On Tue, Jan 5, 2021 at 8:47 AM Amir Goldstein [off-list ref] wrote:

On Tue, Jan 5, 2021 at 2:36 AM Icenowy Zheng [off-list ref] wrote:

quoted

The function ovl_dir_real_file() currently uses the semaphore of the
inode to synchronize write to the upperfile cache field.

Although the inode lock is a rw_sem it is referred to as the "inode lock"
and you also left semaphore in the commit subject.
No need to re-post. This can be fixed on commit.

quoted

However, this function will get called by ovl_ioctl_set_flags(), which
utilizes the inode semaphore too. In this case ovl_dir_real_file() will
try to claim a lock that is owned by a function in its call stack, which
won't get released before ovl_dir_real_file() returns.

Define a dedicated semaphore for the upperfile cache, so that the
deadlock won't happen.

Fixes: 61536bed2149 ("ovl: support [S|G]ETFLAGS and FS[S|G]ETXATTR ioctls for directories")
Cc: stable@vger.kernel.org # v5.10
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
---
Changes in v2:
- Fixed missing replacement in error handling path.
Changes in v3:
- Use mutex instead of semaphore.

 fs/overlayfs/readdir.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
index 01620ebae1bd..3980f9982f34 100644
--- a/fs/overlayfs/readdir.c
+++ b/fs/overlayfs/readdir.c

@@ -56,6 +56,7 @@ struct ovl_dir_file {
        struct list_head *cursor;
        struct file *realfile;
        struct file *upperfile;
+       struct mutex upperfile_mutex;

That's a very specific name.
This mutex protects members of struct ovl_dir_file, which could evolve
into struct ovl_file one day (because no reason to cache only dir upper file),
so I would go with a more generic name, but let's leave it to Miklos to decide.

He could have a different idea altogether for fixing this bug.

Miklos,

Please fast track this or an alternative fix.
It fixes an easy to reproduce deadlock introduced in 5.10.
Icenowy Zheng has written a simple xfstest reproducer, but it wasn't
posted - best to avoid hanging tester's machines until a fix is merged...

Thanks,
Amir.