linux-kernel - Re: [PATCH v5 6/9] KVM: Add a helper function to check if a gpa is in writable memselot

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Message-ID: <CAHBxVyE7Tp97GfbH=deaA7gqWQXByno3O3OHbHDJCJ=J7FUQvw@mail.gmail.com>
Date: Fri, 5 Sep 2025 12:34:48 -0700
From: Atish Kumar Patra <atishp@...osinc.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Anup Patel <anup@...infault.org>, Will Deacon <will@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, Paul Walmsley <paul.walmsley@...ive.com>, 
	Palmer Dabbelt <palmer@...belt.com>, Mayuresh Chitale <mchitale@...tanamicro.com>, 
	linux-riscv@...ts.infradead.org, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	kvm-riscv@...ts.infradead.org
Subject: Re: [PATCH v5 6/9] KVM: Add a helper function to check if a gpa is in
 writable memselot

On Fri, Sep 5, 2025 at 1:23 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Wed, Sep 03, 2025, Atish Kumar Patra wrote:
> > On Fri, Aug 29, 2025 at 1:47 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > >
> > > On Fri, Aug 29, 2025, Atish Patra wrote:
> > > > +static inline bool kvm_is_gpa_in_writable_memslot(struct kvm *kvm, gpa_t gpa)
> > > > +{
> > > > +     bool writable;
> > > > +     unsigned long hva = gfn_to_hva_prot(kvm, gpa_to_gfn(gpa), &writable);
> > > > +
> > > > +     return !kvm_is_error_hva(hva) && writable;
> > >
> > > I don't hate this API, but I don't love it either.  Because knowing that the
> > > _memslot_ is writable doesn't mean all that much.  E.g. in this usage:
> > >
> > >         hva = kvm_vcpu_gfn_to_hva_prot(vcpu, shmem >> PAGE_SHIFT, &writable);
> > >         if (kvm_is_error_hva(hva) || !writable)
> > >                 return SBI_ERR_INVALID_ADDRESS;
> > >
> > >         ret = kvm_vcpu_write_guest(vcpu, shmem, &zero_sta, sizeof(zero_sta));
> > >         if (ret)
> > >                 return SBI_ERR_FAILURE;
> > >
> > > the error code returned to the guest will be different if the memslot is read-only
> > > versus if the VMA is read-only (or not even mapped!).  Unless every read-only
> > > memslot is explicitly communicated as such to the guest, I don't see how the guest
> > > can *know* that a memslot is read-only, so returning INVALID_ADDRESS in that case
> > > but not when the underlying VMA isn't writable seems odd.
> > >
> > > It's also entirely possible the memslot could be replaced with a read-only memslot
> > > after the check, or vice versa, i.e. become writable after being rejected.  Is it
> > > *really* a problem to return FAILURE if the guest attempts to setup steal-time in
> > > a read-only memslot?  I.e. why not do this and call it good?
> > >
> >
> > Reposting the response as gmail converted my previous response as
> > html. Sorry for the spam.
> >
> > From a functionality pov, that should be fine. However, we have
> > explicit error conditions for read only memory defined in the SBI STA
> > specification[1].
> > Technically, we will violate the spec if we return FAILURE instead of
> > INVALID_ADDRESS for read only memslot.
>
> But KVM is already violating the spec, as kvm_vcpu_write_guest() redoes the
> memslot lookup and so could encounter a read-only memslot (if it races with
> a memslot update), and because the underlying memory could be read-only even if
> the memslot is writable.
>

Ahh. Thanks for clarifying that.

> Why not simply return SBI_ERR_INVALID_ADDRESS on kvm_vcpu_write_guest() failure?
> The only downside of that is KVM will also return SBI_ERR_INVALID_ADDRESS if the
> userspace mapping is completely missing, but AFAICT that doesn't seem to be an
> outright spec violation.

Yes. That's correct. That can still be considered as invalid address.
I will revise the patch according to this.
Thanks for the suggestions.