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 3/8] arm64: mte: Sync tags for pages where PTE is untagged
Date: Wed, 19 May 2021 18:48:08 +0100 [thread overview]
Message-ID: <20210519174808.GD21619@arm.com> (raw)
In-Reply-To: <f3a3f560-4d2b-9cd3-bbf4-ea8135ab4d17@arm.com>
On Wed, May 19, 2021 at 10:32:01AM +0100, Steven Price wrote:
> On 17/05/2021 17:14, Marc Zyngier wrote:
> > On Mon, 17 May 2021 13:32:34 +0100,
> > Steven Price <steven.price@arm.com> wrote:
> >>
> >> A KVM guest could store tags in a page even if the VMM hasn't mapped
> >> the page with PROT_MTE. So when restoring pages from swap we will
> >> need to check to see if there are any saved tags even if !pte_tagged().
> >>
> >> However don't check pages for which pte_access_permitted() returns false
> >> as these will not have been swapped out.
> >>
> >> Signed-off-by: Steven Price <steven.price@arm.com>
> >> ---
> >> arch/arm64/include/asm/pgtable.h | 9 +++++++--
> >> arch/arm64/kernel/mte.c | 16 ++++++++++++++--
> >> 2 files changed, 21 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> >> index 0b10204e72fc..275178a810c1 100644
> >> --- a/arch/arm64/include/asm/pgtable.h
> >> +++ b/arch/arm64/include/asm/pgtable.h
> >> @@ -314,8 +314,13 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
> >> if (pte_present(pte) && pte_user_exec(pte) && !pte_special(pte))
> >> __sync_icache_dcache(pte);
> >>
> >> - if (system_supports_mte() &&
> >> - pte_present(pte) && pte_tagged(pte) && !pte_special(pte))
> >> + /*
> >> + * If the PTE would provide user space access to the tags associated
> >> + * with it then ensure that the MTE tags are synchronised. Exec-only
> >> + * mappings don't expose tags (instruction fetches don't check tags).
> >
> > I'm not sure I understand this comment. Of course, execution doesn't
> > match tags. But the memory could still have tags associated with
> > it. Does this mean such a page would lose its tags is swapped out?
>
> Hmm, I probably should have reread that - the context of the comment is
> lost.
>
> I added the comment when changing to pte_access_permitted(), and the
> comment on pte_access_permitted() explains a potential gotcha:
>
> * p??_access_permitted() is true for valid user mappings (PTE_USER
> * bit set, subject to the write permission check). For execute-only
> * mappings, like PROT_EXEC with EPAN (both PTE_USER and PTE_UXN bits
> * not set) must return false. PROT_NONE mappings do not have the
> * PTE_VALID bit set.
>
> So execute-only mappings return false even though that is effectively a
> type of user access. However, because MTE checks are not performed by
> the PE for instruction fetches this doesn't matter. I'll update the
> comment, how about:
>
> /*
> * If the PTE would provide user space access to the tags associated
> * with it then ensure that the MTE tags are synchronised. Although
> * pte_access_permitted() returns false for exec only mappings, they
> * don't expose tags (instruction fetches don't check tags).
> */
This looks fine to me. We basically want to check the PTE_VALID and
PTE_USER bits and pte_access_permitted() does this (we could come up
with a new macro name like pte_valid_user() but since we don't care
about execute-only, it gets unnecessarily complicated).
--
Catalin
next prev parent reply other threads:[~2021-05-19 17:48 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 [this message]
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
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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