Patch "lock_parent() needs to recheck if dentry got __dentry_kill'ed under it" has been added to the 4.9-stable tree

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

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

to the 4.9-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     lock_parent-needs-to-recheck-if-dentry-got-__dentry_kill-ed-under-it.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


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

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;
 }
 


Patches currently in stable-queue which might be from viro@zeniv.linux.org.uk are

queue-4.9/fs-teach-path_connected-to-handle-nfs-filesystems-with-multiple-roots.patch
queue-4.9/lock_parent-needs-to-recheck-if-dentry-got-__dentry_kill-ed-under-it.patch