fs: fix superblock iteration race

list_for_each_entry_safe is not suitable to protect against concurrent
modification of the list. 6754af6 introduced a race in sb walking.

list_for_each_entry can use the trick of pinning the current entry in
the list before we drop and retake the lock because it subsequently
follows cur->next. However list_for_each_entry_safe saves n=cur->next
for following before entering the loop body, so when the lock is
dropped, n may be deleted.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: John Stultz <johnstul@us.ibm.com>
Cc: Frank Mayhar <fmayhar@google.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 0 deletions

diff --git a/fs/super.c b/fs/super.c
index 5c35bc7a499e..938119ab8dcb 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -374,6 +374,8 @@ void sync_supers(void)
                         up_read(&sb->s_umount);
 
                         spin_lock(&sb_lock);
+                        /* lock was dropped, must reset next */
+                        list_safe_reset_next(sb, n, s_list);
                         __put_super(sb);
                 }
         }
@@ -405,6 +407,8 @@ void iterate_supers(void (*f)(struct super_block *, void *), void *arg)
                 up_read(&sb->s_umount);
 
                 spin_lock(&sb_lock);
+                /* lock was dropped, must reset next */
+                list_safe_reset_next(sb, n, s_list);
                 __put_super(sb);
         }
         spin_unlock(&sb_lock);
@@ -585,6 +589,8 @@ static void do_emergency_remount(struct work_struct *work)
                 }
                 up_write(&sb->s_umount);
                 spin_lock(&sb_lock);
+                /* lock was dropped, must reset next */
+                list_safe_reset_next(sb, n, s_list);
                 __put_super(sb);
         }
         spin_unlock(&sb_lock);