From e72b9dd6a5f17d0fb51f16f8685f3004361e83d0 Mon Sep 17 00:00:00 2001

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

Date: Sun, 3 Nov 2019 13:45:04 -0500

Subject: [Backport e72b9dd6a5f1] ecryptfs_lookup_interpose():

 lower_dentry->d_inode is not stable

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.

Cc: stable@vger.kernel.org

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

---

 src/inode.c | 12 ++++++++++--

 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/src/inode.c b/src/inode.c

index a905d5f4f3b0726303595e39ba40309d68baa422..3c229872135987fb4d0a1a36a910651d5f37a08b 100644

--- a/src/inode.c

+++ b/src/inode.c

@@ -319,7 +319,7 @@ static int ecryptfs_i_size_read(struct dentry *dentry, struct inode *inode)

 static struct dentry *ecryptfs_lookup_interpose(struct dentry *dentry,

 				     struct dentry *lower_dentry)

 {

-	struct inode *inode, *lower_inode = d_inode(lower_dentry);

+	struct inode *inode, *lower_inode;

 	struct ecryptfs_dentry_info *dentry_info;

 	struct vfsmount *lower_mnt;

 	int rc = 0;

@@ -339,7 +339,15 @@ static struct dentry *ecryptfs_lookup_interpose(struct dentry *dentry,

 	dentry_info->lower_path.mnt = lower_mnt;

 	dentry_info->lower_path.dentry = lower_dentry;

-	if (d_really_is_negative(lower_dentry)) {

+	/*

+	 * 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 = READ_ONCE(lower_dentry->d_inode);

+

+	if (!lower_inode) {

 		/* We want to add because we couldn't find in lower */

 		d_add(dentry, NULL);

 		return NULL;

-- 

2.31.1