From 3b821409632ab778d46e807516b457dfa72736ed Mon Sep 17 00:00:00 2001
From: Al Viro <viro@zeniv.linux.org.uk>
Date: Fri, 23 Feb 2018 20:47:17 -0500
Subject: [PATCH] lock_parent() needs to recheck if dentry got __dentry_kill'ed
under it
Git-commit: 3b821409632ab778d46e807516b457dfa72736ed
Patch-mainline: v4.16-rc6
References: bsc#1052766
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>
Acked-by: Jan Kara <jack@suse.cz>
---
fs/dcache.c | 11 ++++++++---
1 file 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 @@ static inline struct dentry *lock_parent(struct dentry *dentry)
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;
}
--
2.13.6