Commit 8b5c89d1 authored Oct 11, 2016 by Mark Rutland Committed by Amit Pundir Oct 28, 2016

BACKPORT: 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.

Link: http://lkml.kernel.org/r/1472221903-31181-1-git-send-email-mark.rutland@arm.com


Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Bug: 31374226
Change-Id: I898e4e9f19307e37a9be497cb1a0d7f1e3911661
(cherry picked from commit bf90e56e467ed5766722972d483e6711889ed1b0)
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>

parent a4dce63c

lib/strncpy_from_user.c

+2 −0

Original line number	Diff line number	Diff line
		#include <linux/module.h>
		#include <linux/thread_info.h>
		#include <linux/uaccess.h>
		#include <linux/kernel.h>
		#include <linux/errno.h>
		@@ -108,6 +109,7 @@ long strncpy_from_user(char dst, const char __user src, long count)
		unsigned long max = max_addr - src_addr;
		long retval;

		check_object_size(dst, count, false);
		user_access_begin();
		retval = do_strncpy_from_user(dst, src, count, max);
		user_access_end();