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Message-ID: <20171027204410.4jdg2uwuj2mlw5sd@salmiak>
Date: Fri, 27 Oct 2017 21:44:11 +0100
From: Mark Rutland <mark.rutland@....com>
To: Will Deacon <will.deacon@....com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
kernel-hardening@...ts.openwall.com,
Catalin Marinas <catalin.marinas@....com>,
Kees Cook <keescook@...omium.org>,
Laura Abbott <labbott@...hat.com>
Subject: Re: [RFC PATCH 0/2] arm64: optional paranoid __{get,put}_user checks
On Fri, Oct 27, 2017 at 04:41:13PM +0100, Will Deacon wrote:
> On Thu, Oct 26, 2017 at 10:09:40AM +0100, Mark Rutland wrote:
> > Hi,
> >
> > In Prague, Kees mentioned that it would be nice to have a mechanism to
> > catch bad __{get,put}_user uses, such as the recent CVE-2017-5123 [1,2]
> > issue with unsafe_put_user() in waitid().
> >
> > These patches allow an optional access_ok() check to be dropped in
> > arm64's __{get,put}_user() primitives. These will then BUG() if a bad
> > user pointer is passed (which should only happen in the absence of an
> > earlier access_ok() check).
> >
> > The first patch rewrites the arm64 access_ok() check in C. This gives
> > the compiler the visibility it needs to elide redundant access_ok()
> > checks, so in the common case:
> >
> > get_user()
> > access_ok()
> > __get_user()
> > BUG_ON(!access_ok())
> > <uaccess asm>
> >
> > ... the compiler can determine that the second access_ok() must return
> > true, and can elide it along with the BUG_ON(), leaving:
> >
> > get_user()
> > access_ok()
> > __get_user()
> > <uaccess asm>
> >
> > ... and thus this sanity check can have no cost in the common case.
>
> Probably a stupid question, but why not just move the access_ok check
> into __{get,put}_user and remove it from {get,put}_user?
Good question.
I was considering this as a debug option, making it possible to catch unsafe
__{get,put}_user() uses via fuzzing or at build time.
As a hardening option, it would make more sense to always have the check in
__{get,put}_user().
> We can also then move the uaccess_{enable,disable}_not_uao calls out from the
> __ variants so that we can implement user_access_{begin,end}.
Mhmm. I'll take a look at this for v2, afer I've figured out precisely what
I've broken with this RFC.
I'd still like the option to scream on unsafe __{get,put}_user() calls, but it
should be possible to handle both cases with minimal IS_ENABLED() usage.
Thanks,
Mark.
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