[PATCH 3.14 12/29] fix d_walk()/non-delayed __d_free() race

Greg Kroah-Hartman
2016-06-22T23:28:47Z

From: Al Viro <viro@zeniv.linux.org.uk>

commit 3d56c25e3bb0726a5c5e16fc2d9e38f8ed763085 upstream.

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.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/dcache.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1500,7 +1500,7 @@ struct dentry *d_alloc(struct dentry * p
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
@@ -2352,7 +2352,6 @@ static void __d_rehash(struct dentry * e
{
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);
}
@@ -2536,6 +2535,7 @@ static void __d_move(struct dentry * den

/* ... and switch the parents */
if (IS_ROOT(dentry)) {
+ dentry->d_flags |= DCACHE_RCUACCESS;
dentry->d_parent = target->d_parent;
target->d_parent = target;
INIT_LIST_HEAD(&target->d_child);