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Date: Fri, 26 Aug 2016 14:57:58 -0400
From: Kees Cook <>
To: "" <>
Cc: LKML <>, Mark Rutland <>, 
	Andrew Morton <>
Subject: Re: [PATCH] lib: harden strncpy_from_user

On Fri, Aug 26, 2016 at 10:31 AM, Mark Rutland <> wrote:
> 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 <>
> Cc: Andrew Morton <>
> Cc: Kees Cook <>
> ---
>  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

Ah, yes, good catch! (And to repeat what you mentioned to me in
passing in the hall: there appear to be other users of get_user() in a
loop in other places in the kernel that will likely need some
attention too.)

I'll get this into the hardened usercopy tree when I get back from the
Security Summit. This will likely need to grow knowledge about
builtin-const "count" arguments like we need to the architectures that
are missing them in the copy_*_user calls.



Kees Cook
Nexus Security

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