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 bf90e56e46)
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>

lib/strncpy_from_user.c

@@ -1,5 +1,6 @@
 #include <linux/compiler.h>
 #include <linux/export.h>
+#include <linux/thread_info.h>
 #include <linux/uaccess.h>
 #include <linux/kernel.h>
 #include <linux/errno.h>
@@ -109,6 +110,7 @@ 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();