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Subject: Re: [RFC PATCH 0/2] arm64: optional paranoid __{get,put}_user checks
To: Mark Rutland <mark.rutland@arm.com>
Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        kernel-hardening@lists.openwall.com,
        Catalin Marinas <catalin.marinas@arm.com>,
        Kees Cook <keescook@chromium.org>, Will Deacon <will.deacon@arm.com>
References: <20171026090942.7041-1-mark.rutland@arm.com>
 <77c80381-cf68-aa1a-9112-e057c068eeb6@redhat.com>
 <20171101120555.yvb65g3wgtxskfh3@lakrids.cambridge.arm.com>
From: Laura Abbott <labbott@redhat.com>
Message-ID: <0f2d5f89-2939-06ec-9b59-b19f828d8968@redhat.com>
Date: Wed, 1 Nov 2017 14:13:45 -0700
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On 11/01/2017 05:05 AM, Mark Rutland wrote:
> On Tue, Oct 31, 2017 at 04:56:39PM -0700, Laura Abbott wrote:
>> On 10/26/2017 02:09 AM, Mark Rutland wrote:
>>> 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).
> 
>> Turning on the option fails as soon as we hit userspace. On my buildroot
>> based environment I get the help text for ld.so (????) and then a message
>> about attempting to kill init. 
> 
> Ouch. Thanks for the report, and sorry about this.
> 
> The problem is that I evaluate the ptr argument twice in
> __{get,put}_user(), and this may have side effects.
> 
> e.g. when the ELF loader does things like:
> 
>   __put_user((elf_addr_t)p, sp++)
> 
> ... we increment sp twice, and write to the wrong user address, leaving
> sp corrupt.
> 
> I have an additional patch [1] to fix this, which is in my
> arm64/access-ok branch [2].
> 
> Thanks,
> Mark.
> 
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/commit/?h=arm64/access-ok&id=ebb7ff83eb53b8810395d5cf48712a4ae6d678543
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/access-ok
> 

Thanks, the updated patch works. I wrote an LKDTM test to verify
the expected behavior (__{get,put}_user panic whereas {get,put}_user
do not). You're welcome to add Tested-by or I can wait for v2.

Thanks,
Laura