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

Ben Hutchings
2019-12-17T00:52:07Z

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

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@zeniv.linux.org.uk>
[bwh: Backported to 3.16:
- Open-code d_inode()
- Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
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;

/*