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Message-ID: <20161017130412.GG29095@leverpostej>
Date: Mon, 17 Oct 2016 14:04:12 +0100
From: Mark Rutland <mark.rutland@....com>
To: Kees Cook <keescook@...omium.org>
Cc: "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
LKML <linux-kernel@...r.kernel.org>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] lib: harden strncpy_from_user
On Fri, Aug 26, 2016 at 02:57:58PM -0400, Kees Cook wrote:
> On Fri, Aug 26, 2016 at 10:31 AM, Mark Rutland <mark.rutland@....com> 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.
[...]
> 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 was reminded of this as it just hit mainline; is it worth dropping a
TODO on the KSPP wiki? I suspect I won't have the time to delve much
further into this in the near term, and it might be a good intro task
for someone.
Thanks,
Mark.
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