[PATCH 3.16 126/136] ecryptfs_lookup_interpose(): lower_dentry->d_inode is not stable

Ben Hutchings
2019-12-17T00:54:42Z

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

commit e72b9dd6a5f17d0fb51f16f8685f3004361e83d0 upstream.

lower_dentry can't go from positive to negative (we have it pinned),
but it *can* go from negative to positive. So fetching ->d_inode
into a local variable, doing a blocking allocation, checking that
now ->d_inode is non-NULL and feeding the value we'd fetched
earlier to a function that won't accept NULL is not a good idea.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
[bwh: Backported to 3.16:
- Use ACCESS_ONCE() instead of READ_ONCE()
- Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/ecryptfs/inode.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)

--- a/fs/ecryptfs/inode.c
+++ b/fs/ecryptfs/inode.c
@@ -341,7 +341,7 @@ static int ecryptfs_lookup_interpose(str
struct dentry *lower_dentry,
struct inode *dir_inode)
{
- struct inode *inode, *lower_inode = lower_dentry->d_inode;
+ struct inode *inode, *lower_inode;
struct ecryptfs_dentry_info *dentry_info;
struct vfsmount *lower_mnt;
int rc = 0;
@@ -363,7 +363,15 @@ static int ecryptfs_lookup_interpose(str
dentry_info->lower_path.mnt = lower_mnt;
dentry_info->lower_path.dentry = lower_dentry;

- if (!lower_dentry->d_inode) {
+ /*
+ * negative dentry can go positive under us here - its parent is not
+ * locked. That's OK and that could happen just as we return from
+ * ecryptfs_lookup() anyway. Just need to be careful and fetch
+ * ->d_inode only once - it's not stable here.
+ */
+ lower_inode = ACCESS_ONCE(lower_dentry->d_inode);
+
+ if (!lower_inode) {
/* We want to add because we couldn't find in lower */
d_add(dentry, NULL);
return 0;