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Date: Fri, 26 Aug 2016 15:31:43 +0100
From: Mark Rutland <mark.rutland@....com>
To: linux-kernel@...r.kernel.org,
kernel-hardening@...ts.openwall.com
Cc: Mark Rutland <mark.rutland@....com>,
Andrew Morton <akpm@...ux-foundation.org>,
Kees Cook <keescook@...omium.org>
Subject: [PATCH] lib: harden strncpy_from_user
The strncpy_from_user() accessor is effectively a copy_from_user()
specialised to copy strings, terminating early at a NUL byte if
possible. In other respects it is identical, and can be used to copy an
arbitrarily large buffer from userspace into the kernel. Conceptually,
it exposes a similar attack surface.
As with copy_from_user(), we check the destination range when the kernel
is built with KASAN, but unlike copy_from_user() we do not check the
destination buffer when using HARDENED_USERCOPY. As strncpy_from_user()
calls get_user() in a loop, we must call check_object_size() explicitly.
This patch adds this instrumentation to strncpy_from_user(), per the
same rationale as with the regular copy_from_user(). In the absence of
hardened usercopy this will have no impact as the instrumentation
expands to an empty static inline function.
Signed-off-by: Mark Rutland <mark.rutland@....com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Kees Cook <keescook@...omium.org>
---
lib/strncpy_from_user.c | 2 ++
1 file changed, 2 insertions(+)
diff --git a/lib/strncpy_from_user.c b/lib/strncpy_from_user.c
index 9c5fe81..7e35fc4 100644
--- a/lib/strncpy_from_user.c
+++ b/lib/strncpy_from_user.c
@@ -1,6 +1,7 @@
#include <linux/compiler.h>
#include <linux/export.h>
#include <linux/kasan-checks.h>
+#include <linux/thread_info.h>
#include <linux/uaccess.h>
#include <linux/kernel.h>
#include <linux/errno.h>
@@ -111,6 +112,7 @@ long strncpy_from_user(char *dst, const char __user *src, long count)
long retval;
kasan_check_write(dst, count);
+ check_object_size(dst, count, false);
user_access_begin();
retval = do_strncpy_from_user(dst, src, count, max);
user_access_end();
--
2.7.4
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