From: Steven Price <steven.price@arm.com>
To: Marc Zyngier <maz@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
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 10:32:01 +0100 [thread overview]
Message-ID: <f3a3f560-4d2b-9cd3-bbf4-ea8135ab4d17@arm.com> (raw)
In-Reply-To: <87y2cdtk09.wl-maz@kernel.org>
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).
*/
Thanks,
Steve
> Thanks,
>
> M.
>
>> + */
>> + if (system_supports_mte() && pte_present(pte) &&
>> + pte_access_permitted(pte, false) && !pte_special(pte))
>> mte_sync_tags(ptep, pte);
>>
>> __check_racy_pte_update(mm, ptep, pte);
>> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
>> index c88e778c2fa9..a604818c52c1 100644
>> --- a/arch/arm64/kernel/mte.c
>> +++ b/arch/arm64/kernel/mte.c
>> @@ -33,11 +33,15 @@ DEFINE_STATIC_KEY_FALSE(mte_async_mode);
>> EXPORT_SYMBOL_GPL(mte_async_mode);
>> #endif
>>
>> -static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
>> +static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap,
>> + bool pte_is_tagged)
>> {
>> unsigned long flags;
>> pte_t old_pte = READ_ONCE(*ptep);
>>
>> + if (!is_swap_pte(old_pte) && !pte_is_tagged)
>> + return;
>> +
>> spin_lock_irqsave(&tag_sync_lock, flags);
>>
>> /* Recheck with the lock held */
>> @@ -53,6 +57,9 @@ static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
>> }
>> }
>>
>> + if (!pte_is_tagged)
>> + goto out;
>> +
>> page_kasan_tag_reset(page);
>> /*
>> * We need smp_wmb() in between setting the flags and clearing the
>> @@ -76,10 +83,15 @@ void mte_sync_tags(pte_t *ptep, pte_t pte)
>> bool check_swap = nr_pages == 1;
>> bool pte_is_tagged = pte_tagged(pte);
>>
>> + /* Early out if there's nothing to do */
>> + if (!check_swap && !pte_is_tagged)
>> + return;
>> +
>> /* if PG_mte_tagged is set, tags have already been initialised */
>> for (i = 0; i < nr_pages; i++, page++) {
>> if (!test_bit(PG_mte_tagged, &page->flags))
>> - mte_sync_page_tags(page, ptep, check_swap);
>> + mte_sync_page_tags(page, ptep, check_swap,
>> + pte_is_tagged);
>> }
>> }
>>
>> --
>> 2.20.1
>>
>>
>
next prev parent reply other threads:[~2021-05-19 9:32 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 [this message]
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
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=f3a3f560-4d2b-9cd3-bbf4-ea8135ab4d17@arm.com \
--to=steven.price@arm.com \
--cc=Dave.Martin@arm.com \
--cc=Haibo.Xu@arm.com \
--cc=catalin.marinas@arm.com \
--cc=dgilbert@redhat.com \
--cc=drjones@redhat.com \
--cc=james.morse@arm.com \
--cc=julien.thierry.kdev@gmail.com \
--cc=kvmarm@lists.cs.columbia.edu \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mark.rutland@arm.com \
--cc=maz@kernel.org \
--cc=peter.maydell@linaro.org \
--cc=qemu-devel@nongnu.org \
--cc=quintela@redhat.com \
--cc=richard.henderson@linaro.org \
--cc=suzuki.poulose@arm.com \
--cc=tglx@linutronix.de \
--cc=will@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).