[PATCH] ocfs2: remove entry once instead of null-ptr-dereference in ocfs2_xa_remove()

[Andrew Kanner <andrew.kanner@gmail.com>]

Syzkaller is able to provoke null-ptr-dereference in ocfs2_xa_remove():

[   57.319872] (a.out,1161,7):ocfs2_xa_remove:2028 ERROR: status = -12
[   57.320420] (a.out,1161,7):ocfs2_xa_cleanup_value_truncate:1999 ERROR: Partial truncate while removing xattr overlay.upper.  Leaking 1 clusters and removing the entry
[   57.321727] BUG: kernel NULL pointer dereference, address: 0000000000000004
[...]
[   57.325727] RIP: 0010:ocfs2_xa_block_wipe_namevalue+0x2a/0xc0
[...]
[   57.331328] Call Trace:
[   57.331477]  <TASK>
[...]
[   57.333511]  ? do_user_addr_fault+0x3e5/0x740
[   57.333778]  ? exc_page_fault+0x70/0x170
[   57.334016]  ? asm_exc_page_fault+0x2b/0x30
[   57.334263]  ? __pfx_ocfs2_xa_block_wipe_namevalue+0x10/0x10
[   57.334596]  ? ocfs2_xa_block_wipe_namevalue+0x2a/0xc0
[   57.334913]  ocfs2_xa_remove_entry+0x23/0xc0
[   57.335164]  ocfs2_xa_set+0x704/0xcf0
[   57.335381]  ? _raw_spin_unlock+0x1a/0x40
[   57.335620]  ? ocfs2_inode_cache_unlock+0x16/0x20
[   57.335915]  ? trace_preempt_on+0x1e/0x70
[   57.336153]  ? start_this_handle+0x16c/0x500
[   57.336410]  ? preempt_count_sub+0x50/0x80
[   57.336656]  ? _raw_read_unlock+0x20/0x40
[   57.336906]  ? start_this_handle+0x16c/0x500
[   57.337162]  ocfs2_xattr_block_set+0xa6/0x1e0
[   57.337424]  __ocfs2_xattr_set_handle+0x1fd/0x5d0
[   57.337706]  ? ocfs2_start_trans+0x13d/0x290
[   57.337971]  ocfs2_xattr_set+0xb13/0xfb0
[   57.338207]  ? dput+0x46/0x1c0
[   57.338393]  ocfs2_xattr_trusted_set+0x28/0x30
[   57.338665]  ? ocfs2_xattr_trusted_set+0x28/0x30
[   57.338948]  __vfs_removexattr+0x92/0xc0
[   57.339182]  __vfs_removexattr_locked+0xd5/0x190
[   57.339456]  ? preempt_count_sub+0x50/0x80
[   57.339705]  vfs_removexattr+0x5f/0x100
[...]

Reproducer uses faultinject facility to fail ocfs2_xa_remove() ->
ocfs2_xa_value_truncate() with -ENOMEM.

In this case the comment mentions that we can return 0 if
ocfs2_xa_cleanup_value_truncate() is going to wipe the entry
anyway. But the following 'rc' check is wrong and execution flow do
'ocfs2_xa_remove_entry(loc);' twice:
* 1st: in ocfs2_xa_cleanup_value_truncate();
* 2nd: returning back to ocfs2_xa_remove() instead of going to 'out'.

Fix this by skipping the 2nd removal of the same entry and making
syzkaller repro happy.

Cc: stable@vger.kernel.org
Fixes: 399ff3a748cf ("ocfs2: Handle errors while setting external xattr values.")
Reported-by: syzbot+386ce9e60fa1b18aac5b@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/671e13ab.050a0220.2b8c0f.01d0.GAE@google.com/T/
Tested-by: syzbot+386ce9e60fa1b18aac5b@syzkaller.appspotmail.com
Signed-off-by: Andrew Kanner <andrew.kanner@gmail.com>
---
 fs/ocfs2/xattr.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ocfs2/xattr.c b/fs/ocfs2/xattr.c
index dd0a05365e79..5bc4d660e15a 100644
--- a/fs/ocfs2/xattr.c
+++ b/fs/ocfs2/xattr.c
@@ -2036,7 +2036,7 @@ static int ocfs2_xa_remove(struct ocfs2_xa_loc *loc,
 				rc = 0;
 			ocfs2_xa_cleanup_value_truncate(loc, "removing",
 							orig_clusters);
-			if (rc)
+			if (rc == 0)
 				goto out;
 		}
 	}
-- 
2.43.5