[PATCH 6.18.y 3/3] fuse: fix missing barrier when checking io-uring readiness

[Sasha Levin <sashal@kernel.org>]

From: Joanne Koong <joannelkoong@gmail.com>

[ Upstream commit edb310bc27f0ad83e7fd558a3caf1a94ca511654 ]

fuse_block_alloc() reads fch->initialized and then fch->io_uring.
fch->io_uring is set before fch->initialized, ordered by the smp_wmb()
in fuse_chan_set_intialized(), but fuse_block_alloc() has no matching
read barrier between the two loads.

This may lead a CPU to observe fch->initialized=1 but fch->io_uring=0,
and skip the check that blocks request allocation until the io-uring
queues are ready. This can reintroduce the lock-order inversion deadlock
that commit 3393ff964e0f prevents.

Add an smp_rmb() barrier to pair with the smp_wmb() in
fuse_chan_set_initialized() to prevent this.

Fixes: 3393ff964e0f ("fuse: block request allocation until io-uring init is complete")
Cc: stable@vger.kernel.org
Reviewed-by: Bernd Schubert <bernd@bsbernd.com>
Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
[ renamed `fch`/`struct fuse_chan` to `fc`/`struct fuse_conn` and `fuse_chan_set_initialized()` to `fuse_set_initialized()` in the comment ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/fuse/dev.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 8a38a263295fe..33e01d67e5151 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -172,7 +172,13 @@ void fuse_set_initialized(struct fuse_conn *fc)
 
 static bool fuse_block_alloc(struct fuse_conn *fc, bool for_background)
 {
-	return !fc->initialized || (for_background && fc->blocked) ||
+	if (!fc->initialized)
+		return true;
+
+	/* Pairs with smp_wmb() in fuse_set_initialized() */
+	smp_rmb();
+
+	return (for_background && fc->blocked) ||
 	       (fc->io_uring && fc->connected && !fuse_uring_ready(fc));
 }
 
@@ -212,8 +218,6 @@ static struct fuse_req *fuse_get_req(struct mnt_idmap *idmap,
 				(TASK_KILLABLE | TASK_FREEZABLE)))
 			goto out;
 	}
-	/* Matches smp_wmb() in fuse_set_initialized() */
-	smp_rmb();
 
 	err = -ENOTCONN;
 	if (!fc->connected)
-- 
2.53.0