TCP_SYN_RECV) {
> 		mask |= mptcp_check_readable(msk);
> -- 
> 2.26.3
>
>
>

--
Mat Martineau
Intel

Re: [PATCH v2 mptcp-net] mptcp: fix possible stall on recvmsg()

[Paolo Abeni]

On Fri, 2021-09-24 at 15:01 -0700, Mat Martineau wrote:
> On Fri, 24 Sep 2021, Paolo Abeni wrote:
> 
> > recvmsg() can enter an infinite loop if the caller provides the
> > MSG_WAITALL, the data present in the receive queue is not
> > sufficient to fulfill the request and no more data is received by
> > the peer.
> > 
> > When the above happens, mptcp_wait_data() will always return with
> > no wait, as the MPTCP_DATA_READY flag checked by such function is
> > set and never cleared in such code path.
> > 
> > Leveraging the above syzbot was able to trigger an RCU stall:
> > 
> > rcu: INFO: rcu_preempt self-detected stall on CPU
> > rcu:    0-...!: (10499 ticks this GP) idle=0af/1/0x4000000000000000 softirq=10678/10678 fqs=1
> >        (t=10500 jiffies g=13089 q=109)
> > rcu: rcu_preempt kthread starved for 10497 jiffies! g13089 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=1
> > rcu:    Unless rcu_preempt kthread gets sufficient CPU time, OOM is now expected behavior.
> > rcu: RCU grace-period kthread stack dump:
> > task:rcu_preempt     state:R  running task     stack:28696 pid:   14 ppid:     2 flags:0x00004000
> > Call Trace:
> > context_switch kernel/sched/core.c:4955 [inline]
> > __schedule+0x940/0x26f0 kernel/sched/core.c:6236
> > schedule+0xd3/0x270 kernel/sched/core.c:6315
> > schedule_timeout+0x14a/0x2a0 kernel/time/timer.c:1881
> > rcu_gp_fqs_loop+0x186/0x810 kernel/rcu/tree.c:1955
> > rcu_gp_kthread+0x1de/0x320 kernel/rcu/tree.c:2128
> > kthread+0x405/0x4f0 kernel/kthread.c:327
> > ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
> > rcu: Stack dump where RCU GP kthread last ran:
> > Sending NMI from CPU 0 to CPUs 1:
> > NMI backtrace for cpu 1
> > CPU: 1 PID: 8510 Comm: syz-executor827 Not tainted 5.15.0-rc2-next-20210920-syzkaller #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > RIP: 0010:bytes_is_nonzero mm/kasan/generic.c:84 [inline]
> > RIP: 0010:memory_is_nonzero mm/kasan/generic.c:102 [inline]
> > RIP: 0010:memory_is_poisoned_n mm/kasan/generic.c:128 [inline]
> > RIP: 0010:memory_is_poisoned mm/kasan/generic.c:159 [inline]
> > RIP: 0010:check_region_inline mm/kasan/generic.c:180 [inline]
> > RIP: 0010:kasan_check_range+0xc8/0x180 mm/kasan/generic.c:189
> > Code: 38 00 74 ed 48 8d 50 08 eb 09 48 83 c0 01 48 39 d0 74 7a 80 38 00 74 f2 48 89 c2 b8 01 00 00 00 48 85 d2 75 56 5b 5d 41 5c c3 <48> 85 d2 74 5e 48 01 ea eb 09 48 83 c0 01 48 39 d0 74 50 80 38 00
> > RSP: 0018:ffffc9000cd676c8 EFLAGS: 00000283
> > RAX: ffffed100e9a110e RBX: ffffed100e9a110f RCX: ffffffff88ea062a
> > RDX: 0000000000000001 RSI: 0000000000000008 RDI: ffff888074d08870
> > RBP: ffffed100e9a110e R08: 0000000000000001 R09: ffff888074d08877
> > R10: ffffed100e9a110e R11: 0000000000000000 R12: ffff888074d08000
> > R13: ffff888074d08000 R14: ffff888074d08088 R15: ffff888074d08000
> > FS:  0000555556d8e300(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
> > S:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 0000000020000180 CR3: 0000000068909000 CR4: 00000000001506e0
> > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > Call Trace:
> > instrument_atomic_read_write include/linux/instrumented.h:101 [inline]
> > test_and_clear_bit include/asm-generic/bitops/instrumented-atomic.h:83 [inline]
> > mptcp_release_cb+0x14a/0x210 net/mptcp/protocol.c:3016
> > release_sock+0xb4/0x1b0 net/core/sock.c:3204
> > mptcp_wait_data net/mptcp/protocol.c:1770 [inline]
> > mptcp_recvmsg+0xfd1/0x27b0 net/mptcp/protocol.c:2080
> > inet6_recvmsg+0x11b/0x5e0 net/ipv6/af_inet6.c:659
> > sock_recvmsg_nosec net/socket.c:944 [inline]
> > ____sys_recvmsg+0x527/0x600 net/socket.c:2626
> > ___sys_recvmsg+0x127/0x200 net/socket.c:2670
> > do_recvmmsg+0x24d/0x6d0 net/socket.c:2764
> > __sys_recvmmsg net/socket.c:2843 [inline]
> > __do_sys_recvmmsg net/socket.c:2866 [inline]
> > __se_sys_recvmmsg net/socket.c:2859 [inline]
> > __x64_sys_recvmmsg+0x20b/0x260 net/socket.c:2859
> > do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> > do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
> > entry_SYSCALL_64_after_hwframe+0x44/0xae
> > RIP: 0033:0x7fc200d2dc39
> > Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 41 15 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 c0 ff ff ff f7 d8 64 89 01 48
> > RSP: 002b:00007ffc5758e5a8 EFLAGS: 00000246 ORIG_RAX: 000000000000012b
> > RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007fc200d2dc39
> > RDX: 0000000000000002 RSI: 00000000200017c0 RDI: 0000000000000003
> > RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000f0b5ff
> > R10: 0000000000000100 R11: 0000000000000246 R12: 0000000000000003
> > R13: 00007ffc5758e5d0 R14: 00007ffc5758e5c0 R15: 0000000000000003
> > 
> > Fix the issue replacing the MPTCP_DATA_READY bit
> 
> for incoming data
> 
> > with direct
> > inspection of the msk receive queue.
> 
> (since MPTCP_DATA_READY is still used for incoming connections)
> 
> > Reported-and-tested-by: syzbot+3360da629681aa0d22fe@syzkaller.appspotmail.com
> > Fixes: 7a6a6cbc3e59 ("mptcp: recvmsg() can drain data from multiple subflow")
> > Signed-off-by: Paolo Abeni <pabeni@redhat.com>
> > ---
> > v1 -> v2:
> > - instead of do more fiddling with the DATA_READY bit, just
> >   check sk_receive_queue
> > ---
> > net/mptcp/protocol.c | 52 ++++++++++----------------------------------
> > 1 file changed, 12 insertions(+), 40 deletions(-)
> > 
> > diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
> > index e5df0b5971c8..cc996e8973b3 100644
> > --- a/net/mptcp/protocol.c
> > +++ b/net/mptcp/protocol.c
> > @@ -528,7 +528,6 @@ static bool mptcp_check_data_fin(struct sock *sk)
> > 
> > 		sk->sk_shutdown |= RCV_SHUTDOWN;
> > 		smp_mb__before_atomic(); /* SHUTDOWN must be visible first */
> > -		set_bit(MPTCP_DATA_READY, &msk->flags);
> > 
> > 		switch (sk->sk_state) {
> > 		case TCP_ESTABLISHED:
> > @@ -742,10 +741,9 @@ void mptcp_data_ready(struct sock *sk, struct sock *ssk)
> > 
> > 	/* Wake-up the reader only for in-sequence data */
> > 	mptcp_data_lock(sk);
> > -	if (move_skbs_to_msk(msk, ssk)) {
> > -		set_bit(MPTCP_DATA_READY, &msk->flags);
> > +	if (move_skbs_to_msk(msk, ssk))
> > 		sk->sk_data_ready(sk);
> > -	}
> > +
> > 	mptcp_data_unlock(sk);
> > }
> > 
> > @@ -847,7 +845,6 @@ static void mptcp_check_for_eof(struct mptcp_sock *msk)
> > 		sk->sk_shutdown |= RCV_SHUTDOWN;
> > 
> > 		smp_mb__before_atomic(); /* SHUTDOWN must be visible first */
> > -		set_bit(MPTCP_DATA_READY, &msk->flags);
> > 		sk->sk_data_ready(sk);
> > 	}
> > 
> > @@ -1759,21 +1756,6 @@ static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
> > 	return copied ? : ret;
> > }
> > 
> > -static void mptcp_wait_data(struct sock *sk, long *timeo)
> > -{
> > -	DEFINE_WAIT_FUNC(wait, woken_wake_function);
> > -	struct mptcp_sock *msk = mptcp_sk(sk);
> > -
> > -	add_wait_queue(sk_sleep(sk), &wait);
> > -	sk_set_bit(SOCKWQ_ASYNC_WAITDATA, sk);
> > -
> > -	sk_wait_event(sk, timeo,
> > -		      test_bit(MPTCP_DATA_READY, &msk->flags), &wait);
> > -
> > -	sk_clear_bit(SOCKWQ_ASYNC_WAITDATA, sk);
> > -	remove_wait_queue(sk_sleep(sk), &wait);
> > -}
> > -
> > static int __mptcp_recvmsg_mskq(struct mptcp_sock *msk,
> > 				struct msghdr *msg,
> > 				size_t len, int flags,
> > @@ -2077,19 +2059,7 @@ static int mptcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
> > 		}
> > 
> > 		pr_debug("block timeout %ld", timeo);
> > -		mptcp_wait_data(sk, &timeo);
> > -	}
> > -
> > -	if (skb_queue_empty_lockless(&sk->sk_receive_queue) &&
> > -	    skb_queue_empty(&msk->receive_queue)) {
> > -		/* entire backlog drained, clear DATA_READY. */
> > -		clear_bit(MPTCP_DATA_READY, &msk->flags);
> > -
> > -		/* .. race-breaker: ssk might have gotten new data
> > -		 * after last __mptcp_move_skbs() returned false.
> > -		 */
> > -		if (unlikely(__mptcp_move_skbs(msk)))
> > -			set_bit(MPTCP_DATA_READY, &msk->flags);
> > +		sk_wait_data(sk, &timeo, NULL);
> > 	}
> > 
> > out_err:
> > @@ -2098,9 +2068,9 @@ static int mptcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
> > 			tcp_recv_timestamp(msg, sk, &tss);
> > 	}
> > 
> > -	pr_debug("msk=%p data_ready=%d rx queue empty=%d copied=%d",
> > -		 msk, test_bit(MPTCP_DATA_READY, &msk->flags),
> > -		 skb_queue_empty_lockless(&sk->sk_receive_queue), copied);
> > +	pr_debug("msk=%p rx queue empty=%d:%d copied=%d",
> > +		 msk, skb_queue_empty_lockless(&sk->sk_receive_queue),
> > +		 skb_queue_empty(&msk->receive_queue), copied);
> > 	if (!(flags & MSG_PEEK))
> > 		mptcp_rcv_space_adjust(msk, copied);
> > 
> > @@ -2368,7 +2338,6 @@ static void mptcp_check_fastclose(struct mptcp_sock *msk)
> > 	inet_sk_state_store(sk, TCP_CLOSE);
> > 	sk->sk_shutdown = SHUTDOWN_MASK;
> > 	smp_mb__before_atomic(); /* SHUTDOWN must be visible first */
> > -	set_bit(MPTCP_DATA_READY, &msk->flags);
> > 	set_bit(MPTCP_WORK_CLOSE_SUBFLOW, &msk->flags);
> > 
> > 	mptcp_close_wake_up(sk);
> > @@ -3385,8 +3354,11 @@ static int mptcp_stream_accept(struct socket *sock, struct socket *newsock,
> > 
> > static __poll_t mptcp_check_readable(struct mptcp_sock *msk)
> > {
> > -	return test_bit(MPTCP_DATA_READY, &msk->flags) ? EPOLLIN | EPOLLRDNORM :
> > -	       0;
> > +	if (skb_queue_empty_lockless(&((struct sock *)msk)->sk_receive_queue) &&
> > +            skb_queue_empty_lockless(&msk->receive_queue))
> 
> This is the tricky bit, as it has potential to be racy. I _think_ it works 
> out ok because skbs are moved from the msk->sk_receive_queue to 
> msk->receive_queue by skb_queue_splice_tail_init().
> 
> skb_queue_splice_tail_init() will reinitialize sk_receive_queue only after 
> the splice is complete (although __skb_queue_head_init() unfortunately 
> doesn't use WRITE_ONCE()). If both sk_receive_queue and receive_queue 
> appear to be non-empty while a splice is in progress, that's not a problem 
> since the code above will bypass the second check and still correctly 
> return EPOLLIN | EPOLLRDNORM.
> 
> Do you agree? Seems like this should be explained in a comment in 
> mptcp_check_readable() too.

Yes, this part is a bit tricky. Still I think avoding MPTCP_DATA_READY
usage in this case simplifies the code make the receive path more
clear.

My understanding is that poll() is not supposed to work reliably when
another process is doing a concurrent read operation on the same socket
- AFAICS, it can't without locking. 

Only a concurrent read() can remove packets from sk_receive_queue()
and/or from msk->receive_queue(). So my take is that we can just ignore
such scenario.

As for an additional comment:

	/* both queues can be empty only when a concurrent read splices
	 * sk_receive_queue into receive_queue
	 */

WDYT?

Thanks,

Paolo

Re: [PATCH v2 mptcp-net] mptcp: fix possible stall on recvmsg()

[Mat Martineau]

On Mon, 27 Sep 2021, Paolo Abeni wrote:

> On Fri, 2021-09-24 at 15:01 -0700, Mat Martineau wrote:
>> On Fri, 24 Sep 2021, Paolo Abeni wrote:
>>
>>> recvmsg() can enter an infinite loop if the caller provides the
>>> MSG_WAITALL, the data present in the receive queue is not
>>> sufficient to fulfill the request and no more data is received by
>>> the peer.
>>>
>>> When the above happens, mptcp_wait_data() will always return with
>>> no wait, as the MPTCP_DATA_READY flag checked by such function is
>>> set and never cleared in such code path.
>>>
>>> Leveraging the above syzbot was able to trigger an RCU stall:
>>>
>>> rcu: INFO: rcu_preempt self-detected stall on CPU
>>> rcu:    0-...!: (10499 ticks this GP) idle=0af/1/0x4000000000000000 softirq=10678/10678 fqs=1
>>>        (t=10500 jiffies g=13089 q=109)
>>> rcu: rcu_preempt kthread starved for 10497 jiffies! g13089 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=1
>>> rcu:    Unless rcu_preempt kthread gets sufficient CPU time, OOM is now expected behavior.
>>> rcu: RCU grace-period kthread stack dump:
>>> task:rcu_preempt     state:R  running task     stack:28696 pid:   14 ppid:     2 flags:0x00004000
>>> Call Trace:
>>> context_switch kernel/sched/core.c:4955 [inline]
>>> __schedule+0x940/0x26f0 kernel/sched/core.c:6236
>>> schedule+0xd3/0x270 kernel/sched/core.c:6315
>>> schedule_timeout+0x14a/0x2a0 kernel/time/timer.c:1881
>>> rcu_gp_fqs_loop+0x186/0x810 kernel/rcu/tree.c:1955
>>> rcu_gp_kthread+0x1de/0x320 kernel/rcu/tree.c:2128
>>> kthread+0x405/0x4f0 kernel/kthread.c:327
>>> ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
>>> rcu: Stack dump where RCU GP kthread last ran:
>>> Sending NMI from CPU 0 to CPUs 1:
>>> NMI backtrace for cpu 1
>>> CPU: 1 PID: 8510 Comm: syz-executor827 Not tainted 5.15.0-rc2-next-20210920-syzkaller #0
>>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
>>> RIP: 0010:bytes_is_nonzero mm/kasan/generic.c:84 [inline]
>>> RIP: 0010:memory_is_nonzero mm/kasan/generic.c:102 [inline]
>>> RIP: 0010:memory_is_poisoned_n mm/kasan/generic.c:128 [inline]
>>> RIP: 0010:memory_is_poisoned mm/kasan/generic.c:159 [inline]
>>> RIP: 0010:check_region_inline mm/kasan/generic.c:180 [inline]
>>> RIP: 0010:kasan_check_range+0xc8/0x180 mm/kasan/generic.c:189
>>> Code: 38 00 74 ed 48 8d 50 08 eb 09 48 83 c0 01 48 39 d0 74 7a 80 38 00 74 f2 48 89 c2 b8 01 00 00 00 48 85 d2 75 56 5b 5d 41 5c c3 <48> 85 d2 74 5e 48 01 ea eb 09 48 83 c0 01 48 39 d0 74 50 80 38 00
>>> RSP: 0018:ffffc9000cd676c8 EFLAGS: 00000283
>>> RAX: ffffed100e9a110e RBX: ffffed100e9a110f RCX: ffffffff88ea062a
>>> RDX: 0000000000000001 RSI: 0000000000000008 RDI: ffff888074d08870
>>> RBP: ffffed100e9a110e R08: 0000000000000001 R09: ffff888074d08877
>>> R10: ffffed100e9a110e R11: 0000000000000000 R12: ffff888074d08000
>>> R13: ffff888074d08000 R14: ffff888074d08088 R15: ffff888074d08000
>>> FS:  0000555556d8e300(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
>>> S:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>> CR2: 0000000020000180 CR3: 0000000068909000 CR4: 00000000001506e0
>>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>>> Call Trace:
>>> instrument_atomic_read_write include/linux/instrumented.h:101 [inline]
>>> test_and_clear_bit include/asm-generic/bitops/instrumented-atomic.h:83 [inline]
>>> mptcp_release_cb+0x14a/0x210 net/mptcp/protocol.c:3016
>>> release_sock+0xb4/0x1b0 net/core/sock.c:3204
>>> mptcp_wait_data net/mptcp/protocol.c:1770 [inline]
>>> mptcp_recvmsg+0xfd1/0x27b0 net/mptcp/protocol.c:2080
>>> inet6_recvmsg+0x11b/0x5e0 net/ipv6/af_inet6.c:659
>>> sock_recvmsg_nosec net/socket.c:944 [inline]
>>> ____sys_recvmsg+0x527/0x600 net/socket.c:2626
>>> ___sys_recvmsg+0x127/0x200 net/socket.c:2670
>>> do_recvmmsg+0x24d/0x6d0 net/socket.c:2764
>>> __sys_recvmmsg net/socket.c:2843 [inline]
>>> __do_sys_recvmmsg net/socket.c:2866 [inline]
>>> __se_sys_recvmmsg net/socket.c:2859 [inline]
>>> __x64_sys_recvmmsg+0x20b/0x260 net/socket.c:2859
>>> do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>>> do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
>>> entry_SYSCALL_64_after_hwframe+0x44/0xae
>>> RIP: 0033:0x7fc200d2dc39
>>> Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 41 15 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 c0 ff ff ff f7 d8 64 89 01 48
>>> RSP: 002b:00007ffc5758e5a8 EFLAGS: 00000246 ORIG_RAX: 000000000000012b
>>> RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007fc200d2dc39
>>> RDX: 0000000000000002 RSI: 00000000200017c0 RDI: 0000000000000003
>>> RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000f0b5ff
>>> R10: 0000000000000100 R11: 0000000000000246 R12: 0000000000000003
>>> R13: 00007ffc5758e5d0 R14: 00007ffc5758e5c0 R15: 0000000000000003
>>>
>>> Fix the issue replacing the MPTCP_DATA_READY bit
>>
>> for incoming data
>>
>>> with direct
>>> inspection of the msk receive queue.
>>
>> (since MPTCP_DATA_READY is still used for incoming connections)
>>
>>> Reported-and-tested-by: syzbot+3360da629681aa0d22fe@syzkaller.appspotmail.com
>>> Fixes: 7a6a6cbc3e59 ("mptcp: recvmsg() can drain data from multiple subflow")
>>> Signed-off-by: Paolo Abeni <pabeni@redhat.com>
>>> ---
>>> v1 -> v2:
>>> - instead of do more fiddling with the DATA_READY bit, just
>>>   check sk_receive_queue
>>> ---
>>> net/mptcp/protocol.c | 52 ++++++++++----------------------------------
>>> 1 file changed, 12 insertions(+), 40 deletions(-)
>>>
>>> diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
>>> index e5df0b5971c8..cc996e8973b3 100644
>>> --- a/net/mptcp/protocol.c
>>> +++ b/net/mptcp/protocol.c
>>> @@ -528,7 +528,6 @@ static bool mptcp_check_data_fin(struct sock *sk)
>>>
>>> 		sk->sk_shutdown |= RCV_SHUTDOWN;
>>> 		smp_mb__before_atomic(); /* SHUTDOWN must be visible first */
>>> -		set_bit(MPTCP_DATA_READY, &msk->flags);
>>>
>>> 		switch (sk->sk_state) {
>>> 		case TCP_ESTABLISHED:
>>> @@ -742,10 +741,9 @@ void mptcp_data_ready(struct sock *sk, struct sock *ssk)
>>>
>>> 	/* Wake-up the reader only for in-sequence data */
>>> 	mptcp_data_lock(sk);
>>> -	if (move_skbs_to_msk(msk, ssk)) {
>>> -		set_bit(MPTCP_DATA_READY, &msk->flags);
>>> +	if (move_skbs_to_msk(msk, ssk))
>>> 		sk->sk_data_ready(sk);
>>> -	}
>>> +
>>> 	mptcp_data_unlock(sk);
>>> }
>>>
>>> @@ -847,7 +845,6 @@ static void mptcp_check_for_eof(struct mptcp_sock *msk)
>>> 		sk->sk_shutdown |= RCV_SHUTDOWN;
>>>
>>> 		smp_mb__before_atomic(); /* SHUTDOWN must be visible first */
>>> -		set_bit(MPTCP_DATA_READY, &msk->flags);
>>> 		sk->sk_data_ready(sk);
>>> 	}
>>>
>>> @@ -1759,21 +1756,6 @@ static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
>>> 	return copied ? : ret;
>>> }
>>>
>>> -static void mptcp_wait_data(struct sock *sk, long *timeo)
>>> -{
>>> -	DEFINE_WAIT_FUNC(wait, woken_wake_function);
>>> -	struct mptcp_sock *msk = mptcp_sk(sk);
>>> -
>>> -	add_wait_queue(sk_sleep(sk), &wait);
>>> -	sk_set_bit(SOCKWQ_ASYNC_WAITDATA, sk);
>>> -
>>> -	sk_wait_event(sk, timeo,
>>> -		      test_bit(MPTCP_DATA_READY, &msk->flags), &wait);
>>> -
>>> -	sk_clear_bit(SOCKWQ_ASYNC_WAITDATA, sk);
>>> -	remove_wait_queue(sk_sleep(sk), &wait);
>>> -}
>>> -
>>> static int __mptcp_recvmsg_mskq(struct mptcp_sock *msk,
>>> 				struct msghdr *msg,
>>> 				size_t len, int flags,
>>> @@ -2077,19 +2059,7 @@ static int mptcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
>>> 		}
>>>
>>> 		pr_debug("block timeout %ld", timeo);
>>> -		mptcp_wait_data(sk, &timeo);
>>> -	}
>>> -
>>> -	if (skb_queue_empty_lockless(&sk->sk_receive_queue) &&
>>> -	    skb_queue_empty(&msk->receive_queue)) {
>>> -		/* entire backlog drained, clear DATA_READY. */
>>> -		clear_bit(MPTCP_DATA_READY, &msk->flags);
>>> -
>>> -		/* .. race-breaker: ssk might have gotten new data
>>> -		 * after last __mptcp_move_skbs() returned false.
>>> -		 */
>>> -		if (unlikely(__mptcp_move_skbs(msk)))
>>> -			set_bit(MPTCP_DATA_READY, &msk->flags);
>>> +		sk_wait_data(sk, &timeo, NULL);
>>> 	}
>>>
>>> out_err:
>>> @@ -2098,9 +2068,9 @@ static int mptcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
>>> 			tcp_recv_timestamp(msg, sk, &tss);
>>> 	}
>>>
>>> -	pr_debug("msk=%p data_ready=%d rx queue empty=%d copied=%d",
>>> -		 msk, test_bit(MPTCP_DATA_READY, &msk->flags),
>>> -		 skb_queue_empty_lockless(&sk->sk_receive_queue), copied);
>>> +	pr_debug("msk=%p rx queue empty=%d:%d copied=%d",
>>> +		 msk, skb_queue_empty_lockless(&sk->sk_receive_queue),
>>> +		 skb_queue_empty(&msk->receive_queue), copied);
>>> 	if (!(flags & MSG_PEEK))
>>> 		mptcp_rcv_space_adjust(msk, copied);
>>>
>>> @@ -2368,7 +2338,6 @@ static void mptcp_check_fastclose(struct mptcp_sock *msk)
>>> 	inet_sk_state_store(sk, TCP_CLOSE);
>>> 	sk->sk_shutdown = SHUTDOWN_MASK;
>>> 	smp_mb__before_atomic(); /* SHUTDOWN must be visible first */
>>> -	set_bit(MPTCP_DATA_READY, &msk->flags);
>>> 	set_bit(MPTCP_WORK_CLOSE_SUBFLOW, &msk->flags);
>>>
>>> 	mptcp_close_wake_up(sk);
>>> @@ -3385,8 +3354,11 @@ static int mptcp_stream_accept(struct socket *sock, struct socket *newsock,
>>>
>>> static __poll_t mptcp_check_readable(struct mptcp_sock *msk)
>>> {
>>> -	return test_bit(MPTCP_DATA_READY, &msk->flags) ? EPOLLIN | EPOLLRDNORM :
>>> -	       0;
>>> +	if (skb_queue_empty_lockless(&((struct sock *)msk)->sk_receive_queue) &&
>>> +            skb_queue_empty_lockless(&msk->receive_queue))
>>
>> This is the tricky bit, as it has potential to be racy. I _think_ it works
>> out ok because skbs are moved from the msk->sk_receive_queue to
>> msk->receive_queue by skb_queue_splice_tail_init().
>>
>> skb_queue_splice_tail_init() will reinitialize sk_receive_queue only after
>> the splice is complete (although __skb_queue_head_init() unfortunately
>> doesn't use WRITE_ONCE()). If both sk_receive_queue and receive_queue
>> appear to be non-empty while a splice is in progress, that's not a problem
>> since the code above will bypass the second check and still correctly
>> return EPOLLIN | EPOLLRDNORM.
>>
>> Do you agree? Seems like this should be explained in a comment in
>> mptcp_check_readable() too.
>
> Yes, this part is a bit tricky. Still I think avoding MPTCP_DATA_READY
> usage in this case simplifies the code make the receive path more
> clear.
>

I agree, it's clearer.

> My understanding is that poll() is not supposed to work reliably when
> another process is doing a concurrent read operation on the same socket
> - AFAICS, it can't without locking.
>
> Only a concurrent read() can remove packets from sk_receive_queue()
> and/or from msk->receive_queue(). So my take is that we can just ignore
> such scenario.
>

To clarify, my concern was not so much with concurrent reads but with the 
splice that happens in mptcp_data_ready() when data arrives on a subflow. 
If the splice implementation were to clear the source queue before 
updating the destination, or if receive_queue was checked before 
sk_receive_queue, the lockless poll could "see" two empty queues even 
though there was data to read.

Both the ordering in the code above and the implentation of the 
splice/init make sure that the only slight weirdness due to concurrency 
would be to see two non-empty queues - which is fine for polling.

> As for an additional comment:
>
> 	/* both queues can be empty only when a concurrent read splices
> 	 * sk_receive_queue into receive_queue
> 	 */
>
> WDYT?

How about "Concurrent splices from sk_receive_queue into receive_queue 
will always show at least one non-empty queue when checked in this order." 
?


--
Mat Martineau
Intel

Re: [PATCH v2 mptcp-net] mptcp: fix possible stall on recvmsg()

[Paolo Abeni]

On Mon, 2021-09-27 at 11:05 -0700, Mat Martineau wrote:
> On Mon, 27 Sep 2021, Paolo Abeni wrote:
> 
> > On Fri, 2021-09-24 at 15:01 -0700, Mat Martineau wrote:
> > > On Fri, 24 Sep 2021, Paolo Abeni wrote:
> > > 
> > > > recvmsg() can enter an infinite loop if the caller provides the
> > > > MSG_WAITALL, the data present in the receive queue is not
> > > > sufficient to fulfill the request and no more data is received by
> > > > the peer.
> > > > 
> > > > When the above happens, mptcp_wait_data() will always return with
> > > > no wait, as the MPTCP_DATA_READY flag checked by such function is
> > > > set and never cleared in such code path.
> > > > 
> > > > Leveraging the above syzbot was able to trigger an RCU stall:
> > > > 
> > > > rcu: INFO: rcu_preempt self-detected stall on CPU
> > > > rcu:    0-...!: (10499 ticks this GP) idle=0af/1/0x4000000000000000 softirq=10678/10678 fqs=1
> > > >        (t=10500 jiffies g=13089 q=109)
> > > > rcu: rcu_preempt kthread starved for 10497 jiffies! g13089 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=1
> > > > rcu:    Unless rcu_preempt kthread gets sufficient CPU time, OOM is now expected behavior.
> > > > rcu: RCU grace-period kthread stack dump:
> > > > task:rcu_preempt     state:R  running task     stack:28696 pid:   14 ppid:     2 flags:0x00004000
> > > > Call Trace:
> > > > context_switch kernel/sched/core.c:4955 [inline]
> > > > __schedule+0x940/0x26f0 kernel/sched/core.c:6236
> > > > schedule+0xd3/0x270 kernel/sched/core.c:6315
> > > > schedule_timeout+0x14a/0x2a0 kernel/time/timer.c:1881
> > > > rcu_gp_fqs_loop+0x186/0x810 kernel/rcu/tree.c:1955
> > > > rcu_gp_kthread+0x1de/0x320 kernel/rcu/tree.c:2128
> > > > kthread+0x405/0x4f0 kernel/kthread.c:327
> > > > ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
> > > > rcu: Stack dump where RCU GP kthread last ran:
> > > > Sending NMI from CPU 0 to CPUs 1:
> > > > NMI backtrace for cpu 1
> > > > CPU: 1 PID: 8510 Comm: syz-executor827 Not tainted 5.15.0-rc2-next-20210920-syzkaller #0
> > > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > > > RIP: 0010:bytes_is_nonzero mm/kasan/generic.c:84 [inline]
> > > > RIP: 0010:memory_is_nonzero mm/kasan/generic.c:102 [inline]
> > > > RIP: 0010:memory_is_poisoned_n mm/kasan/generic.c:128 [inline]
> > > > RIP: 0010:memory_is_poisoned mm/kasan/generic.c:159 [inline]
> > > > RIP: 0010:check_region_inline mm/kasan/generic.c:180 [inline]
> > > > RIP: 0010:kasan_check_range+0xc8/0x180 mm/kasan/generic.c:189
> > > > Code: 38 00 74 ed 48 8d 50 08 eb 09 48 83 c0 01 48 39 d0 74 7a 80 38 00 74 f2 48 89 c2 b8 01 00 00 00 48 85 d2 75 56 5b 5d 41 5c c3 <48> 85 d2 74 5e 48 01 ea eb 09 48 83 c0 01 48 39 d0 74 50 80 38 00
> > > > RSP: 0018:ffffc9000cd676c8 EFLAGS: 00000283
> > > > RAX: ffffed100e9a110e RBX: ffffed100e9a110f RCX: ffffffff88ea062a
> > > > RDX: 0000000000000001 RSI: 0000000000000008 RDI: ffff888074d08870
> > > > RBP: ffffed100e9a110e R08: 0000000000000001 R09: ffff888074d08877
> > > > R10: ffffed100e9a110e R11: 0000000000000000 R12: ffff888074d08000
> > > > R13: ffff888074d08000 R14: ffff888074d08088 R15: ffff888074d08000
> > > > FS:  0000555556d8e300(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
> > > > S:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > > CR2: 0000000020000180 CR3: 0000000068909000 CR4: 00000000001506e0
> > > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > > Call Trace:
> > > > instrument_atomic_read_write include/linux/instrumented.h:101 [inline]
> > > > test_and_clear_bit include/asm-generic/bitops/instrumented-atomic.h:83 [inline]
> > > > mptcp_release_cb+0x14a/0x210 net/mptcp/protocol.c:3016
> > > > release_sock+0xb4/0x1b0 net/core/sock.c:3204
> > > > mptcp_wait_data net/mptcp/protocol.c:1770 [inline]
> > > > mptcp_recvmsg+0xfd1/0x27b0 net/mptcp/protocol.c:2080
> > > > inet6_recvmsg+0x11b/0x5e0 net/ipv6/af_inet6.c:659
> > > > sock_recvmsg_nosec net/socket.c:944 [inline]
> > > > ____sys_recvmsg+0x527/0x600 net/socket.c:2626
> > > > ___sys_recvmsg+0x127/0x200 net/socket.c:2670
> > > > do_recvmmsg+0x24d/0x6d0 net/socket.c:2764
> > > > __sys_recvmmsg net/socket.c:2843 [inline]
> > > > __do_sys_recvmmsg net/socket.c:2866 [inline]
> > > > __se_sys_recvmmsg net/socket.c:2859 [inline]
> > > > __x64_sys_recvmmsg+0x20b/0x260 net/socket.c:2859
> > > > do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> > > > do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
> > > > entry_SYSCALL_64_after_hwframe+0x44/0xae
> > > > RIP: 0033:0x7fc200d2dc39
> > > > Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 41 15 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 c0 ff ff ff f7 d8 64 89 01 48
> > > > RSP: 002b:00007ffc5758e5a8 EFLAGS: 00000246 ORIG_RAX: 000000000000012b
> > > > RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007fc200d2dc39
> > > > RDX: 0000000000000002 RSI: 00000000200017c0 RDI: 0000000000000003
> > > > RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000f0b5ff
> > > > R10: 0000000000000100 R11: 0000000000000246 R12: 0000000000000003
> > > > R13: 00007ffc5758e5d0 R14: 00007ffc5758e5c0 R15: 0000000000000003
> > > > 
> > > > Fix the issue replacing the MPTCP_DATA_READY bit
> > > 
> > > for incoming data
> > > 
> > > > with direct
> > > > inspection of the msk receive queue.
> > > 
> > > (since MPTCP_DATA_READY is still used for incoming connections)
> > > 
> > > > Reported-and-tested-by: syzbot+3360da629681aa0d22fe@syzkaller.appspotmail.com
> > > > Fixes: 7a6a6cbc3e59 ("mptcp: recvmsg() can drain data from multiple subflow")
> > > > Signed-off-by: Paolo Abeni <pabeni@redhat.com>
> > > > ---
> > > > v1 -> v2:
> > > > - instead of do more fiddling with the DATA_READY bit, just
> > > >   check sk_receive_queue
> > > > ---
> > > > net/mptcp/protocol.c | 52 ++++++++++----------------------------------
> > > > 1 file changed, 12 insertions(+), 40 deletions(-)
> > > > 
> > > > diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
> > > > index e5df0b5971c8..cc996e8973b3 100644
> > > > --- a/net/mptcp/protocol.c
> > > > +++ b/net/mptcp/protocol.c
> > > > @@ -528,7 +528,6 @@ static bool mptcp_check_data_fin(struct sock *sk)
> > > > 
> > > > 		sk->sk_shutdown |= RCV_SHUTDOWN;
> > > > 		smp_mb__before_atomic(); /* SHUTDOWN must be visible first */
> > > > -		set_bit(MPTCP_DATA_READY, &msk->flags);
> > > > 
> > > > 		switch (sk->sk_state) {
> > > > 		case TCP_ESTABLISHED:
> > > > @@ -742,10 +741,9 @@ void mptcp_data_ready(struct sock *sk, struct sock *ssk)
> > > > 
> > > > 	/* Wake-up the reader only for in-sequence data */
> > > > 	mptcp_data_lock(sk);
> > > > -	if (move_skbs_to_msk(msk, ssk)) {
> > > > -		set_bit(MPTCP_DATA_READY, &msk->flags);
> > > > +	if (move_skbs_to_msk(msk, ssk))
> > > > 		sk->sk_data_ready(sk);
> > > > -	}
> > > > +
> > > > 	mptcp_data_unlock(sk);
> > > > }
> > > > 
> > > > @@ -847,7 +845,6 @@ static void mptcp_check_for_eof(struct mptcp_sock *msk)
> > > > 		sk->sk_shutdown |= RCV_SHUTDOWN;
> > > > 
> > > > 		smp_mb__before_atomic(); /* SHUTDOWN must be visible first */
> > > > -		set_bit(MPTCP_DATA_READY, &msk->flags);
> > > > 		sk->sk_data_ready(sk);
> > > > 	}
> > > > 
> > > > @@ -1759,21 +1756,6 @@ static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
> > > > 	return copied ? : ret;
> > > > }
> > > > 
> > > > -static void mptcp_wait_data(struct sock *sk, long *timeo)
> > > > -{
> > > > -	DEFINE_WAIT_FUNC(wait, woken_wake_function);
> > > > -	struct mptcp_sock *msk = mptcp_sk(sk);
> > > > -
> > > > -	add_wait_queue(sk_sleep(sk), &wait);
> > > > -	sk_set_bit(SOCKWQ_ASYNC_WAITDATA, sk);
> > > > -
> > > > -	sk_wait_event(sk, timeo,
> > > > -		      test_bit(MPTCP_DATA_READY, &msk->flags), &wait);
> > > > -
> > > > -	sk_clear_bit(SOCKWQ_ASYNC_WAITDATA, sk);
> > > > -	remove_wait_queue(sk_sleep(sk), &wait);
> > > > -}
> > > > -
> > > > static int __mptcp_recvmsg_mskq(struct mptcp_sock *msk,
> > > > 				struct msghdr *msg,
> > > > 				size_t len, int flags,
> > > > @@ -2077,19 +2059,7 @@ static int mptcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
> > > > 		}
> > > > 
> > > > 		pr_debug("block timeout %ld", timeo);
> > > > -		mptcp_wait_data(sk, &timeo);
> > > > -	}
> > > > -
> > > > -	if (skb_queue_empty_lockless(&sk->sk_receive_queue) &&
> > > > -	    skb_queue_empty(&msk->receive_queue)) {
> > > > -		/* entire backlog drained, clear DATA_READY. */
> > > > -		clear_bit(MPTCP_DATA_READY, &msk->flags);
> > > > -
> > > > -		/* .. race-breaker: ssk might have gotten new data
> > > > -		 * after last __mptcp_move_skbs() returned false.
> > > > -		 */
> > > > -		if (unlikely(__mptcp_move_skbs(msk)))
> > > > -			set_bit(MPTCP_DATA_READY, &msk->flags);
> > > > +		sk_wait_data(sk, &timeo, NULL);
> > > > 	}
> > > > 
> > > > out_err:
> > > > @@ -2098,9 +2068,9 @@ static int mptcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
> > > > 			tcp_recv_timestamp(msg, sk, &tss);
> > > > 	}
> > > > 
> > > > -	pr_debug("msk=%p data_ready=%d rx queue empty=%d copied=%d",
> > > > -		 msk, test_bit(MPTCP_DATA_READY, &msk->flags),
> > > > -		 skb_queue_empty_lockless(&sk->sk_receive_queue), copied);
> > > > +	pr_debug("msk=%p rx queue empty=%d:%d copied=%d",
> > > > +		 msk, skb_queue_empty_lockless(&sk->sk_receive_queue),
> > > > +		 skb_queue_empty(&msk->receive_queue), copied);
> > > > 	if (!(flags & MSG_PEEK))
> > > > 		mptcp_rcv_space_adjust(msk, copied);
> > > > 
> > > > @@ -2368,7 +2338,6 @@ static void mptcp_check_fastclose(struct mptcp_sock *msk)
> > > > 	inet_sk_state_store(sk, TCP_CLOSE);
> > > > 	sk->sk_shutdown = SHUTDOWN_MASK;
> > > > 	smp_mb__before_atomic(); /* SHUTDOWN must be visible first */
> > > > -	set_bit(MPTCP_DATA_READY, &msk->flags);
> > > > 	set_bit(MPTCP_WORK_CLOSE_SUBFLOW, &msk->flags);
> > > > 
> > > > 	mptcp_close_wake_up(sk);
> > > > @@ -3385,8 +3354,11 @@ static int mptcp_stream_accept(struct socket *sock, struct socket *newsock,
> > > > 
> > > > static __poll_t mptcp_check_readable(struct mptcp_sock *msk)
> > > > {
> > > > -	return test_bit(MPTCP_DATA_READY, &msk->flags) ? EPOLLIN | EPOLLRDNORM :
> > > > -	       0;
> > > > +	if (skb_queue_empty_lockless(&((struct sock *)msk)->sk_receive_queue) &&
> > > > +            skb_queue_empty_lockless(&msk->receive_queue))
> > > 
> > > This is the tricky bit, as it has potential to be racy. I _think_ it works
> > > out ok because skbs are moved from the msk->sk_receive_queue to
> > > msk->receive_queue by skb_queue_splice_tail_init().
> > > 
> > > skb_queue_splice_tail_init() will reinitialize sk_receive_queue only after
> > > the splice is complete (although __skb_queue_head_init() unfortunately
> > > doesn't use WRITE_ONCE()). If both sk_receive_queue and receive_queue
> > > appear to be non-empty while a splice is in progress, that's not a problem
> > > since the code above will bypass the second check and still correctly
> > > return EPOLLIN | EPOLLRDNORM.
> > > 
> > > Do you agree? Seems like this should be explained in a comment in
> > > mptcp_check_readable() too.
> > 
> > Yes, this part is a bit tricky. Still I think avoding MPTCP_DATA_READY
> > usage in this case simplifies the code make the receive path more
> > clear.
> > 
> 
> I agree, it's clearer.
> 
> > My understanding is that poll() is not supposed to work reliably when
> > another process is doing a concurrent read operation on the same socket
> > - AFAICS, it can't without locking.
> > 
> > Only a concurrent read() can remove packets from sk_receive_queue()
> > and/or from msk->receive_queue(). So my take is that we can just ignore
> > such scenario.
> > 
> 
> To clarify, my concern was not so much with concurrent reads but with the 
> splice that happens in mptcp_data_ready() when data arrives on a subflow. 

uhm... I don't see that splice in the receive path?!?
skb_queue_splice_tail_init() is called only by mptcp_destroy_common()
and __mptcp_splice_receive_queue(). The latter is invoked by only
mptcp_recvmsg() and __mptcp_move_skbs(). Finally __mptcp_move_skbs() is
invoked only by mptcp_recvmsg().

To splice the sk_receive_queue into msk->receive_queue, we need both
the msk socket lock and the msk data lock, so we can't do that from the
plain receive path (data lock only acquired).

> If the splice implementation were to clear the source queue before 
> updating the destination, or if receive_queue was checked before 
> sk_receive_queue, the lockless poll could "see" two empty queues even 
> though there was data to read.
> 
> Both the ordering in the code above and the implentation of the 
> splice/init make sure that the only slight weirdness due to concurrency 
> would be to see two non-empty queues - which is fine for polling.
> 
> > As for an additional comment:
> > 
> > 	/* both queues can be empty only when a concurrent read splices
> > 	 * sk_receive_queue into receive_queue
> > 	 */
> > 
> > WDYT?
> 
> How about "Concurrent splices from sk_receive_queue into receive_queue 
> will always show at least one non-empty queue when checked in this order." 
> ?

Fine by me.

Thanks!

Paolo

Re: [PATCH v2 mptcp-net] mptcp: fix possible stall on recvmsg()

[Mat Martineau]

On Tue, 28 Sep 2021, Paolo Abeni wrote:

> On Mon, 2021-09-27 at 11:05 -0700, Mat Martineau wrote:
>> On Mon, 27 Sep 2021, Paolo Abeni wrote:
>>
>>> On Fri, 2021-09-24 at 15:01 -0700, Mat Martineau wrote:
>>>> On Fri, 24 Sep 2021, Paolo Abeni wrote:
>>>>
>>>>> recvmsg() can enter an infinite loop if the caller provides the
>>>>> MSG_WAITALL, the data present in the receive queue is not
>>>>> sufficient to fulfill the request and no more data is received by
>>>>> the peer.
>>>>>
>>>>> When the above happens, mptcp_wait_data() will always return with
>>>>> no wait, as the MPTCP_DATA_READY flag checked by such function is
>>>>> set and never cleared in such code path.
>>>>>
>>>>> Leveraging the above syzbot was able to trigger an RCU stall:
>>>>>
>>>>> rcu: INFO: rcu_preempt self-detected stall on CPU
>>>>> rcu:    0-...!: (10499 ticks this GP) idle=0af/1/0x4000000000000000 softirq=10678/10678 fqs=1
>>>>>        (t=10500 jiffies g=13089 q=109)
>>>>> rcu: rcu_preempt kthread starved for 10497 jiffies! g13089 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=1
>>>>> rcu:    Unless rcu_preempt kthread gets sufficient CPU time, OOM is now expected behavior.
>>>>> rcu: RCU grace-period kthread stack dump:
>>>>> task:rcu_preempt     state:R  running task     stack:28696 pid:   14 ppid:     2 flags:0x00004000
>>>>> Call Trace:
>>>>> context_switch kernel/sched/core.c:4955 [inline]
>>>>> __schedule+0x940/0x26f0 kernel/sched/core.c:6236
>>>>> schedule+0xd3/0x270 kernel/sched/core.c:6315
>>>>> schedule_timeout+0x14a/0x2a0 kernel/time/timer.c:1881
>>>>> rcu_gp_fqs_loop+0x186/0x810 kernel/rcu/tree.c:1955
>>>>> rcu_gp_kthread+0x1de/0x320 kernel/rcu/tree.c:2128
>>>>> kthread+0x405/0x4f0 kernel/kthread.c:327
>>>>> ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
>>>>> rcu: Stack dump where RCU GP kthread last ran:
>>>>> Sending NMI from CPU 0 to CPUs 1:
>>>>> NMI backtrace for cpu 1
>>>>> CPU: 1 PID: 8510 Comm: syz-executor827 Not tainted 5.15.0-rc2-next-20210920-syzkaller #0
>>>>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
>>>>> RIP: 0010:bytes_is_nonzero mm/kasan/generic.c:84 [inline]
>>>>> RIP: 0010:memory_is_nonzero mm/kasan/generic.c:102 [inline]
>>>>> RIP: 0010:memory_is_poisoned_n mm/kasan/generic.c:128 [inline]
>>>>> RIP: 0010:memory_is_poisoned mm/kasan/generic.c:159 [inline]
>>>>> RIP: 0010:check_region_inline mm/kasan/generic.c:180 [inline]
>>>>> RIP: 0010:kasan_check_range+0xc8/0x180 mm/kasan/generic.c:189
>>>>> Code: 38 00 74 ed 48 8d 50 08 eb 09 48 83 c0 01 48 39 d0 74 7a 80 38 00 74 f2 48 89 c2 b8 01 00 00 00 48 85 d2 75 56 5b 5d 41 5c c3 <48> 85 d2 74 5e 48 01 ea eb 09 48 83 c0 01 48 39 d0 74 50 80 38 00
>>>>> RSP: 0018:ffffc9000cd676c8 EFLAGS: 00000283
>>>>> RAX: ffffed100e9a110e RBX: ffffed100e9a110f RCX: ffffffff88ea062a
>>>>> RDX: 0000000000000001 RSI: 0000000000000008 RDI: ffff888074d08870
>>>>> RBP: ffffed100e9a110e R08: 0000000000000001 R09: ffff888074d08877
>>>>> R10: ffffed100e9a110e R11: 0000000000000000 R12: ffff888074d08000
>>>>> R13: ffff888074d08000 R14: ffff888074d08088 R15: ffff888074d08000
>>>>> FS:  0000555556d8e300(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
>>>>> S:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>>> CR2: 0000000020000180 CR3: 0000000068909000 CR4: 00000000001506e0
>>>>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>>>>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>>>>> Call Trace:
>>>>> instrument_atomic_read_write include/linux/instrumented.h:101 [inline]
>>>>> test_and_clear_bit include/asm-generic/bitops/instrumented-atomic.h:83 [inline]
>>>>> mptcp_release_cb+0x14a/0x210 net/mptcp/protocol.c:3016
>>>>> release_sock+0xb4/0x1b0 net/core/sock.c:3204
>>>>> mptcp_wait_data net/mptcp/protocol.c:1770 [inline]
>>>>> mptcp_recvmsg+0xfd1/0x27b0 net/mptcp/protocol.c:2080
>>>>> inet6_recvmsg+0x11b/0x5e0 net/ipv6/af_inet6.c:659
>>>>> sock_recvmsg_nosec net/socket.c:944 [inline]
>>>>> ____sys_recvmsg+0x527/0x600 net/socket.c:2626
>>>>> ___sys_recvmsg+0x127/0x200 net/socket.c:2670
>>>>> do_recvmmsg+0x24d/0x6d0 net/socket.c:2764
>>>>> __sys_recvmmsg net/socket.c:2843 [inline]
>>>>> __do_sys_recvmmsg net/socket.c:2866 [inline]
>>>>> __se_sys_recvmmsg net/socket.c:2859 [inline]
>>>>> __x64_sys_recvmmsg+0x20b/0x260 net/socket.c:2859
>>>>> do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>>>>> do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
>>>>> entry_SYSCALL_64_after_hwframe+0x44/0xae
>>>>> RIP: 0033:0x7fc200d2dc39
>>>>> Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 41 15 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 c0 ff ff ff f7 d8 64 89 01 48
>>>>> RSP: 002b:00007ffc5758e5a8 EFLAGS: 00000246 ORIG_RAX: 000000000000012b
>>>>> RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007fc200d2dc39
>>>>> RDX: 0000000000000002 RSI: 00000000200017c0 RDI: 0000000000000003
>>>>> RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000f0b5ff
>>>>> R10: 0000000000000100 R11: 0000000000000246 R12: 0000000000000003
>>>>> R13: 00007ffc5758e5d0 R14: 00007ffc5758e5c0 R15: 0000000000000003
>>>>>
>>>>> Fix the issue replacing the MPTCP_DATA_READY bit
>>>>
>>>> for incoming data
>>>>
>>>>> with direct
>>>>> inspection of the msk receive queue.
>>>>
>>>> (since MPTCP_DATA_READY is still used for incoming connections)
>>>>
>>>>> Reported-and-tested-by: syzbot+3360da629681aa0d22fe@syzkaller.appspotmail.com
>>>>> Fixes: 7a6a6cbc3e59 ("mptcp: recvmsg() can drain data from multiple subflow")
>>>>> Signed-off-by: Paolo Abeni <pabeni@redhat.com>
>>>>> ---
>>>>> v1 -> v2:
>>>>> - instead of do more fiddling with the DATA_READY bit, just
>>>>>   check sk_receive_queue
>>>>> ---
>>>>> net/mptcp/protocol.c | 52 ++++++++++----------------------------------
>>>>> 1 file changed, 12 insertions(+), 40 deletions(-)
>>>>>
>>>>> diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
>>>>> index e5df0b5971c8..cc996e8973b3 100644
>>>>> --- a/net/mptcp/protocol.c
>>>>> +++ b/net/mptcp/protocol.c
>>>>> @@ -528,7 +528,6 @@ static bool mptcp_check_data_fin(struct sock *sk)
>>>>>
>>>>> 		sk->sk_shutdown |= RCV_SHUTDOWN;
>>>>> 		smp_mb__before_atomic(); /* SHUTDOWN must be visible first */
>>>>> -		set_bit(MPTCP_DATA_READY, &msk->flags);
>>>>>
>>>>> 		switch (sk->sk_state) {
>>>>> 		case TCP_ESTABLISHED:
>>>>> @@ -742,10 +741,9 @@ void mptcp_data_ready(struct sock *sk, struct sock *ssk)
>>>>>
>>>>> 	/* Wake-up the reader only for in-sequence data */
>>>>> 	mptcp_data_lock(sk);
>>>>> -	if (move_skbs_to_msk(msk, ssk)) {
>>>>> -		set_bit(MPTCP_DATA_READY, &msk->flags);
>>>>> +	if (move_skbs_to_msk(msk, ssk))
>>>>> 		sk->sk_data_ready(sk);
>>>>> -	}
>>>>> +
>>>>> 	mptcp_data_unlock(sk);
>>>>> }
>>>>>
>>>>> @@ -847,7 +845,6 @@ static void mptcp_check_for_eof(struct mptcp_sock *msk)
>>>>> 		sk->sk_shutdown |= RCV_SHUTDOWN;
>>>>>
>>>>> 		smp_mb__before_atomic(); /* SHUTDOWN must be visible first */
>>>>> -		set_bit(MPTCP_DATA_READY, &msk->flags);
>>>>> 		sk->sk_data_ready(sk);
>>>>> 	}
>>>>>
>>>>> @@ -1759,21 +1756,6 @@ static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
>>>>> 	return copied ? : ret;
>>>>> }
>>>>>
>>>>> -static void mptcp_wait_data(struct sock *sk, long *timeo)
>>>>> -{
>>>>> -	DEFINE_WAIT_FUNC(wait, woken_wake_function);
>>>>> -	struct mptcp_sock *msk = mptcp_sk(sk);
>>>>> -
>>>>> -	add_wait_queue(sk_sleep(sk), &wait);
>>>>> -	sk_set_bit(SOCKWQ_ASYNC_WAITDATA, sk);
>>>>> -
>>>>> -	sk_wait_event(sk, timeo,
>>>>> -		      test_bit(MPTCP_DATA_READY, &msk->flags), &wait);
>>>>> -
>>>>> -	sk_clear_bit(SOCKWQ_ASYNC_WAITDATA, sk);
>>>>> -	remove_wait_queue(sk_sleep(sk), &wait);
>>>>> -}
>>>>> -
>>>>> static int __mptcp_recvmsg_mskq(struct mptcp_sock *msk,
>>>>> 				struct msghdr *msg,
>>>>> 				size_t len, int flags,
>>>>> @@ -2077,19 +2059,7 @@ static int mptcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
>>>>> 		}
>>>>>
>>>>> 		pr_debug("block timeout %ld", timeo);
>>>>> -		mptcp_wait_data(sk, &timeo);
>>>>> -	}
>>>>> -
>>>>> -	if (skb_queue_empty_lockless(&sk->sk_receive_queue) &&
>>>>> -	    skb_queue_empty(&msk->receive_queue)) {
>>>>> -		/* entire backlog drained, clear DATA_READY. */
>>>>> -		clear_bit(MPTCP_DATA_READY, &msk->flags);
>>>>> -
>>>>> -		/* .. race-breaker: ssk might have gotten new data
>>>>> -		 * after last __mptcp_move_skbs() returned false.
>>>>> -		 */
>>>>> -		if (unlikely(__mptcp_move_skbs(msk)))
>>>>> -			set_bit(MPTCP_DATA_READY, &msk->flags);
>>>>> +		sk_wait_data(sk, &timeo, NULL);
>>>>> 	}
>>>>>
>>>>> out_err:
>>>>> @@ -2098,9 +2068,9 @@ static int mptcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
>>>>> 			tcp_recv_timestamp(msg, sk, &tss);
>>>>> 	}
>>>>>
>>>>> -	pr_debug("msk=%p data_ready=%d rx queue empty=%d copied=%d",
>>>>> -		 msk, test_bit(MPTCP_DATA_READY, &msk->flags),
>>>>> -		 skb_queue_empty_lockless(&sk->sk_receive_queue), copied);
>>>>> +	pr_debug("msk=%p rx queue empty=%d:%d copied=%d",
>>>>> +		 msk, skb_queue_empty_lockless(&sk->sk_receive_queue),
>>>>> +		 skb_queue_empty(&msk->receive_queue), copied);
>>>>> 	if (!(flags & MSG_PEEK))
>>>>> 		mptcp_rcv_space_adjust(msk, copied);
>>>>>
>>>>> @@ -2368,7 +2338,6 @@ static void mptcp_check_fastclose(struct mptcp_sock *msk)
>>>>> 	inet_sk_state_store(sk, TCP_CLOSE);
>>>>> 	sk->sk_shutdown = SHUTDOWN_MASK;
>>>>> 	smp_mb__before_atomic(); /* SHUTDOWN must be visible first */
>>>>> -	set_bit(MPTCP_DATA_READY, &msk->flags);
>>>>> 	set_bit(MPTCP_WORK_CLOSE_SUBFLOW, &msk->flags);
>>>>>
>>>>> 	mptcp_close_wake_up(sk);
>>>>> @@ -3385,8 +3354,11 @@ static int mptcp_stream_accept(struct socket *sock, struct socket *newsock,
>>>>>
>>>>> static __poll_t mptcp_check_readable(struct mptcp_sock *msk)
>>>>> {
>>>>> -	return test_bit(MPTCP_DATA_READY, &msk->flags) ? EPOLLIN | EPOLLRDNORM :
>>>>> -	       0;
>>>>> +	if (skb_queue_empty_lockless(&((struct sock *)msk)->sk_receive_queue) &&
>>>>> +            skb_queue_empty_lockless(&msk->receive_queue))
>>>>
>>>> This is the tricky bit, as it has potential to be racy. I _think_ it works
>>>> out ok because skbs are moved from the msk->sk_receive_queue to
>>>> msk->receive_queue by skb_queue_splice_tail_init().
>>>>
>>>> skb_queue_splice_tail_init() will reinitialize sk_receive_queue only after
>>>> the splice is complete (although __skb_queue_head_init() unfortunately
>>>> doesn't use WRITE_ONCE()). If both sk_receive_queue and receive_queue
>>>> appear to be non-empty while a splice is in progress, that's not a problem
>>>> since the code above will bypass the second check and still correctly
>>>> return EPOLLIN | EPOLLRDNORM.
>>>>
>>>> Do you agree? Seems like this should be explained in a comment in
>>>> mptcp_check_readable() too.
>>>
>>> Yes, this part is a bit tricky. Still I think avoding MPTCP_DATA_READY
>>> usage in this case simplifies the code make the receive path more
>>> clear.
>>>
>>
>> I agree, it's clearer.
>>
>>> My understanding is that poll() is not supposed to work reliably when
>>> another process is doing a concurrent read operation on the same socket
>>> - AFAICS, it can't without locking.
>>>
>>> Only a concurrent read() can remove packets from sk_receive_queue()
>>> and/or from msk->receive_queue(). So my take is that we can just ignore
>>> such scenario.
>>>
>>
>> To clarify, my concern was not so much with concurrent reads but with the
>> splice that happens in mptcp_data_ready() when data arrives on a subflow.
>
> uhm... I don't see that splice in the receive path?!?
> skb_queue_splice_tail_init() is called only by mptcp_destroy_common()
> and __mptcp_splice_receive_queue(). The latter is invoked by only
> mptcp_recvmsg() and __mptcp_move_skbs(). Finally __mptcp_move_skbs() is
> invoked only by mptcp_recvmsg().
>
> To splice the sk_receive_queue into msk->receive_queue, we need both
> the msk socket lock and the msk data lock, so we can't do that from the
> plain receive path (data lock only acquired).
>

Yeah, you're right. Looks like I got mixed up between __mptcp_move_skbs() 
(does splice, not in rx path) and __mptcp_move_skb() (no splice, in rx 
path).

>> If the splice implementation were to clear the source queue before
>> updating the destination, or if receive_queue was checked before
>> sk_receive_queue, the lockless poll could "see" two empty queues even
>> though there was data to read.
>>
>> Both the ordering in the code above and the implentation of the
>> splice/init make sure that the only slight weirdness due to concurrency
>> would be to see two non-empty queues - which is fine for polling.
>>
>>> As for an additional comment:
>>>
>>> 	/* both queues can be empty only when a concurrent read splices
>>> 	 * sk_receive_queue into receive_queue
>>> 	 */
>>>
>>> WDYT?
>>
>> How about "Concurrent splices from sk_receive_queue into receive_queue
>> will always show at least one non-empty queue when checked in this order."
>> ?
>
> Fine by me.
>

--
Mat Martineau
Intel