linux-kernel - Re: [PATCH v2] powerpc/rtas: Keep MSR[RI] set when calling RTAS

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Message-ID: <f498f098-8b87-26bd-9967-2315bbc231f3@linux.ibm.com>
Date:   Wed, 4 May 2022 13:01:18 +0200
From:   Laurent Dufour <ldufour@...ux.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        stable@...r.kernel.org, Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH v2] powerpc/rtas: Keep MSR[RI] set when calling RTAS

On 04/05/2022, 07:59:29, Michael Ellerman wrote:
> Laurent Dufour <ldufour@...ux.ibm.com> writes:
>> On 03/05/2022, 17:06:41, Michael Ellerman wrote:
>>> Laurent Dufour <ldufour@...ux.ibm.com> writes:
> ...
>>>> diff --git a/arch/powerpc/kernel/rtas.c b/arch/powerpc/kernel/rtas.c
>>>> index 1f42aabbbab3..d7775b8c8853 100644
>>>> --- a/arch/powerpc/kernel/rtas.c
>>>> +++ b/arch/powerpc/kernel/rtas.c
>>>> @@ -49,6 +49,11 @@ void enter_rtas(unsigned long);
>>>>  
>>>>  static inline void do_enter_rtas(unsigned long args)
>>>>  {
>>>> +	unsigned long msr;
>>>> +
>>>> +	msr = mfmsr();
>>>> +	BUG_ON(!(msr & MSR_RI));
>>>
>>> I'm not sure about this.
>>>
>>> We call RTAS in some low-level places, so if we ever hit this BUG_ON
>>> then it might cause us to crash badly, or recursively BUG.
>>>
>>> A WARN_ON_ONCE() might be safer?
>>
>> I'm afraid a BUG_ON is required here. Since MSR[RI] is set on RTAS exit so
>> if it was not set when calling RTAS, that's a real issue and should
>> generate unexpected behaviour.
>>
>> Do you have places in mind where RTAS could be called with !MSR[RI]?
> 
> The main one I can think of is if someone is using
> CONFIG_UDBG_RTAS_CONSOLE, then udbg_rtascon_putc() is wired up as
> udbg_putc() and that might be called from anywhere, including xmon.
> 
> There's also RTAS calls in low-level xics interrupt code, that might get
> called during panic/crash.
> 
> I don't expect any of those places to be called with MSR[RI] unset, but
> I'm worried that if we're already crashing and for some reason MSR[RI]
> is unset, then that BUG_ON will just make things worse.
> 
> eg. imagine taking a BUG_ON() for every character we try to print as
> part of an oops.
> 
> Admittedly CONFIG_UDBG_RTAS_CONSOLE is old and probably not used much
> anymore, but I'm still a bit paranoid :)

I think you're right to be paranoid :)

This part of code can be really sensitive.
I boot a kernel built with CONFIG_UDBG_RTAS_CONSOLE, xmon is working fine,
but I cannot pretend this is covering all the RTAS call cases.

My hope with BUG_ON() is to raise the issue, as soon as possible, so it can
be addressed during the test phase.