fs: fix dropping of rcu-walk from force_reval_path

As J. R. Okajima noted, force_reval_path passes in the same dentry to
d_revalidate as the one in the nameidata structure (other callers pass in a
child), so the locking breaks. This can oops with a chrooted nfs mount, for
example. Similarly there can be other problems with revalidating a dentry
which is already in nameidata of the path walk.

Signed-off-by: Nick Piggin <npiggin@kernel.dk>

1 files changed, 8 insertions, 0 deletions

diff --git a/fs/namei.c b/fs/namei.c
index 0f02359ce685..14c73edca9ce 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -479,6 +479,14 @@ static int nameidata_dentry_drop_rcu(struct nameidata *nd, struct dentry *dentry
         struct fs_struct *fs = current->fs;
         struct dentry *parent = nd->path.dentry;
 
+        /*
+         * It can be possible to revalidate the dentry that we started
+         * the path walk with. force_reval_path may also revalidate the
+         * dentry already committed to the nameidata.
+         */
+        if (unlikely(parent == dentry))
+                return nameidata_drop_rcu(nd);
+
         BUG_ON(!(nd->flags & LOOKUP_RCU));
         if (nd->root.mnt) {
                 spin_lock(&fs->lock);