linux-kernel - Re: [PATCH v2 2/2] powerpc/rtas: block error injection when locked down

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Message-ID: <87y1u3pixf.fsf@linux.ibm.com>
Date:   Wed, 28 Sep 2022 11:23:08 -0500
From:   Nathan Lynch <nathanl@...ux.ibm.com>
To:     Andrew Donnellan <ajd@...ux.ibm.com>
Cc:     jmorris@...ei.org, mpe@...erman.id.au, paul@...l-moore.com,
        serge@...lyn.com, gcwilson@...ux.ibm.com, nayna@...ux.ibm.com,
        linuxppc-dev@...ts.ozlabs.org,
        linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] powerpc/rtas: block error injection when locked
 down

Andrew Donnellan <ajd@...ux.ibm.com> writes:

> On Mon, 2022-09-26 at 08:16 -0500, Nathan Lynch wrote:
>> The error injection facility on pseries VMs allows corruption of
>> arbitrary guest memory, potentially enabling a sufficiently
>> privileged
>> user to disable lockdown or perform other modifications of the
>> running
>> kernel via the rtas syscall.
>> 
>> Block the PAPR error injection facility from being opened or called
>> when locked down.
>> 
>> Signed-off-by: Nathan Lynch <nathanl@...ux.ibm.com>
>
> Is there any circumstance (short of arbitrary code execution etc) where
> the rtas_call() check will actually trigger rather than the sys_rtas()
> check? (Not that it matters, defence in depth is good.)

Fair question! There are no in-kernel users of rtas_call() that pass the
error injection tokens as far as I could tell. Nor am I aware of any
out-of-tree users, for that matter. But rtas_call() is the likely most
appropriate place to have the lockdown gate should that situation change
(as it might, see https://github.com/ibm-power-utilities/librtas/issues/29).