4.9-stable review patch. If anyone has any objections, please let me know.
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From: Al Viro viro@zeniv.linux.org.uk
commit 3b821409632ab778d46e807516b457dfa72736ed upstream.
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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/dcache.c | 11 ++++++++--- 1 file changed, 8 insertions(+), 3 deletions(-)
--- a/fs/dcache.c +++ b/fs/dcache.c @@ -637,11 +637,16 @@ 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; }