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Message-ID: <20210326185653.GG5126@arm.com>
Date: Fri, 26 Mar 2021 18:56:54 +0000
From: Catalin Marinas <catalin.marinas@....com>
To: Steven Price <steven.price@....com>
Cc: Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
James Morse <james.morse@....com>,
Julien Thierry <julien.thierry.kdev@...il.com>,
Suzuki K Poulose <suzuki.poulose@....com>,
kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
linux-kernel@...r.kernel.org, Dave Martin <Dave.Martin@....com>,
Mark Rutland <mark.rutland@....com>,
Thomas Gleixner <tglx@...utronix.de>, qemu-devel@...gnu.org,
Juan Quintela <quintela@...hat.com>,
"Dr. David Alan Gilbert" <dgilbert@...hat.com>,
Richard Henderson <richard.henderson@...aro.org>,
Peter Maydell <peter.maydell@...aro.org>,
Haibo Xu <Haibo.Xu@....com>, Andrew Jones <drjones@...hat.com>
Subject: Re: [PATCH v10 1/6] arm64: mte: Sync tags for pages where PTE is
untagged
Hi Steven,
On Fri, Mar 12, 2021 at 03:18:57PM +0000, 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.
>
> Signed-off-by: Steven Price <steven.price@....com>
> ---
> arch/arm64/include/asm/pgtable.h | 2 +-
> arch/arm64/kernel/mte.c | 16 ++++++++++++----
> 2 files changed, 13 insertions(+), 5 deletions(-)
>
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index e17b96d0e4b5..84166625c989 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_valid_user(pte) && !pte_special(pte))
> mte_sync_tags(ptep, pte);
With the EPAN patches queued in for-next/epan, pte_valid_user()
disappeared as its semantics weren't very clear.
So this relies on the set_pte_at() being done on the VMM address space.
I wonder, if the VMM did an mprotect(PROT_NONE), can the VM still access
it via stage 2? If yes, the pte_valid_user() test wouldn't work. We need
something like pte_present() && addr <= user_addr_max().
BTW, ignoring virtualisation, can we ever bring a page in from swap on a
PROT_NONE mapping (say fault-around)? It's not too bad if we keep the
metadata around for when the pte becomes accessible but I suspect we
remove it if the page is removed from swap.
--
Catalin
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