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

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>
[bwh: Backported to 3.2:
 - Adjust context
 - Also set the flag in __d_materialise_dentry())]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/fs/dcache.c b/fs/dcache.c
index 46265f5..01c68ae 100644 (file)
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1261,7 +1261,7 @@ 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
@@ -2011,7 +2011,6 @@ 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);
 }
@@ -2195,6 +2194,7 @@ static void __d_move(struct dentry * dentry, struct dentry * target)
 
        /* ... 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);
@@ -2311,6 +2311,7 @@ static void __d_materialise_dentry(struct dentry *dentry, struct dentry *anon)
        switch_names(dentry, anon);
        swap(dentry->d_name.hash, anon->d_name.hash);
 
+       dentry->d_flags |= DCACHE_RCUACCESS;
        dentry->d_parent = (aparent == anon) ? dentry : aparent;
        list_del(&dentry->d_child);
        if (!IS_ROOT(dentry))