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Message-ID: <aN_TjloudttqBUtf@google.com>
Date: Fri, 3 Oct 2025 14:45:50 +0100
From: Vincent Donnefort <vdonnefort@...gle.com>
To: Marc Zyngier <maz@...nel.org>
Cc: Oliver Upton <oliver.upton@...ux.dev>, joey.gouly@....com,
suzuki.poulose@....com, yuzenghui@...wei.com,
catalin.marinas@....com, will@...nel.org, qperret@...gle.com,
sebastianene@...gle.com, keirf@...gle.com,
linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v2] KVM: arm64: Check range args for pKVM mem transitions
[...]
> > > > > > +static bool check_range_args(u64 start, u64 nr_pages, u64 *size)
> > > > > > +{
> > > > > > + if (check_mul_overflow(nr_pages, PAGE_SIZE, size))
> > > > > > + return false;
> > > > > > +
> > > > > > + return start < (start + *size);
> > > > >
> > > > > I will echo Oliver's concern on v1: you probably want to convert the
> > > > > boundary check to be inclusive of the end of the range. Otherwise, a
> > > > > range that ends at the top of the 64bit range will be represented as
> > > > > 0, and fail the check despite being perfectly valid.
> > > >
> > > > Do you mean allowing something like start == 0xfffffffffffff000 and size ==
> > > > 4096?
> > >
> > > Yes, this is what I was alluding to on v1.
> > >
> > > > But I guess that would still put all the following checks using "addr + size" at
> > > > risk. Also, I believe even the code in pgtable.c wouldn't support a such range
> > > > as it is also using a u64 end boundary.
> > >
> > > I'm not sure I follow. Ranges are pretty commonly expressed as a range
> > > terminated by an exclusive value. This just hasn't been an issue yet as
> > > the page table code is only ever dealing with TTBR0 or VTTBR
> > > translations.
> >
> > If I do exclude the end boundary, evading checks would be as simple as making
> > sure we overflow the end boundary?
> >
> > e.g. __pkvm_host_share_guest(phys = 0xfffffffffffff000, size = 4096)
> >
> > check_range_allowed_memory(phys, phys + size) /* nop */
> > ....
> > for_each_hyp_page(page, phys, size) { /* nop */
> > ...
> > }
> > ...
> > /* Install a valid mapping to phys */
> > kvm_pgtable_stage2_map(&vm->pgt, ipa, size, phys, ...)
>
> Why shouldn't this be as simple as this:
>
> static bool check_range_args(u64 start, u64 nr_pages, u64 *size)
> {
> if (check_mul_overflow(nr_pages, PAGE_SIZE, size))
> return false;
>
> return start < (start + *size - 1);
> }
>
> which correctly deals with the boundary issue?
I am concerned about allowing ranges that will still overflow "phys + size".
e.g. phys=0xfffffffffffff000 and size=4096 would pass check_range_args().
But in __pkvm_host_share_guest() that would mean:
bypassing check_range_allowed_memory()
bypassing for_each_hyp_page()
but installing a valid mapping to phys with:
kvm_pgtable_stage2_map(&vm->pgt, ipa, size, phys, ...)
>
> M.
>
> --
> Without deviation from the norm, progress is not possible.
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