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Subject[PATCH][RFC] 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>

diff --git a/fs/ecryptfs/inode.c b/fs/ecryptfs/inode.c
index 3c2298721359..e23752d9a79f 100644
--- a/fs/ecryptfs/inode.c
+++ b/fs/ecryptfs/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;

/*
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