From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-14.0 required=3.0 tests=BAYES_00,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 54B89C48BCD for ; Wed, 9 Jun 2021 11:19:39 +0000 (UTC) Received: from mm01.cs.columbia.edu (mm01.cs.columbia.edu [128.59.11.253]) by mail.kernel.org (Postfix) with ESMTP id B600C61364 for ; Wed, 9 Jun 2021 11:19:38 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B600C61364 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kvmarm-bounces@lists.cs.columbia.edu Received: from localhost (localhost [127.0.0.1]) by mm01.cs.columbia.edu (Postfix) with ESMTP id 41E4D49FB0; Wed, 9 Jun 2021 07:19:38 -0400 (EDT) X-Virus-Scanned: at lists.cs.columbia.edu Received: from mm01.cs.columbia.edu ([127.0.0.1]) by localhost (mm01.cs.columbia.edu [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id xs+QArL31wEX; Wed, 9 Jun 2021 07:19:37 -0400 (EDT) Received: from mm01.cs.columbia.edu (localhost [127.0.0.1]) by mm01.cs.columbia.edu (Postfix) with ESMTP id 0F44949E8A; Wed, 9 Jun 2021 07:19:37 -0400 (EDT) Received: from localhost (localhost [127.0.0.1]) by mm01.cs.columbia.edu (Postfix) with ESMTP id 8F16449E5F for ; Wed, 9 Jun 2021 07:19:36 -0400 (EDT) X-Virus-Scanned: at lists.cs.columbia.edu Received: from mm01.cs.columbia.edu ([127.0.0.1]) by localhost (mm01.cs.columbia.edu [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id 9BKuZwUNetpO for ; Wed, 9 Jun 2021 07:19:35 -0400 (EDT) Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by mm01.cs.columbia.edu (Postfix) with ESMTPS id 53FC840489 for ; Wed, 9 Jun 2021 07:19:35 -0400 (EDT) Received: from disco-boy.misterjones.org (disco-boy.misterjones.org [51.254.78.96]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 54E4361040; Wed, 9 Jun 2021 11:19:34 +0000 (UTC) Received: from 82-132-234-177.dab.02.net ([82.132.234.177] helo=why.misterjones.org) by disco-boy.misterjones.org with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.94.2) (envelope-from ) id 1lqwFA-006T4w-5o; Wed, 09 Jun 2021 12:19:32 +0100 Date: Wed, 09 Jun 2021 12:19:31 +0100 Message-ID: <874ke7z3ng.wl-maz@kernel.org> From: Marc Zyngier To: Steven Price Subject: Re: [PATCH v14 1/8] arm64: mte: Handle race when synchronising tags In-Reply-To: References: <20210607110816.25762-1-steven.price@arm.com> <20210607110816.25762-2-steven.price@arm.com> <875yynz5wp.wl-maz@kernel.org> User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue) FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/27.1 (x86_64-pc-linux-gnu) MULE/6.0 (HANACHIRUSATO) MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") X-SA-Exim-Connect-IP: 82.132.234.177 X-SA-Exim-Rcpt-To: steven.price@arm.com, catalin.marinas@arm.com, will@kernel.org, james.morse@arm.com, julien.thierry.kdev@gmail.com, suzuki.poulose@arm.com, kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Dave.Martin@arm.com, mark.rutland@arm.com, tglx@linutronix.de, qemu-devel@nongnu.org, quintela@redhat.com, dgilbert@redhat.com, richard.henderson@linaro.org, peter.maydell@linaro.org, Haibo.Xu@arm.com, drjones@redhat.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false Cc: "Dr. David Alan Gilbert" , qemu-devel@nongnu.org, Catalin Marinas , Juan Quintela , Richard Henderson , linux-kernel@vger.kernel.org, Dave Martin , linux-arm-kernel@lists.infradead.org, Thomas Gleixner , Will Deacon , kvmarm@lists.cs.columbia.edu X-BeenThere: kvmarm@lists.cs.columbia.edu X-Mailman-Version: 2.1.14 Precedence: list List-Id: Where KVM/ARM decisions are made List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: kvmarm-bounces@lists.cs.columbia.edu Sender: kvmarm-bounces@lists.cs.columbia.edu On Wed, 09 Jun 2021 11:51:34 +0100, Steven Price wrote: > > On 09/06/2021 11:30, Marc Zyngier wrote: > > On Mon, 07 Jun 2021 12:08:09 +0100, > > Steven Price wrote: > >> > >> mte_sync_tags() used test_and_set_bit() to set the PG_mte_tagged flag > >> before restoring/zeroing the MTE tags. However if another thread were to > >> race and attempt to sync the tags on the same page before the first > >> thread had completed restoring/zeroing then it would see the flag is > >> already set and continue without waiting. This would potentially expose > >> the previous contents of the tags to user space, and cause any updates > >> that user space makes before the restoring/zeroing has completed to > >> potentially be lost. > >> > >> Since this code is run from atomic contexts we can't just lock the page > >> during the process. Instead implement a new (global) spinlock to protect > >> the mte_sync_page_tags() function. > >> > >> Fixes: 34bfeea4a9e9 ("arm64: mte: Clear the tags when a page is mapped in user-space with PROT_MTE") > >> Reviewed-by: Catalin Marinas > >> Signed-off-by: Steven Price > >> --- > >> arch/arm64/kernel/mte.c | 20 +++++++++++++++++--- > >> 1 file changed, 17 insertions(+), 3 deletions(-) > >> > >> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c > >> index 125a10e413e9..a3583a7fd400 100644 > >> --- a/arch/arm64/kernel/mte.c > >> +++ b/arch/arm64/kernel/mte.c > >> @@ -25,6 +25,7 @@ > >> u64 gcr_kernel_excl __ro_after_init; > >> > >> static bool report_fault_once = true; > >> +static DEFINE_SPINLOCK(tag_sync_lock); > >> > >> #ifdef CONFIG_KASAN_HW_TAGS > >> /* Whether the MTE asynchronous mode is enabled. */ > >> @@ -34,13 +35,22 @@ EXPORT_SYMBOL_GPL(mte_async_mode); > >> > >> static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap) > >> { > >> + unsigned long flags; > >> pte_t old_pte = READ_ONCE(*ptep); > >> > >> + spin_lock_irqsave(&tag_sync_lock, flags); > > > > having though a bit more about this after an offline discussion with > > Catalin: why can't this lock be made per mm? We can't really share > > tags across processes anyway, so this is limited to threads from the > > same process. > > Currently there's nothing stopping processes sharing tags (mmap(..., > PROT_MTE, MAP_SHARED)) - I agree making use of this is tricky and it > would have been nice if this had just been prevented from the > beginning. I don't think it should be prevented. I think it should be made clear that it is unreliable and that it will result in tag corruption. > Given the above, clearly the lock can't be per mm and robust. I don't think we need to make it robust. The architecture actively prevents sharing if the tags are also shared, just like we can't really expect the VMM to share tags with the guest. > > I'd also like it to be documented that page sharing can only reliably > > work with tagging if only one of the mappings is using tags. > > I'm not entirely clear whether you mean "can only reliably work" to be > "is practically impossible to coordinate tag values", or whether you are > proposing to (purposefully) introduce the race with a per-mm lock? (and > document it). The latter. You can obviously communicate your tags to another task, but this should come with attached restrictions (mlock?). > I guess we could have a per-mm lock and handle the race if user space > screws up with the outcome being lost tags (double clear). > > But it feels to me like it could come back to bite in the future since > VM_SHARED|VM_MTE will almost always work and I fear someone will start > using it since it's permitted by the kernel. I'm really worried that performance is going to suck even on a small system, and this global lock will be heavily contended, even without considering KVM. Catalin, any though? M. -- Without deviation from the norm, progress is not possible. _______________________________________________ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm