lock_parent() needs to recheck if dentry got __dentry_kill'ed under it

In case when dentry passed to lock_parent() is protected from freeing only
by the fact that it's on a shrink list and trylock of parent fails, we
could get hit by __dentry_kill() (and subsequent dentry_kill(parent))
between unlocking dentry and locking presumed parent.  We need to recheck
that dentry is alive once we lock both it and parent *and* postpone
rcu_read_unlock() until after that point.  Otherwise we could return
a pointer to struct dentry that already is rcu-scheduled for freeing, with
->d_lock held on it; caller's subsequent attempt to unlock it can end
up with memory corruption.

Cc: stable@vger.kernel.org # 3.12+, counting backports
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 8 insertions, 3 deletions

diff --git a/fs/dcache.c b/fs/dcache.c
index 7c38f39958bc..32aaab21e648 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -647,11 +647,16 @@ again:
                 spin_unlock(&parent->d_lock);
                 goto again;
         }
-        rcu_read_unlock();
-        if (parent != dentry)
+        if (parent != dentry) {
                 spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
-        else
+                if (unlikely(dentry->d_lockref.count < 0)) {
+                        spin_unlock(&parent->d_lock);
+                        parent = NULL;
+                }
+        } else {
                 parent = NULL;
+        }
+        rcu_read_unlock();
         return parent;
 }