Re: [PATCH 1/2] KVM: arm64: vgic: Forbid invalid userspace Redistributor accesses

[Marc Zyngier <maz@kernel.org>]

On 2020-11-16 13:09, Zenghui Yu wrote:
> Hi Marc,
> 
> On 2020/11/16 1:04, Marc Zyngier wrote:
>> Hi Zenghui,
>> 
>> On 2020-11-13 14:28, Zenghui Yu wrote:
>>> It's expected that users will access registers in the redistributor 
>>> *if*
>>> the RD has been initialized properly. Unfortunately userspace can be 
>>> bogus
>>> enough to access registers before setting the RD base address, and 
>>> KVM
>>> implicitly allows it (we handle the access anyway, regardless of 
>>> whether
>>> the base address is set).
>>> 
>>> Bad thing happens when we're handling the user read of GICR_TYPER. We 
>>> end
>>> up with an oops when deferencing the unset rdreg...
>>> 
>>>     gpa_t last_rdist_typer = rdreg->base + GICR_TYPER +
>>>             (rdreg->free_index - 1) * KVM_VGIC_V3_REDIST_SIZE;
>>> 
>>> Fix this issue by informing userspace what had gone wrong (-ENXIO).
>> 
>> I'm worried about the "implicit" aspect of the access that this patch
>> now forbids.
>> 
>> The problem is that the existing documentation doesn't cover this 
>> case, > and -ENXIO's "Getting or setting this register is not yet 
>> supported"
>> is way too vague.
> 
> Indeed. How about changing to
> 
>     -ENXIO  Getting or setting this register is not yet supported
>             or VGIC not properly configured (e.g., [Re]Distributor base
>             address is unknown)

Looks OK to me.

> 
>> There is a precedent with the ITS, but that's undocumented
>> as well. Also, how about v2? If that's the wasy we are going to fix 
>> this,
>> we also nned to beef up the documentation.
> 
> Sure, I plan to do so and hope it won't break the existing userspace.

Well, at this stage we can only hope.

> 
>> Of course, the other horrible way to address the issue is to return a 
>> value
>> that doesn't have the Last bit set, since we can't synthetise it. It 
>> doesn't
>> change the userspace API, and I can even find some (admittedly  
>> twisted)
>> logic to it (since there is no base address, there is no last RD...).
> 
> I'm fine with it. But I'm afraid that there might be other issues due 
> to
> the "unexpected" accesses since I haven't tested with all registers 
> from
> userspace.

I have had a look at the weekend, and couldn't see any other other GICR
register that would suffer from rdreg being NULL. I haven't looked at
GICD, but I don't anticipate anything bad on that front.

> My take is that only if the "[Re]Distributor base address" is specified
> in the system memory map, will the user-provided kvm_device_attr.offset
> make sense. And we can then handle the access to the register which is
> defined by "base address + offset".

I'd tend to agree, but it is just that this is a large change at -rc4.
I'd rather have a quick fix for 5.10, and a more invasive change for 
5.11,
spanning all the possible vgic devices.

         M.
-- 
Jazz is not dead. It just smells funny...
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