From: Catalin Marinas <catalin.marinas@arm.com>
To: Steven Price <steven.price@arm.com>
Cc: Marc Zyngier <maz@kernel.org>, Will Deacon <will@kernel.org>,
James Morse <james.morse@arm.com>,
Julien Thierry <julien.thierry.kdev@gmail.com>,
Suzuki K Poulose <suzuki.poulose@arm.com>,
kvmarm@lists.cs.columbia.edu,
linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org, Dave Martin <Dave.Martin@arm.com>,
Mark Rutland <mark.rutland@arm.com>,
Thomas Gleixner <tglx@linutronix.de>,
qemu-devel@nongnu.org, Juan Quintela <quintela@redhat.com>,
"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
Richard Henderson <richard.henderson@linaro.org>,
Peter Maydell <peter.maydell@linaro.org>,
Haibo Xu <Haibo.Xu@arm.com>, Andrew Jones <drjones@redhat.com>
Subject: Re: [PATCH v12 7/8] KVM: arm64: ioctl to fetch/store tags in a guest
Date: Thu, 20 May 2021 18:27:16 +0100 [thread overview]
Message-ID: <20210520172713.GF12251@arm.com> (raw)
In-Reply-To: <dd5ab3a0-5a74-b145-2485-d6d871be945b@arm.com>
On Thu, May 20, 2021 at 04:58:01PM +0100, Steven Price wrote:
> On 20/05/2021 13:05, Catalin Marinas wrote:
> > On Mon, May 17, 2021 at 01:32:38PM +0100, Steven Price wrote:
> >> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> >> index e89a5e275e25..4b6c83beb75d 100644
> >> --- a/arch/arm64/kvm/arm.c
> >> +++ b/arch/arm64/kvm/arm.c
> >> @@ -1309,6 +1309,65 @@ static int kvm_vm_ioctl_set_device_addr(struct kvm *kvm,
> >> }
> >> }
> >>
> >> +static int kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
> >> + struct kvm_arm_copy_mte_tags *copy_tags)
> >> +{
> >> + gpa_t guest_ipa = copy_tags->guest_ipa;
> >> + size_t length = copy_tags->length;
> >> + void __user *tags = copy_tags->addr;
> >> + gpa_t gfn;
> >> + bool write = !(copy_tags->flags & KVM_ARM_TAGS_FROM_GUEST);
> >> + int ret = 0;
> >> +
> >> + if (copy_tags->reserved[0] || copy_tags->reserved[1])
> >> + return -EINVAL;
> >> +
> >> + if (copy_tags->flags & ~KVM_ARM_TAGS_FROM_GUEST)
> >> + return -EINVAL;
> >> +
> >> + if (length & ~PAGE_MASK || guest_ipa & ~PAGE_MASK)
> >> + return -EINVAL;
> >> +
> >> + gfn = gpa_to_gfn(guest_ipa);
> >> +
> >> + mutex_lock(&kvm->slots_lock);
> >> +
> >> + while (length > 0) {
> >> + kvm_pfn_t pfn = gfn_to_pfn_prot(kvm, gfn, write, NULL);
> >> + void *maddr;
> >> + unsigned long num_tags = PAGE_SIZE / MTE_GRANULE_SIZE;
> >> +
> >> + if (is_error_noslot_pfn(pfn)) {
> >> + ret = -EFAULT;
> >> + goto out;
> >> + }
> >> +
> >> + maddr = page_address(pfn_to_page(pfn));
> >> +
> >> + if (!write) {
> >> + num_tags = mte_copy_tags_to_user(tags, maddr, num_tags);
> >> + kvm_release_pfn_clean(pfn);
> >
> > Do we need to check if PG_mte_tagged is set? If the page was not faulted
> > into the guest address space but the VMM has the page, does the
> > gfn_to_pfn_prot() guarantee that a kvm_set_spte_gfn() was called? If
> > not, this may read stale tags.
>
> Ah, I hadn't thought about that... No I don't believe gfn_to_pfn_prot()
> will fault it into the guest.
It doesn't indeed. What it does is a get_user_pages() but it's not of
much help since the VMM pte wouldn't be tagged (we would have solved
lots of problems if we required PROT_MTE in the VMM...)
> >> + } else {
> >> + num_tags = mte_copy_tags_from_user(maddr, tags,
> >> + num_tags);
> >> + kvm_release_pfn_dirty(pfn);
> >> + }
> >
> > Same question here, if the we can't guarantee the stage 2 pte being set,
> > we'd need to set PG_mte_tagged.
>
> This is arguably worse as we'll be writing tags into the guest but
> without setting PG_mte_tagged - so they'll be lost when the guest then
> faults the pages in. Which sounds like it should break migration.
>
> I think the below should be safe, and avoids the overhead of setting the
> flag just for reads.
>
> Thanks,
>
> Steve
>
> ----8<----
> page = pfn_to_page(pfn);
> maddr = page_address(page);
>
> if (!write) {
> if (test_bit(PG_mte_tagged, &page->flags))
> num_tags = mte_copy_tags_to_user(tags, maddr,
> MTE_GRANULES_PER_PAGE);
> else
> /* No tags in memory, so write zeros */
> num_tags = MTE_GRANULES_PER_PAGE -
> clear_user(tag, MTE_GRANULES_PER_PAGE);
> kvm_release_pfn_clean(pfn);
For ptrace we return a -EOPNOTSUPP if the address doesn't have VM_MTE
but I don't think it makes sense here, so I'm fine with clearing the
destination and assuming that the tags are zero (as they'd be on
faulting into the guest.
Another thing I forgot to ask, what's guaranteeing that the page
supports tags? Does this ioctl ensure that it would attempt the tag
copying from some device mapping? Do we need some kvm_is_device_pfn()
check? I guess ZONE_DEVICE memory we just refuse to map in an earlier
patch.
> } else {
> num_tags = mte_copy_tags_from_user(maddr, tags,
> MTE_GRANULES_PER_PAGE);
> kvm_release_pfn_dirty(pfn);
> }
>
> if (num_tags != MTE_GRANULES_PER_PAGE) {
> ret = -EFAULT;
> goto out;
> }
>
> if (write)
> test_and_set_bit(PG_mte_tagged, &page->flags);
I think a set_bit() would do, I doubt it's any more efficient. But why
not add it in the 'else' block above where we actually wrote the tags?
The copy function may have failed part-way through. Maybe your logic is
correct though, there are invalid tags in the page. Just add a comment.
--
Catalin
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next prev parent reply other threads:[~2021-05-20 17:29 UTC|newest]
Thread overview: 49+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-17 12:32 [PATCH v12 0/8] MTE support for KVM guest Steven Price
2021-05-17 12:32 ` [PATCH v12 1/8] arm64: mte: Handle race when synchronising tags Steven Price
2021-05-17 14:03 ` Marc Zyngier
2021-05-17 14:56 ` Steven Price
2021-05-19 17:32 ` Catalin Marinas
2021-05-17 12:32 ` [PATCH v12 2/8] arm64: Handle MTE tags zeroing in __alloc_zeroed_user_highpage() Steven Price
2021-05-17 12:32 ` [PATCH v12 3/8] arm64: mte: Sync tags for pages where PTE is untagged Steven Price
2021-05-17 16:14 ` Marc Zyngier
2021-05-19 9:32 ` Steven Price
2021-05-19 17:48 ` Catalin Marinas
2021-05-19 18:06 ` Catalin Marinas
2021-05-20 11:55 ` Steven Price
2021-05-20 12:25 ` Catalin Marinas
2021-05-20 13:02 ` Catalin Marinas
2021-05-20 13:03 ` Steven Price
2021-05-17 12:32 ` [PATCH v12 4/8] arm64: kvm: Introduce MTE VM feature Steven Price
2021-05-17 16:45 ` Marc Zyngier
2021-05-19 10:48 ` Steven Price
2021-05-20 8:51 ` Marc Zyngier
2021-05-20 14:46 ` Steven Price
2021-05-20 11:54 ` Catalin Marinas
2021-05-20 15:05 ` Steven Price
2021-05-20 17:50 ` Catalin Marinas
2021-05-21 9:28 ` Steven Price
2021-05-17 12:32 ` [PATCH v12 5/8] arm64: kvm: Save/restore MTE registers Steven Price
2021-05-17 17:17 ` Marc Zyngier
2021-05-19 13:04 ` Steven Price
2021-05-20 9:46 ` Marc Zyngier
2021-05-20 15:21 ` Steven Price
2021-05-17 12:32 ` [PATCH v12 6/8] arm64: kvm: Expose KVM_ARM_CAP_MTE Steven Price
2021-05-17 17:40 ` Marc Zyngier
2021-05-19 13:26 ` Steven Price
2021-05-20 10:09 ` Marc Zyngier
2021-05-20 10:51 ` Steven Price
2021-05-17 12:32 ` [PATCH v12 7/8] KVM: arm64: ioctl to fetch/store tags in a guest Steven Price
2021-05-17 18:04 ` Marc Zyngier
2021-05-19 13:51 ` Steven Price
2021-05-20 12:05 ` Catalin Marinas
2021-05-20 15:58 ` Steven Price
2021-05-20 17:27 ` Catalin Marinas [this message]
2021-05-21 9:42 ` Steven Price
2021-05-24 18:11 ` Catalin Marinas
2021-05-27 7:50 ` Steven Price
2021-05-27 13:08 ` Catalin Marinas
2021-05-17 12:32 ` [PATCH v12 8/8] KVM: arm64: Document MTE capability and ioctl Steven Price
2021-05-17 18:09 ` Marc Zyngier
2021-05-19 14:09 ` Steven Price
2021-05-20 10:24 ` Marc Zyngier
2021-05-20 10:52 ` Steven Price
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