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Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.94 #2 (Red Hat Linux)) id 1ljiL9-00GLwZ-My for linux-arm-kernel@lists.infradead.org; Thu, 20 May 2021 13:03:53 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id A401011D4; Thu, 20 May 2021 06:03:50 -0700 (PDT) Received: from [192.168.1.179] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id C6E2C3F73B; Thu, 20 May 2021 06:03:47 -0700 (PDT) Subject: Re: [PATCH v12 3/8] arm64: mte: Sync tags for pages where PTE is untagged To: Catalin Marinas Cc: Marc Zyngier , Will Deacon , James Morse , Julien Thierry , Suzuki K Poulose , kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Dave Martin , Mark Rutland , Thomas Gleixner , qemu-devel@nongnu.org, Juan Quintela , "Dr. David Alan Gilbert" , Richard Henderson , Peter Maydell , Haibo Xu , Andrew Jones References: <20210517123239.8025-1-steven.price@arm.com> <20210517123239.8025-4-steven.price@arm.com> <20210519180610.GE21619@arm.com> <3bac3a47-9f96-c7bf-e401-fdef60dcc9d8@arm.com> <20210520122550.GD12251@arm.com> From: Steven Price Message-ID: <85946169-3670-c33e-bd49-abd16dce3fa1@arm.com> Date: Thu, 20 May 2021 14:03:42 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: <20210520122550.GD12251@arm.com> Content-Language: en-GB X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210520_060351_876379_356C8312 X-CRM114-Status: GOOD ( 42.33 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 20/05/2021 13:25, Catalin Marinas wrote: > On Thu, May 20, 2021 at 12:55:21PM +0100, Steven Price wrote: >> On 19/05/2021 19:06, Catalin Marinas wrote: >>> On Mon, May 17, 2021 at 01:32:34PM +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 for which pte_access_permitted() returns false >>>> as these will not have been swapped out. >>>> >>>> Signed-off-by: Steven Price >>>> --- >>>> 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). >>>> + */ >>>> + if (system_supports_mte() && pte_present(pte) && >>>> + pte_access_permitted(pte, false) && !pte_special(pte)) >>>> mte_sync_tags(ptep, pte); >>> >>> Looking at the mte_sync_page_tags() logic, we bail out early if it's the >>> old pte is not a swap one and the new pte is not tagged. So we only need >>> to call mte_sync_tags() if it's a tagged new pte or the old one is swap. >>> What about changing the set_pte_at() test to: >>> >>> if (system_supports_mte() && pte_present(pte) && !pte_special(pte) && >>> (pte_tagged(pte) || is_swap_pte(READ_ONCE(*ptep)))) >>> mte_sync_tags(ptep, pte); >>> >>> We can even change mte_sync_tags() to take the old pte directly: >>> >>> if (system_supports_mte() && pte_present(pte) && !pte_special(pte)) { >>> pte_t old_pte = READ_ONCE(*ptep); >>> if (pte_tagged(pte) || is_swap_pte(old_pte)) >>> mte_sync_tags(old_pte, pte); >>> } >>> >>> It would save a function call in most cases where the page is not >>> tagged. >> >> Yes that looks like a good optimisation - although you've missed the >> pte_access_permitted() part of the check ;) > > I was actually wondering if we could remove it. I don't think it buys us > much as we have a pte_present() check already, so we know it is pointing > to a valid page. Currently we'd only get a tagged pte on user mappings, > same with swap entries. Actually the other way round makes more sense surely? pte_access_permitted() is true if both PTE_VALID & PTE_USER are set. pte_present() is true if *either* PTE_VALID or PTE_PROT_NONE are set. So the pte_present() is actually redundant. > When vmalloc kasan_hw will be added, I think we have a set_pte_at() with > a tagged pte but init_mm and high address (we might as well add a > warning if addr > TASK_SIZE_64 on the mte_sync_tags path so that we > don't forget). While we might not yet have tagged kernel pages - I'm not sure there's much point weakening the check to have to then check addr as well in the future. >> The problem I hit is one of include dependencies: >> >> is_swap_pte() is defined (as a static inline) in >> include/linux/swapops.h. However the definition depends on >> pte_none()/pte_present() which are defined in pgtable.h - so there's a >> circular dependency. >> >> Open coding is_swap_pte() in set_pte_at() works, but it's a bit ugly. >> Any ideas on how to improve on the below? >> >> if (system_supports_mte() && pte_present(pte) && >> pte_access_permitted(pte, false) && !pte_special(pte)) { >> pte_t old_pte = READ_ONCE(*ptep); >> /* >> * We only need to synchronise if the new PTE has tags enabled >> * or if swapping in (in which case another mapping may have >> * set tags in the past even if this PTE isn't tagged). >> * (!pte_none() && !pte_present()) is an open coded version of >> * is_swap_pte() >> */ >> if (pte_tagged(pte) || (!pte_none(pte) && !pte_present(pte))) >> mte_sync_tags(old_pte, pte); >> } > > That's why I avoided testing my suggestion ;). I think we should just > add !pte_none() in there with a comment that it may be a swap pte and > use the is_swap_pte() again on the mte_sync_tags() path. We already have > the pte_present() check. Well of course I didn't test the above beyond building - and I've screwed up because the open coded is_swap_pte() should have been called on old_pte not pte! So the pte_present() check above (which I've just removed...) is for the *new* PTE. So I think we need to keep both here. Steve _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel