linux-kernel - [PATCH 4.4 045/112] ovl: Fix info leak in ovl_lookup

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Date:   Wed, 26 Oct 2016 14:22:28 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Richard Weinberger <richard@....at>,
        Kees Cook <keescook@...omium.org>,
        Miklos Szeredi <mszeredi@...hat.com>
Subject: [PATCH 4.4 045/112] ovl: Fix info leak in ovl_lookup_temp()

4.4-stable review patch.  If anyone has any objections, please let me know.

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

From: Richard Weinberger <richard@....at>

commit 6a45b3628ce4dcf7498b39c87d475bab6e2a9b24 upstream.

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@....at>
Reviewed-by: Kees Cook <keescook@...omium.org>
Signed-off-by: Miklos Szeredi <mszeredi@...hat.com>
Fixes: e9be9d5e76e3 ("overlay filesystem")
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 fs/overlayfs/dir.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -12,6 +12,7 @@
 #include <linux/xattr.h>
 #include <linux/security.h>
 #include <linux/cred.h>
+#include <linux/atomic.h>
 #include "overlayfs.h"
 
 void ovl_cleanup(struct inode *wdir, struct dentry *wdentry)
@@ -35,8 +36,10 @@ struct dentry *ovl_lookup_temp(struct de
 {
 	struct dentry *temp;
 	char name[20];
+	static atomic_t temp_id = ATOMIC_INIT(0);
 
-	snprintf(name, sizeof(name), "#%lx", (unsigned long) dentry);
+	/* counter is allowed to wrap, since temp dentries are ephemeral */
+	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) {