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

["Alejandro Vallejo" <alejandro.vallejo@cloud.com>]

On Wed Sep 25, 2024 at 7:35 AM BST, 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.

Sure.

>
> >  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

Cheers,
Alejandro