[PATCH 3.16 127/136] ecryptfs_lookup_interpose(): lower_dentry->d_parent is not stable either

[Ben Hutchings]

3.16.80-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Al Viro <viro@xxxxxxxxxxxxxxxxxx>

commit 762c69685ff7ad5ad7fee0656671e20a0c9c864d upstream.

We need to get the underlying dentry of parent; sure, absent the races
it is the parent of underlying dentry, but there's nothing to prevent
losing a timeslice to preemtion in the middle of evaluation of
lower_dentry->d_parent->d_inode, having another process move lower_dentry
around and have its (ex)parent not pinned anymore and freed on memory
pressure.  Then we regain CPU and try to fetch ->d_inode from memory
that is freed by that point.

dentry->d_parent *is* stable here - it's an argument of ->lookup() and
we are guaranteed that it won't be moved anywhere until we feed it
to d_add/d_splice_alias.  So we safely go that way to get to its
underlying dentry.

Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
[bwh: Backported to 3.16:
 - Open-code d_inode()
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 fs/ecryptfs/inode.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

--- a/fs/ecryptfs/inode.c
+++ b/fs/ecryptfs/inode.c
@@ -341,9 +341,9 @@ static int ecryptfs_lookup_interpose(str
 				     struct dentry *lower_dentry,
 				     struct inode *dir_inode)
 {
+	struct path *path = ecryptfs_dentry_to_lower_path(dentry->d_parent);
 	struct inode *inode, *lower_inode;
 	struct ecryptfs_dentry_info *dentry_info;
-	struct vfsmount *lower_mnt;
 	int rc = 0;
 
 	dentry_info = kmem_cache_alloc(ecryptfs_dentry_info_cache, GFP_KERNEL);
@@ -355,12 +355,11 @@ static int ecryptfs_lookup_interpose(str
 		return -ENOMEM;
 	}
 
-	lower_mnt = mntget(ecryptfs_dentry_to_lower_mnt(dentry->d_parent));
-	fsstack_copy_attr_atime(dir_inode, lower_dentry->d_parent->d_inode);
+	fsstack_copy_attr_atime(dir_inode, path->dentry->d_inode);
 	BUG_ON(!d_count(lower_dentry));
 
 	ecryptfs_set_dentry_private(dentry, dentry_info);
-	dentry_info->lower_path.mnt = lower_mnt;
+	dentry_info->lower_path.mnt = mntget(path->mnt);
 	dentry_info->lower_path.dentry = lower_dentry;
 
 	/*