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 v11 1/6] arm64: mte: Sync tags for pages where PTE is untagged
Date: Tue, 27 Apr 2021 18:43:58 +0100 [thread overview]
Message-ID: <20210427174357.GA17872@arm.com> (raw)
In-Reply-To: <20210416154309.22129-2-steven.price@arm.com>
On Fri, Apr 16, 2021 at 04:43:04PM +0100, Steven Price 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 which are !pte_valid_user() as these will
> not have been swapped out.
You should remove the pte_valid_user() mention from the commit log as
well.
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index e17b96d0e4b5..cf4b52a33b3c 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -312,7 +312,7 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
> __sync_icache_dcache(pte);
>
> if (system_supports_mte() &&
> - pte_present(pte) && pte_tagged(pte) && !pte_special(pte))
> + pte_present(pte) && (pte_val(pte) & PTE_USER) && !pte_special(pte))
I would add a pte_user() macro here or, if we restore the tags only when
the page is readable, use pte_access_permitted(pte, false). Also add a
comment why we do this.
There's also the pte_user_exec() case which may not have the PTE_USER
set (exec-only permission) but I don't think it matters. We don't do tag
checking on instruction fetches, so if the user adds a PROT_READ to it,
it would go through set_pte_at() again. I'm not sure KVM does anything
special with exec-only mappings at stage 2, I suspect they won't be
accessible by the guest (but needs checking).
> 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 b3c70a612c7a..e016ab57ea36 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -26,17 +26,23 @@ u64 gcr_kernel_excl __ro_after_init;
>
> static bool report_fault_once = true;
>
> -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)
> {
> pte_t old_pte = READ_ONCE(*ptep);
>
> if (check_swap && is_swap_pte(old_pte)) {
> swp_entry_t entry = pte_to_swp_entry(old_pte);
>
> - if (!non_swap_entry(entry) && mte_restore_tags(entry, page))
> + if (!non_swap_entry(entry) && mte_restore_tags(entry, page)) {
> + set_bit(PG_mte_tagged, &page->flags);
> return;
> + }
> }
>
> + if (!pte_is_tagged || test_and_set_bit(PG_mte_tagged, &page->flags))
> + return;
I don't think we need another test_bit() here, it was done in the
caller (bar potential races which need more thought).
> +
> page_kasan_tag_reset(page);
> /*
> * We need smp_wmb() in between setting the flags and clearing the
> @@ -54,11 +60,13 @@ void mte_sync_tags(pte_t *ptep, pte_t pte)
> struct page *page = pte_page(pte);
> long i, nr_pages = compound_nr(page);
> bool check_swap = nr_pages == 1;
> + bool pte_is_tagged = pte_tagged(pte);
>
> /* if PG_mte_tagged is set, tags have already been initialised */
> for (i = 0; i < nr_pages; i++, page++) {
> - if (!test_and_set_bit(PG_mte_tagged, &page->flags))
> - mte_sync_page_tags(page, ptep, check_swap);
> + if (!test_bit(PG_mte_tagged, &page->flags))
> + mte_sync_page_tags(page, ptep, check_swap,
> + pte_is_tagged);
> }
> }
You were right in the previous thread that if we have a race, it's
already there even without your patches KVM patches.
If it's the same pte in a multithreaded app, we should be ok as the core
code holds the ptl (the arch code also holds the mmap_lock during
exception handling but only as a reader, so you can have multiple
holders).
If there are multiple ptes to the same page, for example mapped with
MAP_ANONYMOUS | MAP_SHARED, metadata recovery is done via
arch_swap_restore() before we even set the pte and with the page locked.
So calling lock_page() again in mte_restore_tags() would deadlock.
I can see that do_swap_page() also holds the page lock around
set_pte_at(), so I think we are covered.
Any other scenario I may have missed? My understanding is that if the
pte is the same, we have the ptl. Otherwise we have the page lock for
shared pages.
--
Catalin
next prev parent reply other threads:[~2021-04-27 17:44 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-16 15:43 [PATCH v11 0/6] MTE support for KVM guest Steven Price
2021-04-16 15:43 ` [PATCH v11 1/6] arm64: mte: Sync tags for pages where PTE is untagged Steven Price
2021-04-27 17:43 ` Catalin Marinas [this message]
2021-04-29 16:06 ` Steven Price
2021-05-04 15:29 ` Catalin Marinas
2021-04-16 15:43 ` [PATCH v11 2/6] arm64: kvm: Introduce MTE VM feature Steven Price
2021-04-28 17:07 ` Catalin Marinas
2021-04-29 16:06 ` Steven Price
2021-05-04 17:40 ` Catalin Marinas
2021-05-06 16:15 ` Steven Price
2021-05-07 18:25 ` Catalin Marinas
2021-05-10 18:35 ` Catalin Marinas
2021-05-12 15:46 ` Steven Price
2021-05-12 17:45 ` Catalin Marinas
2021-05-13 10:57 ` Steven Price
2021-05-13 15:08 ` Catalin Marinas
2021-05-13 15:21 ` Catalin Marinas
2021-04-16 15:43 ` [PATCH v11 3/6] arm64: kvm: Save/restore MTE registers Steven Price
2021-04-16 15:43 ` [PATCH v11 4/6] arm64: kvm: Expose KVM_ARM_CAP_MTE Steven Price
2021-04-16 15:43 ` [PATCH v11 5/6] KVM: arm64: ioctl to fetch/store tags in a guest Steven Price
2021-04-27 17:58 ` Catalin Marinas
2021-04-29 16:06 ` Steven Price
2021-05-04 17:44 ` Catalin Marinas
2021-05-07 9:44 ` Steven Price
2021-05-07 9:59 ` David Laight
2021-04-16 15:43 ` [PATCH v11 6/6] KVM: arm64: Document MTE capability and ioctl 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=20210427174357.GA17872@arm.com \
--to=catalin.marinas@arm.com \
--cc=Dave.Martin@arm.com \
--cc=Haibo.Xu@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=steven.price@arm.com \
--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).