[PATCH] ovl: Fix info leak in ovl_lookup_temp()

From: Richard Weinberger
Date: Fri Sep 16 2016 - 05:46:28 EST

The function uses the memory address of a struct dentry as unique id.
While the address-based directory entry is only visible to root
it is IMHO still worth fixing since the temporary name does not have
to be a kernel address. It can be any unique number. Replace it by an
atomic integer which is allowed to wrap around.

Signed-off-by: Richard Weinberger <richard@xxxxxx>
fs/overlayfs/dir.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
index 1560fdc..6e00a08 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -14,6 +14,7 @@
#include <linux/cred.h>
#include <linux/posix_acl.h>
#include <linux/posix_acl_xattr.h>
+#include <linux/atomic.h>
#include "overlayfs.h"

void ovl_cleanup(struct inode *wdir, struct dentry *wdentry)
@@ -37,8 +38,9 @@ struct dentry *ovl_lookup_temp(struct dentry *workdir, struct dentry *dentry)
struct dentry *temp;
char name[20];
+ static atomic_t temp_id = ATOMIC_INIT(0);

- snprintf(name, sizeof(name), "#%lx", (unsigned long) dentry);
+ snprintf(name, sizeof(name), "#%x", atomic_inc_return(&temp_id));

temp = lookup_one_len(name, workdir, strlen(name));
if (!IS_ERR(temp) && temp->d_inode) {