linux-kernel - Re: [PATCH v15 22/23] KVM: SEV: Fix return code interpretation for RMP nested page faults

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Date: Fri, 10 May 2024 10:36:10 -0500
From: Michael Roth <michael.roth@....com>
To: Sean Christopherson <seanjc@...gle.com>
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Subject: Re: [PATCH v15 22/23] KVM: SEV: Fix return code interpretation for
 RMP nested page faults

On Fri, May 10, 2024 at 06:58:45AM -0700, Sean Christopherson wrote:
> On Thu, May 09, 2024, Michael Roth wrote:
> > The intended logic when handling #NPFs with the RMP bit set (31) is to
> > first check to see if the #NPF requires a shared<->private transition
> > and, if so, to go ahead and let the corresponding KVM_EXIT_MEMORY_FAULT
> > get forwarded on to userspace before proceeding with any handling of
> > other potential RMP fault conditions like needing to PSMASH the RMP
> > entry/etc (which will be done later if the guest still re-faults after
> > the KVM_EXIT_MEMORY_FAULT is processed by userspace).
> > 
> > The determination of whether any userspace handling of
> > KVM_EXIT_MEMORY_FAULT is needed is done by interpreting the return code
> > of kvm_mmu_page_fault(). However, the current code misinterprets the
> > return code, expecting 0 to indicate a userspace exit rather than less
> > than 0 (-EFAULT). This leads to the following unexpected behavior:
> > 
> >   - for KVM_EXIT_MEMORY_FAULTs resulting for implicit shared->private
> >     conversions, warnings get printed from sev_handle_rmp_fault()
> >     because it does not expect to be called for GPAs where
> >     KVM_MEMORY_ATTRIBUTE_PRIVATE is not set. Standard linux guests don't
> >     generally do this, but it is allowed and should be handled
> >     similarly to private->shared conversions rather than triggering any
> >     sort of warnings
> > 
> >   - if gmem support for 2MB folios is enabled (via currently out-of-tree
> >     code), implicit shared<->private conversions will always result in
> >     a PSMASH being attempted, even if it's not actually needed to
> >     resolve the RMP fault. This doesn't cause any harm, but results in a
> >     needless PSMASH and zapping of the sPTE
> > 
> > Resolve these issues by calling sev_handle_rmp_fault() only when
> > kvm_mmu_page_fault()'s return code is greater than or equal to 0,
> > indicating a KVM_MEMORY_EXIT_FAULT/-EFAULT isn't needed. While here,
> > simplify the code slightly and fix up the associated comments for better
> > clarity.
> > 
> > Fixes: ccc9d836c5c3 ("KVM: SEV: Add support to handle RMP nested page faults")
> > 
> > Signed-off-by: Michael Roth <michael.roth@....com>
> > ---
> >  arch/x86/kvm/svm/svm.c | 10 ++++------
> >  1 file changed, 4 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index 426ad49325d7..9431ce74c7d4 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -2070,14 +2070,12 @@ static int npf_interception(struct kvm_vcpu *vcpu)
> >  				svm->vmcb->control.insn_len);
> >  
> >  	/*
> > -	 * rc == 0 indicates a userspace exit is needed to handle page
> > -	 * transitions, so do that first before updating the RMP table.
> > +	 * rc < 0 indicates a userspace exit may be needed to handle page
> > +	 * attribute updates, so deal with that first before handling other
> > +	 * potential RMP fault conditions.
> >  	 */
> > -	if (error_code & PFERR_GUEST_RMP_MASK) {
> > -		if (rc == 0)
> > -			return rc;
> > +	if (rc >= 0 && error_code & PFERR_GUEST_RMP_MASK)
> 
> This isn't correct either.  A return of '0' also indiciates "exit to userspace",
> it just doesn't happen with SNP because '0' is returned only when KVM attempts
> emulation, and that too gets short-circuited by svm_check_emulate_instruction().
> 
> And I would honestly drop the comment, KVM's less-than-pleasant 1/0/-errno return
> values overload is ubiquitous enough that it should be relatively self-explanatory.
> 
> Or if you prefer to keep a comment, drop the part that specifically calls out
> attributes updates, because that incorrectly implies that's the _only_ reason
> why KVM checks the return.  But my vote is to drop the comment, because it
> essentially becomes "don't proceed to step 2 if step 1 failed", which kind of
> makes the reader go "well, yeah".

Ok, I think I was just paranoid after missing this. I've gone ahead and
dropped the comment, and hopefully it's now drilled into my head enough
that it's obvious to me now as well :) I've also changed the logic to
skip the extra RMP handling for rc==0 as well (should that ever arise
for any future reason):

  https://github.com/mdroth/linux/commit/0a0ba0d7f7571a31f0abc68acc51f24c2a14a8cf

Thanks!

-Mike