Re: [PATCH v2] x86/traps: Re-enable interrupts after reading cr2 in the #PF handler

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 25/09/2024 7:35 am, Jan Beulich wrote:
> On 24.09.2024 20:36, Andrew Cooper wrote:
>> On 23/09/2024 2:03 pm, Jan Beulich wrote:
>>> On 23.09.2024 12:14, Alejandro Vallejo wrote:
>>>> On Fri Sep 20, 2024 at 3:12 PM BST, Roger Pau Monné wrote:
>>>>> On Wed, Sep 18, 2024 at 02:05:54PM +0100, Alejandro Vallejo wrote:
>>>>>> Moves sti directly after the cr2 read and immediately after the #PF
>>>>>> handler.
>>>>> I think you need to add some context about why this is needed, iow:
>>>>> avoid corrupting %cr2 if a nested 3PF happens.
>>>> I can send a v3 with:
>>>>
>>>> ```
>>>>   Hitting a page fault clobbers %cr2, so if a page fault is handled while
>>>>   handling a previous page fault then %cr2 will hold the address of the 
>>>> latter
>>>>   fault rather than the former. This patch makes the page fault path delay
>>>>   re-enabling IRQs until %cr2 has been read in order to ensure it stays
>>>>   consistent.
>>> And under what conditions would we experience #PF while already processing
>>> an earlier #PF? If an interrupt kicks in, that's not supposed to by raising
>>> any #PF itself. Which isn't to say that the change isn't worthwhile to make,
>>> but it would be nice if it was explicit whether there are active issues, or
>>> whether this is merely to be on the safe side going forward.
>> My understanding is that this came from code inspection, not an active
>> issue.
>>
>> The same is true for %dr6 and #DB, and MSR_XFD_ERR and #NM.
>>
>> I think we can safely agree to veto the use of AMX in the #NM handler,
>> and IST exceptions don't re-enable interrupts[1], so #PF is the only
>> problem case.
>>
>> Debug keys happen off the back of plain IRQs, and we can get #PF when
>> interrogating guest stacks.
> Hmm, yes, this looks like a case that is actively being fixed here. Wants
> mentioning, likely wants a respective Fixes: tag, and then also wants
> backporting.

I'd also like to see at least a brief mention of #DB/#NM and why they're
safe.
>>   Also, I'm far from certain we're safe to
>> spurious #PF's from updating Xen mappings, so I think there are a bunch
>> of risky corner cases that we might be exposed to.
> Spurious #PF are possible, but __page_fault_type() explicitly excludes
> the in_irq() case.

Right, but %cr2 is clobbered when the spurious #PF occurs, whatever
__page_fault_type() subsequently decides to do.

~Andrew