Re: [PATCH] powerpc/powernv/idle: Restore IAMR after idle

[Michael Ellerman <mpe@ellerman.id.au>]

Nicholas Piggin <npiggin@gmail.com> writes:
> Russell Currey's on February 6, 2019 4:28 pm:
>> Without restoring the IAMR after idle, execution prevention on POWER9
>> with Radix MMU is overwritten and the kernel can freely execute userspace without
>> faulting.
>> 
>> This is necessary when returning from any stop state that modifies user
>> state, as well as hypervisor state.
>> 
>> To test how this fails without this patch, load the lkdtm driver and
>> do the following:
>> 
>>    echo EXEC_USERSPACE > /sys/kernel/debug/provoke-crash/DIRECT
>> 
>> which won't fault, then boot the kernel with powersave=off, where it
>> will fault.  Applying this patch will fix this.
>> 
>> Fixes: 3b10d0095a1e ("powerpc/mm/radix: Prevent kernel execution of user
>> space")
>> Cc: <stable@vger.kernel.org>
>> Signed-off-by: Russell Currey <ruscur@russell.cc>
>
> Good catch and debugging. This really should be a quirk, we don't want 
> to have to restore this thing on a thread switch.

I'm not sure I follow. We don't context switch it on Radix, but we do
on hash if pkeys are enabled.

> Can we put it under a CONFIG option if we're not using IAMR?

We'll always be using it with Radix, and we might be using it for pkeys
on hash, unless pkeys are compiled out. But I don't really expect anyone
to be running with pkeys compiled out.

So I think the only case we could optimise is that we're on hash and the
current thread has an IAMR of 0, then we could just not restore
(assuming we come out of idle with IAMR=0).

But maybe I'm not understanding.

cheers