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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

>  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.

Jan