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 4/8] arm64: kvm: Introduce MTE VM feature
Date: Thu, 20 May 2021 18:50:38 +0100 [thread overview]
Message-ID: <20210520175037.GG12251@arm.com> (raw)
In-Reply-To: <5f0996d6-0a6e-ebcd-afcd-8290faba6780@arm.com>
On Thu, May 20, 2021 at 04:05:46PM +0100, Steven Price wrote:
> On 20/05/2021 12:54, Catalin Marinas wrote:
> > On Mon, May 17, 2021 at 01:32:35PM +0100, Steven Price wrote:
> >> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> >> index c5d1f3c87dbd..8660f6a03f51 100644
> >> --- a/arch/arm64/kvm/mmu.c
> >> +++ b/arch/arm64/kvm/mmu.c
> >> @@ -822,6 +822,31 @@ transparent_hugepage_adjust(struct kvm_memory_slot *memslot,
> >> return PAGE_SIZE;
> >> }
> >>
> >> +static int sanitise_mte_tags(struct kvm *kvm, unsigned long size,
> >> + kvm_pfn_t pfn)
> >> +{
> >> + if (kvm_has_mte(kvm)) {
> >> + /*
> >> + * The page will be mapped in stage 2 as Normal Cacheable, so
> >> + * the VM will be able to see the page's tags and therefore
> >> + * they must be initialised first. If PG_mte_tagged is set,
> >> + * tags have already been initialised.
> >> + */
> >> + unsigned long i, nr_pages = size >> PAGE_SHIFT;
> >> + struct page *page = pfn_to_online_page(pfn);
> >> +
> >> + if (!page)
> >> + return -EFAULT;
> >
> > IIRC we ended up with pfn_to_online_page() to reject ZONE_DEVICE pages
> > that may be mapped into a guest and we have no idea whether they support
> > MTE. It may be worth adding a comment, otherwise, as Marc said, the page
> > wouldn't disappear.
>
> I'll add a comment.
>
> >> +
> >> + for (i = 0; i < nr_pages; i++, page++) {
> >> + if (!test_and_set_bit(PG_mte_tagged, &page->flags))
> >> + mte_clear_page_tags(page_address(page));
> >
> > We started the page->flags thread and ended up fixing it for the host
> > set_pte_at() as per the first patch:
> >
> > https://lore.kernel.org/r/c3293d47-a5f2-ea4a-6730-f5cae26d8a7e@arm.com
> >
> > Now, can we have a race between the stage 2 kvm_set_spte_gfn() and a
> > stage 1 set_pte_at()? Only the latter takes a lock. Or between two
> > kvm_set_spte_gfn() in different VMs? I think in the above thread we
> > concluded that there's only a problem if the page is shared between
> > multiple VMMs (MAP_SHARED). How realistic is this and what's the
> > workaround?
> >
> > Either way, I think it's worth adding a comment here on the race on
> > page->flags as it looks strange that here it's just a test_and_set_bit()
> > while set_pte_at() uses a spinlock.
> >
>
> Very good point! I should have thought about that. I think splitting the
> test_and_set_bit() in two (as with the cache flush) is sufficient. While
> there technically still is a race which could lead to user space tags
> being clobbered:
>
> a) It's very odd for a VMM to be doing an mprotect() after the fact to
> add PROT_MTE, or to be sharing the memory with another process which
> sets PROT_MTE.
>
> b) The window for the race is incredibly small and the VMM (generally)
> needs to be robust against the guest changing tags anyway.
>
> But I'll add a comment here as well:
>
> /*
> * There is a potential race between sanitising the
> * flags here and user space using mprotect() to add
> * PROT_MTE to access the tags, however by splitting
> * the test/set the only risk is user space tags
> * being overwritten by the mte_clear_page_tags() call.
> */
I think (well, I haven't re-checked), an mprotect() in the VMM ends up
calling set_pte_at_notify() which would call kvm_set_spte_gfn() and that
will map the page in the guest. So the problem only appears between
different VMMs sharing the same page. In principle they can be
MAP_PRIVATE but they'd be CoW so the race wouldn't matter. So it's left
with MAP_SHARED between multiple VMMs.
I think we should just state that this is unsafe and they can delete
each-others tags. If we are really worried, we can export that lock you
added in mte.c.
--
Catalin
next prev parent reply other threads:[~2021-05-20 17:50 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 [this message]
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
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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