Commit bf90e56e authored Oct 11, 2016 by Mark Rutland Committed by Linus Torvalds Oct 11, 2016

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>

parent e0176a2f

lib/strncpy_from_user.c

+2 −0

Original line number	Diff line number	Diff line
		#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();

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