[PATCH v1] ovl: Fix leaked dentry

[Mickaël Salaün]

From: Mickaël Salaün <m...@linux.microsoft.com>

Since commit 6815f479ca90 ("ovl: use only uppermetacopy state in
ovl_lookup()"), overlayfs doesn't put temporary dentry when there is a
metacopy error, which leads to dentry leaks when shutting down the
related superblock:

overlayfs: refusing to follow metacopy origin for (/file0)
...
BUG: Dentry (____ptrval____){i=3f33,n=file3} still in use (1) [unmount of overlay overlay]
...
WARNING: CPU: 1 PID: 432 at umount_check.cold+0x107/0x14d
CPU: 1 PID: 432 Comm: unmount-overlay Not tainted 5.12.0-rc5 #1
...
RIP: 0010:umount_check.cold+0x107/0x14d
...
Call Trace:
d_walk+0x28c/0x950
? dentry_lru_isolate+0x2b0/0x2b0
? __kasan_slab_free+0x12/0x20
do_one_tree+0x33/0x60
shrink_dcache_for_umount+0x78/0x1d0
generic_shutdown_super+0x70/0x440
kill_anon_super+0x3e/0x70
deactivate_locked_super+0xc4/0x160
deactivate_super+0xfa/0x140
cleanup_mnt+0x22e/0x370
__cleanup_mnt+0x1a/0x30
task_work_run+0x139/0x210
do_exit+0xb0c/0x2820
? __kasan_check_read+0x1d/0x30
? find_held_lock+0x35/0x160
? lock_release+0x1b6/0x660
? mm_update_next_owner+0xa20/0xa20
? reacquire_held_locks+0x3f0/0x3f0
? __sanitizer_cov_trace_const_cmp4+0x22/0x30
do_group_exit+0x135/0x380
__do_sys_exit_group.isra.0+0x20/0x20
__x64_sys_exit_group+0x3c/0x50
do_syscall_64+0x45/0x70
entry_SYSCALL_64_after_hwframe+0x44/0xae
...
VFS: Busy inodes after unmount of overlay. Self-destruct in 5 seconds. Have a nice day...

This fix has been tested with a syzkaller reproducer.

Cc: Amir Goldstein <amir...@gmail.com>
Cc: Miklos Szeredi <mik...@szeredi.hu>
Cc: Vivek Goyal <vgo...@redhat.com>
Cc: <sta...@vger.kernel.org> # v5.7+
Reported-by: syzbot <syzk...@googlegroups.com>
Fixes: 6815f479ca90 ("ovl: use only uppermetacopy state in ovl_lookup()")
Signed-off-by: Mickaël Salaün <m...@linux.microsoft.com>
Link: https://lore.kernel.org/r/20210329164907...@digikod.net
---
fs/overlayfs/namei.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
index 3fe05fb5d145..424c594afd79 100644
--- a/fs/overlayfs/namei.c
+++ b/fs/overlayfs/namei.c
@@ -921,6 +921,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
if ((uppermetacopy || d.metacopy) && !ofs->config.metacopy) {
err = -EPERM;
pr_warn_ratelimited("refusing to follow metacopy origin for (%pd2)\n", dentry);
+ dput(this);
goto out_put;
}


base-commit: a5e13c6df0e41702d2b2c77c8ad41677ebb065b3
--
2.30.2

[Mickaël Salaün]

The bogus commit 6815f479ca90 was applied on v5.7-rc2 but it is only
available since v5.8+ .

[Vivek Goyal]

On Mon, Mar 29, 2021 at 06:49:07PM +0200, Mickaël Salaün wrote:
> From: Mickaël Salaün <m...@linux.microsoft.com>
>
> Since commit 6815f479ca90 ("ovl: use only uppermetacopy state in
> ovl_lookup()"), overlayfs doesn't put temporary dentry when there is a
> metacopy error, which leads to dentry leaks when shutting down the
> related superblock:
>

Hi,

Thanks for finding and fixing this bug. Patch looks correct to me. We
need to drop that reference to this.

I am not able to trigger this warning on umount of overlayfs. I copied
up a file with metacopy enabled and then remounted overlay again with
metacopy disabled. That does hit this code and I see the warning.

refusing to follow metacopy origin for (/foo.txt)

This should have lead to leak of dentry pointed by "this".

But after that I unmounted, overlay and that succeeds. Is there any
additional step to be done to trigger this VFS warning.

Vivek

[Vivek Goyal]

On Mon, Mar 29, 2021 at 06:49:07PM +0200, Mickaël Salaün wrote:

Looks good to me. I realized that dentry leak will happen on underlying
filesystem so unmount of underlying filesystem will give this warning. I
created nested overlayfs configuration and could reproduce this error
and tested that this patch fixes it.

Reviewed-by: Vivek Goyal <vgo...@redhat.com>

Vivek

[Miklos Szeredi]

Thanks, applied.

Miklos