Re: [PATCH 14/22] x86/fpu: Eager switch PKRU state

From: Dave Hansen
Date: Fri Mar 08 2019 - 14:01:29 EST

On 3/8/19 10:08 AM, Sebastian Andrzej Siewior wrote:
> On 2019-02-25 10:16:24 [-0800], Dave Hansen wrote:
>>> + if (!cpu_feature_enabled(X86_FEATURE_OSPKE))
>>> + return;
>>> +
>>> + if (current->mm) {
>>> + pk = get_xsave_addr(&new_fpu->state.xsave, XFEATURE_PKRU);
>>> + WARN_ON_ONCE(!pk);
>> This can trip on us of the 'init optimization' is in play because
>> get_xsave_addr() checks xsave->header.xfeatures. That's unlikely today
>> because we usually set PKRU to a restrictive value. But, it's also not
>> *guaranteed*.
>> Userspace could easily do an XRSTOR that puts PKRU back in its init
>> state if it wanted to, then this would end up with pk==NULL.
>> We might actually want a selftest that *does* that. I don't think we
>> have one.
> So you are saying that the above warning might trigger and be "okay"?

Nothing will break, but the warning will trigger, which isn't nice.

> My understanding is that the in-kernel XSAVE will always save everything
> so we should never "lose" the XFEATURE_PKRU no matter what user space
> does.
> So as test case you want
> xsave (-1 & ~XFEATURE_PKRU)
> xrestore (-1 & ~XFEATURE_PKRU)
> in userland and then a context switch to see if the warning above
> triggers?

I think you need an XRSTOR with RFBM=-1 (or at least with the PKRU bit
set) and the PKRU bit in the XFEATURES field in the XSAVE buffer set to 0.

>>> + if (pk)
>>> + pkru_val = pk->pkru;
>>> + }> + __write_pkru(pkru_val);
>>> }
>> A comment above __write_pkru() would be nice to say that it only
>> actually does the slow instruction on changes to the value.
> Could we please not do this? It is a comment above one of the callers
> function and we have two or three. And we have that comment already
> within __write_pkru().

I looked at this code and thought "writing PKRU is slow", and "this
writes PKRU unconditionally", and "the __ version of the function
shoudn't have much logic in it".

I got 2/3 wrong. To me that means this site needs a 1-line comment.
Feel free to move one of the other comments to here if you think it's
over-commented, but this site needs one.

>> BTW, this has the implicit behavior of always trying to do a
>> __write_pkru(0) on switches to kernel threads. That seems a bit weird
>> and it is likely to impose WRPKRU overhead on switches between user and
>> kernel threads.
>> The 0 value is also the most permissive, which is not great considering
>> that user mm's can be active the in page tables when running kernel
>> threads if we're being lazy.
>> Seems like we should either leave PKRU alone or have 'init_pkru_value'
>> be the default. That gives good security properties and is likely to
>> match the application value, removing the WRPKRU overhead.
> Last time we talked about this we agreed (or this was my impression) that
> 0 should be written so that the kernel thread should always be able to
> write to user space in case it borrowed its mm (otherwise it has none
> and it would fail anyway).

We can't write to userspace when borrowing an mm. If the kernel borrows
an mm, we might as well be on the init_mm which has no userspace mappings.

> We didn't want to leave PKRU alone because the outcome (whether or not
> the write by the kernel thread succeeds) should not depend on the last
> running task (and be random) but deterministic.

Right, so let's make it deterministically restrictive: either
init_pkru_value, or -1 since kernel threads shouldn't be touching
userspace in the first place.