Commit 34558d3f authored Jun 07, 2016 by Al Viro Committed by Sasha Levin Jun 19, 2016

fix d_walk()/non-delayed __d_free() race



[ Upstream commit 3d56c25e3bb0726a5c5e16fc2d9e38f8ed763085 ]

Ascend-to-parent logics in d_walk() depends on all encountered child
dentries not getting freed without an RCU delay.  Unfortunately, in
quite a few cases it is not true, with hard-to-hit oopsable race as
the result.

Fortunately, the fix is simiple; right now the rule is "if it ever
been hashed, freeing must be delayed" and changing it to "if it
ever had a parent, freeing must be delayed" closes that hole and
covers all cases the old rule used to cover.  Moreover, pipes and
sockets remain _not_ covered, so we do not introduce RCU delay in
the cases which are the reason for having that delay conditional
in the first place.

Cc: stable@vger.kernel.org # v3.2+ (and watch out for __d_materialise_dentry())
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>

parent d05438b3

fs/dcache.c

+2 −2

Original line number	Diff line number	Diff line
		@@ -1481,7 +1481,7 @@ struct dentry d_alloc(struct dentry parent, const struct qstr *name)
		struct dentry *dentry = __d_alloc(parent->d_sb, name);
		if (!dentry)
		return NULL;

		dentry->d_flags \|= DCACHE_RCUACCESS;
		spin_lock(&parent->d_lock);
		/*
		* don't need child lock because it is not subject
		@@ -2313,7 +2313,6 @@ static void __d_rehash(struct dentry * entry, struct hlist_bl_head *b)
		{
		BUG_ON(!d_unhashed(entry));
		hlist_bl_lock(b);
		entry->d_flags \|= DCACHE_RCUACCESS;
		hlist_bl_add_head_rcu(&entry->d_hash, b);
		hlist_bl_unlock(b);
		}
		@@ -2532,6 +2531,7 @@ static void __d_move(struct dentry dentry, struct dentry target,
		/* ... and switch them in the tree */
		if (IS_ROOT(dentry)) {
		/* splicing a tree */
		dentry->d_flags \|= DCACHE_RCUACCESS;
		dentry->d_parent = target->d_parent;
		target->d_parent = target;
		list_del_init(&target->d_child);