Re: [PATCH] kvm: apply VM_READ/VM_WRITE checks to all VMA types

[Paolo Bonzini]

On Fri, Jul 31, 2026 at 8:46 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> > * if the PTE is present, follow_pfnmap_start() sets args.writable to
> >   false and KVM_PFN_ERR_RO_FAULT is returned;
> >
> > * if no PTE is present, fixup_user_fault(FAULT_FLAG_WRITE) returns
> >   -EFAULT after checking vma_permits_fault(), and hva_to_pfn() ends
> >   up returning KVM_PFN_ERR_FAULT.
> >
> > With this patch KVM_PFN_ERR_RO_FAULT is returned uniformly.
>
> IMO, returning KVM_PFN_ERR_RO_FAULT on a read-only VMA is wrong.  AFAICT, that
> behavior for VM_{IO,PFNMAP} was added by commit bd2fae8da794 ("KVM: do not assume
> PTE is writable after follow_pfn").  Given that that's the only case where KVM
> returns KVM_PFN_ERR_RO_FAULT, I would much prefer to fix that wart and cross our
> fingers nothing has come to rely on the behavior in the last ~5 years.

We can try, but I'd rather not do that in stable releases (while this
patch would be applied there, as a first step towards fixing the DRM
issue that Sergio reported - it avoids the sashiko issue reported for
https://lore.kernel.org/r/20260731160514.1101989-1-pbonzini%40redhat.com/).

> > For regular mappings a PROT_READ VMA, if placed in a writable memslot,
> > would return KVM_PFN_ERR_FAULT instead of KVM_PFN_ERR_RO_FAULT when
> > the guest writes to it.  This would cause a -EFAULT exit to userspace,
> > instead of triggering emulation as the VM_IO|VM_PFNMAP arm would do;
>
> No, arm64 is checking the memslot, not the VMA.
>
>         hva = gfn_to_hva_memslot_prot(memslot, gfn, &writable);
>         write_fault = kvm_is_write_fault(vcpu);
>         if (kvm_is_error_hva(hva) || (write_fault && !writable)) {
>
> Or are you talking about different code?

I am talking about the "arm" of the if/else if/else. :)

>
>         /*
>          * GUP failed.  It could be an inaccessible mapping, a pfnmap one,
>          * or the page might be absent.
>          */
>         if (vma == NULL || unlikely(!(vma->vm_flags & VM_READ)) ||
>             ((kfp->flags & FOLL_WRITE) && unlikely(!(vma->vm_flags & VM_WRITE)))) {
>                 pfn = KVM_PFN_ERR_FAULT;
>         } else if (vma->vm_flags & (VM_IO | VM_PFNMAP)) {
>                 r = hva_to_pfn_remapped(vma, kfp, &pfn);
>                 if (r == -EAGAIN)
>                         goto retry;
>                 if (r < 0)
>                         pfn = KVM_PFN_ERR_FAULT;
>         } else {
>                 pfn = kfp->flags & FOLL_NOWAIT ? KVM_PFN_ERR_NEEDS_IO :
>                                                  KVM_PFN_ERR_FAULT;
>         }

Yes, but I'd do that only in 7.3.

Paolo