[PATCH 3.16 109/134] metag/uaccess: Check access_ok in strncpy_from_user

Ben Hutchings
2017-08-18T13:13:20Z

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

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

From: James Hogan <james.hogan-1AXoQHu6uovQT0dZR+AlfA@public.gmane.org>

commit 3a158a62da0673db918b53ac1440845a5b64fd90 upstream.

The metag implementation of strncpy_from_user() doesn't validate the src
pointer, which could allow reading of arbitrary kernel memory. Add a
short access_ok() check to prevent that.

Its still possible for it to read across the user/kernel boundary, but
it will invariably reach a NUL character after only 9 bytes, leaking
only a static kernel address being loaded into D0Re0 at the beginning of
__start, which is acceptable for the immediate fix.

Reported-by: Al Viro <viro-RmSDqhL/yNMiFSDQTTA3OLVCufUGDwFn@public.gmane.org>
Signed-off-by: James Hogan <james.hogan-1AXoQHu6uovQT0dZR+AlfA@public.gmane.org>
Cc: linux-metag-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
Signed-off-by: Ben Hutchings <ben-/+tVBieCtBitmTQ+vhA3Yw@public.gmane.org>
---
arch/metag/include/asm/uaccess.h | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)

--- a/arch/metag/include/asm/uaccess.h
+++ b/arch/metag/include/asm/uaccess.h
@@ -189,8 +189,13 @@ do {
extern long __must_check __strncpy_from_user(char *dst, const char __user *src,
long count);

-#define strncpy_from_user(dst, src, count) __strncpy_from_user(dst, src, count)
-
+static inline long
+strncpy_from_user(char *dst, const char __user *src, long count)
+{
+ if (!access_ok(VERIFY_READ, src, 1))
+ return -EFAULT;
+ return __strncpy_from_user(dst, src, count);
+}
/*
* Return the size of a string (including the ending 0)
*