[PATCH v3] ext4: fix i_data_sem unlock order in ext4_ind_migrate()

[PATCH v3] ext4: fix i_data_sem unlock order in ext4_ind_migrate()

[Artem Sadovnikov]

Fuzzing reports a possible deadlock in jbd2_log_wait_commit.

This issue is triggered when an EXT4_IOC_MIGRATE ioctl is set to require
synchronous updates because the file descriptor is opened with O_SYNC.
This can lead to the jbd2_journal_stop() function calling
jbd2_might_wait_for_commit(), potentially causing a deadlock if the
EXT4_IOC_MIGRATE call races with a write(2) system call.

This problem only arises when CONFIG_PROVE_LOCKING is enabled. In this
case, the jbd2_might_wait_for_commit macro locks jbd2_handle in the
jbd2_journal_stop function while i_data_sem is locked. This triggers
lockdep because the jbd2_journal_start function might also lock the same
jbd2_handle simultaneously.

Found by Linux Verification Center (linuxtesting.org) with syzkaller.

Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Co-developed-by: Mikhail Ukhin <mish.uxin2012@yandex.ru>
Signed-off-by: Mikhail Ukhin <mish.uxin2012@yandex.ru>
Signed-off-by: Artem Sadovnikov <ancowi69@gmail.com>
---
 v2: New addresses have been added and Ritesh Harjani has been noted as a
 reviewer.
 v3: Description updated.
 fs/ext4/migrate.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ext4/migrate.c b/fs/ext4/migrate.c
index d98ac2af8199..a5e1492bbaaa 100644
--- a/fs/ext4/migrate.c
+++ b/fs/ext4/migrate.c
@@ -663,8 +663,8 @@ int ext4_ind_migrate(struct inode *inode)
 	if (unlikely(ret2 && !ret))
 		ret = ret2;
 errout:
-	ext4_journal_stop(handle);
 	up_write(&EXT4_I(inode)->i_data_sem);
+	ext4_journal_stop(handle);
 out_unlock:
 	ext4_writepages_up_write(inode->i_sb, alloc_ctx);
 	return ret;
-- 
2.43.0

Re: [PATCH v3] ext4: fix i_data_sem unlock order in ext4_ind_migrate()

[Theodore Ts'o]

On Thu, 29 Aug 2024 15:22:09 +0000, Artem Sadovnikov wrote:
> Fuzzing reports a possible deadlock in jbd2_log_wait_commit.
> 
> This issue is triggered when an EXT4_IOC_MIGRATE ioctl is set to require
> synchronous updates because the file descriptor is opened with O_SYNC.
> This can lead to the jbd2_journal_stop() function calling
> jbd2_might_wait_for_commit(), potentially causing a deadlock if the
> EXT4_IOC_MIGRATE call races with a write(2) system call.
> 
> [...]

Applied, thanks!

[1/1] ext4: fix i_data_sem unlock order in ext4_ind_migrate()
      commit: cc749e61c011c255d81b192a822db650c68b313f

Best regards,
-- 
Theodore Ts'o <tytso@mit.edu>