linux-kernel - Re: [PATCH v2 01/13] KVM: Rework KVM_CAP_GUEST_MEMFD_MMAP into KVM_CAP_GUEST_MEMFD

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <diqzh5waelsy.fsf@google.com>
Date: Tue, 07 Oct 2025 09:09:01 -0700
From: Ackerley Tng <ackerleytng@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Christian Borntraeger <borntraeger@...ux.ibm.com>, 
	Janosch Frank <frankja@...ux.ibm.com>, Claudio Imbrenda <imbrenda@...ux.ibm.com>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, David Hildenbrand <david@...hat.com>, 
	Fuad Tabba <tabba@...gle.com>
Subject: Re: [PATCH v2 01/13] KVM: Rework KVM_CAP_GUEST_MEMFD_MMAP into KVM_CAP_GUEST_MEMFD_FLAGS

Sean Christopherson <seanjc@...gle.com> writes:

> On Mon, Oct 06, 2025, Ackerley Tng wrote:
>> Sean Christopherson <seanjc@...gle.com> writes:
>> 
>> > Rework the not-yet-released KVM_CAP_GUEST_MEMFD_MMAP into a more generic
>> > KVM_CAP_GUEST_MEMFD_FLAGS capability so that adding new flags doesn't
>> > require a new capability, and so that developers aren't tempted to bundle
>> > multiple flags into a single capability.
>> >
>> > Note, kvm_vm_ioctl_check_extension_generic() can only return a 32-bit
>> > value, but that limitation can be easily circumvented by adding e.g.
>> > KVM_CAP_GUEST_MEMFD_FLAGS2 in the unlikely event guest_memfd supports more
>> > than 32 flags.
>> 
>> I know you suggested that guest_memfd's HugeTLB sizes shouldn't be
>> squashed into the flags. Just using that as an example, would those
>> kinds of flags (since they're using the upper bits, above the lower 32
>> bits) be awkward to represent in this new model?
>
> Are you asking specifically about flags that use bits 63:32?  If so, no, I don't
> see those as being awkward to deal with.  Hopefully we kill of 32-bit KVM and it's
> a complete non-issue, but even if we have to add KVM_CAP_GUEST_MEMFD_FLAGS2, I
> don't see it being all that awkward for userspace to do:
>
>   uint64_t supported_gmem_flags = kvm_check_extension(KVM_CAP_GUEST_MEMFD_FLAGS) |
>                                   (kvm_check_extension(KVM_CAP_GUEST_MEMFD_FLAGS2) << 32);
>
> We could even mimic what Intel did with 64-bit VMCS fields to handle 32-bit mode,
> and explicitly name the second one KVM_CAP_GUEST_MEMFD_FLAGS_HI:
>
>   uint64_t supported_gmem_flags = kvm_check_extension(KVM_CAP_GUEST_MEMFD_FLAGS) |
>                                   (kvm_check_extension(KVM_CAP_GUEST_MEMFD_FLAGS_HI) << 32);
>

Had the same thing in mind, I guess having a precedent (and seeing it in
code) makes it seem less awkward. Thanks!

> so that if KVM_CAP_GUEST_MEMFD_FLAGS_HI precedes 64-bit-only KVM, it could become
> fully redundant, i.e. where someday this would hold true:
>
>   kvm_check_extension(KVM_CAP_GUEST_MEMFD_FLAGS) == 
>   kvm_check_extension(KVM_CAP_GUEST_MEMFD_FLAGS) | kvm_check_extension(KVM_CAP_GUEST_MEMFD_FLAGS_HI) << 32
>
>> In this model, conditionally valid flags are always set, 
>
> I followed everything except this snippet.
>

I meant "conditionally valid" as in if GUEST_MEMFD_FLAG_BAR was valid
only when GUEST_MEMFD_FLAG_FOO is set, then with this model, when
KVM_CAP_GUEST_MEMFD_FLAGS is queried, would KVM return
GUEST_MEMFD_FLAG_MMAP | GUEST_MEMFD_FLAG_FOO | GUEST_MEMFD_FLAG_BAR,
where GUEST_MEMFD_FLAG_BAR is the conditionally valid flag?

>> but userspace won't be able to do a flags check against the returned 32-bit
>> value. Or do you think when this issue comes up, we'd put the flags in the
>> upper bits in KVM_CAP_GUEST_MEMFD_FLAGS2 and userspace would then check
>> against the OR-ed set of flags instead?
>
> As above, enumerate support for flags 63:32 in a separate capability.

Got it.