[PATCH AUTOSEL 5.15 09/31] ext4: don't set SB_RDONLY after filesystem errors

[Sasha Levin]

From: Jan Kara <jack@xxxxxxx>

[ Upstream commit d3476f3dad4ad68ae5f6b008ea6591d1520da5d8 ]

When the filesystem is mounted with errors=remount-ro, we were setting
SB_RDONLY flag to stop all filesystem modifications. We knew this misses
proper locking (sb->s_umount) and does not go through proper filesystem
remount procedure but it has been the way this worked since early ext2
days and it was good enough for catastrophic situation damage
mitigation. Recently, syzbot has found a way (see link) to trigger
warnings in filesystem freezing because the code got confused by
SB_RDONLY changing under its hands. Since these days we set
EXT4_FLAGS_SHUTDOWN on the superblock which is enough to stop all
filesystem modifications, modifying SB_RDONLY shouldn't be needed. So
stop doing that.

Link: https://lore.kernel.org/all/000000000000b90a8e061e21d12f@xxxxxxxxxx
Reported-by: Christian Brauner <brauner@xxxxxxxxxx>
Signed-off-by: Jan Kara <jack@xxxxxxx>
Reviewed-by: Christian Brauner <brauner@xxxxxxxxxx>
Link: https://patch.msgid.link/20240805201241.27286-1-jack@xxxxxxx
Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 fs/ext4/super.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index b09b7a6b7a154..93eb26c162422 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -674,11 +674,12 @@ static void ext4_handle_error(struct super_block *sb, bool force_ro, int error,
 
 	ext4_msg(sb, KERN_CRIT, "Remounting filesystem read-only");
 	/*
-	 * Make sure updated value of ->s_mount_flags will be visible before
-	 * ->s_flags update
+	 * EXT4_FLAGS_SHUTDOWN was set which stops all filesystem
+	 * modifications. We don't set SB_RDONLY because that requires
+	 * sb->s_umount semaphore and setting it without proper remount
+	 * procedure is confusing code such as freeze_super() leading to
+	 * deadlocks and other problems.
 	 */
-	smp_wmb();
-	sb->s_flags |= SB_RDONLY;
 }
 
 static void flush_stashed_error_work(struct work_struct *work)
-- 
2.43.0