linux-kernel - Re: [PATCH] lkdtm: Test KUAP directional user access unlocks on powerpc

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Message-ID: <c05a4327-0c81-0e3e-d93a-9d62183b146c@c-s.fr>
Date:   Fri, 31 Jan 2020 07:58:16 +0100
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Russell Currey <ruscur@...sell.cc>, keescook@...omium.org,
        mpe@...erman.id.au
Cc:     linux-kernel@...r.kernel.org, dja@...ens.net,
        kernel-hardening@...ts.openwall.com, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] lkdtm: Test KUAP directional user access unlocks on
 powerpc



Le 31/01/2020 à 07:53, Russell Currey a écrit :
> On Fri, 2020-01-31 at 07:44 +0100, Christophe Leroy wrote:
>>
>> Le 31/01/2020 à 06:31, Russell Currey a écrit :
>>> +	pr_info("attempting bad read at %px with write allowed\n",
>>> ptr);
>>> +	tmp = *ptr;
>>> +	tmp += 0xc0dec0de;
>>> +	prevent_write_to_user(ptr, sizeof(unsigned long));
>>
>> Does it work ? I would have thought that if the read fails the
>> process
>> will die and the following test won't be performed.
> 
> Correct, the ACCESS_USERSPACE test does the same thing.  Splitting this
> into separate R and W tests makes sense, even if it is unlikely that
> one would be broken without the other.
> 

Or once we are using user_access_begin() stuff, we can use 
unsafe_put_user() and unsafe_get_user() which should return an error 
instead of killing the caller.

Christophe