Re: [PATCH] rxrpc: fix bad unlock balance in rxrpc_do_sendmsg

[Hawkins Jiawei <yin31149@gmail.com>]

On Mon, 22 Aug 2022 at 16:48, David Howells <dhowells@redhat.com> wrote:
>
> Hawkins Jiawei <yin31149@gmail.com> wrote:
>
> > -             if (mutex_lock_interruptible(&call->user_mutex) < 0)
> > +             if (mutex_lock_interruptible(&call->user_mutex) < 0) {
> > +                     mutex_lock(&call->user_mutex);
>
> Yeah, as Khalid points out that kind of makes the interruptible lock
> pointless.  Either rxrpc_send_data() needs to return a separate indication
> that we returned without the lock held or it needs to always drop the lock on
> error (at least for ERESTARTSYS/EINTR) which can be checked for higher up.
Hi David,

For second option, I think it may meet some difficulty, because we
cannot figure out whether rxrpc_send_data() meets lock error.
To be more specific, rxrpc_send_data() may still returns the number
it has copied even rxrpc_send_data() meets lock error, if
rxrpc_send_data() has successfully dealed some data.(Please correct me
if I am wrong)

So I think the first option seems better. I wonder if we can add an
argument in rxrpc_send_data() as an indication you pointed out? Maybe:

diff --git a/net/rxrpc/sendmsg.c b/net/rxrpc/sendmsg.c
index 1d38e279e2ef..0801325a7c7f 100644
--- a/net/rxrpc/sendmsg.c
+++ b/net/rxrpc/sendmsg.c
@@ -284,13 +284,18 @@ static int rxrpc_queue_packet(struct rxrpc_sock *rx, struct rxrpc_call *call,
 
 /*
  * send data through a socket
+ * @holding_mutex: rxrpc_send_data() will assign this pointer with True
+ * if functions still holds the call user access mutex when returned to caller.
+ * This argument can be NULL, which will effect nothing.
+ *
  * - must be called in process context
  * - The caller holds the call user access mutex, but not the socket lock.
  */
 static int rxrpc_send_data(struct rxrpc_sock *rx,
 			   struct rxrpc_call *call,
 			   struct msghdr *msg, size_t len,
-			   rxrpc_notify_end_tx_t notify_end_tx)
+			   rxrpc_notify_end_tx_t notify_end_tx,
+			   bool *holding_mutex)
 {
 	struct rxrpc_skb_priv *sp;
 	struct sk_buff *skb;
@@ -299,6 +304,9 @@ static int rxrpc_send_data(struct rxrpc_sock *rx,
 	bool more;
 	int ret, copied;
 
+	if (holding_mutex)
+		*holding_mutex = true;
+
 	timeo = sock_sndtimeo(sk, msg->msg_flags & MSG_DONTWAIT);
 
 	/* this should be in poll */
@@ -338,8 +346,11 @@ static int rxrpc_send_data(struct rxrpc_sock *rx,
 				ret = rxrpc_wait_for_tx_window(rx, call,
 							       &timeo,
 							       msg->msg_flags & MSG_WAITALL);
-				if (ret < 0)
+				if (ret < 0) {
+					if (holding_mutex)
+						*holding_mutex = false;
 					goto maybe_error;
+				}
 			}
 
 			/* Work out the maximum size of a packet.  Assume that
@@ -630,6 +641,7 @@ int rxrpc_do_sendmsg(struct rxrpc_sock *rx, struct msghdr *msg, size_t len)
 	struct rxrpc_call *call;
 	unsigned long now, j;
 	int ret;
+	bool holding_user_mutex;
 
 	struct rxrpc_send_params p = {
 		.call.tx_total_len	= -1,
@@ -747,7 +759,9 @@ int rxrpc_do_sendmsg(struct rxrpc_sock *rx, struct msghdr *msg, size_t len)
 		/* Reply phase not begun or not complete for service call. */
 		ret = -EPROTO;
 	} else {
-		ret = rxrpc_send_data(rx, call, msg, len, NULL);
+		ret = rxrpc_send_data(rx, call, msg, len, NULL, &holding_user_mutex);
+		if (!holding_user_mutex)
+			goto error_put;
 	}
 
 out_put_unlock:
@@ -796,7 +810,7 @@ int rxrpc_kernel_send_data(struct socket *sock, struct rxrpc_call *call,
 	case RXRPC_CALL_SERVER_ACK_REQUEST:
 	case RXRPC_CALL_SERVER_SEND_REPLY:
 		ret = rxrpc_send_data(rxrpc_sk(sock->sk), call, msg, len,
-				      notify_end_tx);
+				      notify_end_tx, NULL);
 		break;
 	case RXRPC_CALL_COMPLETE:
 		read_lock_bh(&call->state_lock);


On Mon, 22 Aug 2022 at 17:21, David Howells <dhowells@redhat.com> wrote:
>
> Actually, there's another bug here too: if rxrpc_wait_for_tx_window() drops
> the call mutex then it needs to reload the pending packet state in
> rxrpc_send_data() as it may have raced with another sendmsg().
>
> David
>
After applying the above patch, kernel still panic, but seems not the
bad unlock balance bug before. yet I am not sure if this is the same bug you
mentioned. Kernel output as below:
[   39.115966][ T6508] ====================================
[   39.116940][ T6508] WARNING: syz/6508 still has locks held!
[   39.117978][ T6508] 6.0.0-rc1-00066-g3b06a2755758-dirty #186 Not tainted
[   39.119353][ T6508] ------------------------------------
[   39.120321][ T6508] 1 lock held by syz/6508:
[   39.121122][ T6508]  #0: ffff88801f472b20 (&call->user_mutex){....}-{3:3}0
[   39.123014][ T6508] 
[   39.123014][ T6508] stack backtrace:
[   39.123925][ T6508] CPU: 0 PID: 6508 Comm: syz Not tainted 6.0.0-rc1-00066
[   39.125304][ T6508] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996)4
[   39.125304][ T6508] Call Trace:
[   39.125304][ T6508]  <TASK>
[   39.125304][ T6508]  dump_stack_lvl+0x8e/0xdd
[   39.125304][ T6508]  get_signal+0x1866/0x24d0
[   39.125304][ T6508]  ? lock_acquire+0x172/0x310
[   39.125304][ T6508]  ? exit_signals+0x7b0/0x7b0
[   39.125304][ T6508]  arch_do_signal_or_restart+0x82/0x23f0
[   39.125304][ T6508]  ? __sanitizer_cov_trace_pc+0x1a/0x40
[   39.125304][ T6508]  ? __fget_light+0x20d/0x270
[   39.125304][ T6508]  ? get_sigframe_size+0x10/0x10
[   39.125304][ T6508]  ? __sanitizer_cov_trace_pc+0x1a/0x40
[   39.125304][ T6508]  ? __sys_sendmsg+0x11a/0x1c0
[   39.125304][ T6508]  ? __sys_sendmsg_sock+0x30/0x30
[   39.125304][ T6508]  exit_to_user_mode_prepare+0x146/0x1b0
[   39.125304][ T6508]  syscall_exit_to_user_mode+0x12/0x30
[   39.125304][ T6508]  do_syscall_64+0x42/0xb0
[   39.125304][ T6508]  entry_SYSCALL_64_after_hwframe+0x63/0xcd
[   39.125304][ T6508] RIP: 0033:0x44fbad
[   39.125304][ T6508] Code: c3 e8 97 29 00 00 0f 1f 80 00 00 00 00 f3 0f 1e8
[   39.125304][ T6508] RSP: 002b:00007f4b8ae22d48 EFLAGS: 00000246 ORIG_RAX:e
[   39.125304][ T6508] RAX: fffffffffffffffc RBX: 0000000000000000 RCX: 0000d
[   39.125304][ T6508] RDX: 0000000000000186 RSI: 0000000020000000 RDI: 00003
[   39.125304][ T6508] RBP: 00007f4b8ae22d80 R08: 00007f4b8ae23700 R09: 00000
[   39.125304][ T6508] R10: 00007f4b8ae23700 R11: 0000000000000246 R12: 0000e
[   39.125304][ T6508] R13: 00007ffe483304af R14: 00007ffe48330550 R15: 00000
[   39.125304][ T6508]  </TASK>
====================================

I will make a deeper look and try to patch it.