Re: [PATCH] rxrpc: fix bad unlock balance in rxrpc_do_sendmsg
From: Hawkins Jiawei
Date: Mon Aug 22 2022 - 07:29:43 EST
On Mon, 22 Aug 2022 at 16:48, David Howells <dhowells@xxxxxxxxxx> wrote:
>
> Hawkins Jiawei <yin31149@xxxxxxxxx> 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@xxxxxxxxxx> 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.