From 762c69685ff7ad5ad7fee0656671e20a0c9c864d Mon Sep 17 00:00:00 2001

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

Date: Sun, 3 Nov 2019 13:55:43 -0500

Subject: [Backport 762c69685ff7] ecryptfs_lookup_interpose():

 lower_dentry->d_parent is not stable either

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.

Cc: stable@vger.kernel.org # since 2009 or so

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

---

 src/inode.c | 7 +++----

 1 file changed, 3 insertions(+), 4 deletions(-)

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

index 3c229872135987fb4d0a1a36a910651d5f37a08b..e23752d9a79f3d255345177db1b66950ae8e6607 100644

--- a/src/inode.c

+++ b/src/inode.c

@@ -319,9 +319,9 @@ 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 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);

@@ -330,13 +330,12 @@ static struct dentry *ecryptfs_lookup_interpose(struct dentry *dentry,

 		return ERR_PTR(-ENOMEM);

 	}

-	lower_mnt = mntget(ecryptfs_dentry_to_lower_mnt(dentry->d_parent));

 	fsstack_copy_attr_atime(d_inode(dentry->d_parent),

-				d_inode(lower_dentry->d_parent));

+				d_inode(path->dentry));

 	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;

 	/*

-- 

2.31.1