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

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Message-ID: <87wn9uzhqr.fsf@mpe.ellerman.id.au>
Date:   Fri, 23 Sep 2022 17:12:44 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Paul Moore <paul@...l-moore.com>,
        Nathan Lynch <nathanl@...ux.ibm.com>
Cc:     linuxppc-dev@...ts.ozlabs.org,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, jmorris@...ei.org, serge@...lyn.com,
        ajd@...ux.ibm.com, gcwilson@...ux.ibm.com, nayna@...ux.ibm.com
Subject: Re: [PATCH 2/2] powerpc/rtas: block error injection when locked down

Paul Moore <paul@...l-moore.com> writes:
> On Thu, Sep 22, 2022 at 3:38 PM Nathan Lynch <nathanl@...ux.ibm.com> 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>
>> ---
>>  arch/powerpc/kernel/rtas.c | 25 ++++++++++++++++++++++++-
>>  include/linux/security.h   |  1 +
>>  security/security.c        |  1 +
>>  3 files changed, 26 insertions(+), 1 deletion(-)
>
> ...
>
>> diff --git a/include/linux/security.h b/include/linux/security.h
>> index 1ca8dbacd3cc..b5d5138ae66a 100644
>> --- a/include/linux/security.h
>> +++ b/include/linux/security.h
>> @@ -123,6 +123,7 @@ enum lockdown_reason {
>>         LOCKDOWN_BPF_WRITE_USER,
>>         LOCKDOWN_DBG_WRITE_KERNEL,
>>         LOCKDOWN_DEVICE_TREE,
>> +       LOCKDOWN_RTAS_ERROR_INJECTION,
>
> With the understanding that I've never heard of RTAS until now, are
> there any other RTAS events that would require a lockdown reason?  As
> a follow up, is it important to distinguish between different RTAS
> lockdown reasons?
>
> I'm trying to determine if we can just call it LOCKDOWN_RTAS.

Yes I think we should.

Currently it only locks down the error injection calls, not all of RTAS.

But firmware can/will add new RTAS calls in future, so it's always
possible something will need to be added to the list of things that need
to be blocked during lockdown.

So I think calling it LOCKDOWN_RTAS would be more general and future
proof, and can be read to mean "lockdown the parts of RTAS that need
to be locked down".

Similarly we have LOCKDOWN_ACPI_TABLES which locks down modification to
ACPI data, but doesn't disable ACPI use entirely AIUI.

cheers