linux-kernel - [PATCH 3.16 126/136] ecryptfs_lookup_interpose(): lower_dentry->d

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Message-ID: <lsq.1576543535.985489010@decadent.org.uk>
Date:   Tue, 17 Dec 2019 00:47:40 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Al Viro" <viro@...iv.linux.org.uk>
Subject: [PATCH 3.16 126/136] ecryptfs_lookup_interpose(): lower_dentry->d_inode
 is not stable

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

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

From: Al Viro <viro@...iv.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@...iv.linux.org.uk>
[bwh: Backported to 3.16:
 - Use ACCESS_ONCE() instead of READ_ONCE()
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.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;