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 Received: from lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 5957EEE57F8 for ; Thu, 12 Sep 2024 12:49:18 +0000 (UTC) Received: from boromir.ozlabs.org (localhost [127.0.0.1]) by lists.ozlabs.org (Postfix) with ESMTP id 4X4HM0741Dz2y8B; Thu, 12 Sep 2024 22:49:16 +1000 (AEST) Authentication-Results: lists.ozlabs.org; arc=none smtp.remote-ip=217.140.110.172 ARC-Seal: i=1; a=rsa-sha256; d=lists.ozlabs.org; s=201707; t=1726145356; cv=none; b=J0U67eXuEHaqP9oq/mSlt/ukzfLirpsWBw1+RBip8pQk/hewkMkCf9e7E/tyWqJFxML0X60ZX34xq6jHDlebG4chw4fjqW9qEmC0T0ZFyGftHoNj844X7DjBowM5iLSfLHdskhiHSSxqWpIEBmhKa2JgbT2FoD8yKMjlVj/qk5e/r74esJzS+HByv/S70rVSnghg/VOive2CfTePSSYAynV4AlOkumNkCCifKqpkVJTDiU5QfkzstS1F0o1KEeV9I5gMOofULxwgbWSn1OdwtH+lJZbm1xLHdc4GRSEQh2TeDR7MO7icXKs2sml/JsbFGyEFRYjviOemTmta7aNG6Q== ARC-Message-Signature: i=1; a=rsa-sha256; d=lists.ozlabs.org; s=201707; t=1726145356; c=relaxed/relaxed; bh=rLBLI5dVrkWWSsMh7JaRkvOf4Z8ws21UnPX+021OswE=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=BrdEm11TuF/oAZRKPm3FWLwLGI8yx9RkyuK6m4zCwBKmZeQN0KhgfliPJVR+QlF8UKhQGb0qM4crNrCSlGe/xzG5a9beLUiMygwsHhA9tDMB4YMsOwuNKOvIQZAHakTaYg9KZwoUsIjIwb7XZc/IoDHdxsw32WR9KNIDnlSX/tyyiI/0gO0Y10BkFQ+bjWuHT8hPdvFRf1YIoh++/1MtwTPxJOuGGPwX8Q0BmcvfBvvNiJzHTu1rzq93YWCUtx85i+wi1BrbWOJUT60qQSl8qaaVRY32Su7409FKE2E22wEGovVgOEaUobLLrHniSxHf8j0qRN9VDPs/vBrbpwwG7Q== ARC-Authentication-Results: i=1; lists.ozlabs.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass (client-ip=217.140.110.172; helo=foss.arm.com; envelope-from=joey.gouly@arm.com; receiver=lists.ozlabs.org) smtp.mailfrom=arm.com Authentication-Results: lists.ozlabs.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=arm.com (client-ip=217.140.110.172; helo=foss.arm.com; envelope-from=joey.gouly@arm.com; receiver=lists.ozlabs.org) Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lists.ozlabs.org (Postfix) with ESMTP id 4X4HM04CWzz2xPf for ; Thu, 12 Sep 2024 22:49:16 +1000 (AEST) 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 96090DA7; Thu, 12 Sep 2024 05:49:14 -0700 (PDT) Received: from e124191.cambridge.arm.com (e124191.cambridge.arm.com [10.1.197.45]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 04ED03F73B; Thu, 12 Sep 2024 05:48:40 -0700 (PDT) Date: Thu, 12 Sep 2024 13:48:35 +0100 From: Joey Gouly To: Will Deacon Cc: Dave Hansen , Kevin Brodsky , linux-arm-kernel@lists.infradead.org, nd@arm.com, akpm@linux-foundation.org, aneesh.kumar@kernel.org, aneesh.kumar@linux.ibm.com, anshuman.khandual@arm.com, bp@alien8.de, broonie@kernel.org, catalin.marinas@arm.com, christophe.leroy@csgroup.eu, dave.hansen@linux.intel.com, hpa@zytor.com, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org, maz@kernel.org, mingo@redhat.com, mpe@ellerman.id.au, naveen.n.rao@linux.ibm.com, npiggin@gmail.com, oliver.upton@linux.dev, shuah@kernel.org, skhan@linuxfoundation.org, szabolcs.nagy@arm.com, tglx@linutronix.de, x86@kernel.org, kvmarm@lists.linux.dev, linux-kselftest@vger.kernel.org Subject: Re: [PATCH v5 06/30] arm64: context switch POR_EL0 register Message-ID: <20240912124835.GA1220495@e124191.cambridge.arm.com> References: <20240822151113.1479789-1-joey.gouly@arm.com> <20240822151113.1479789-7-joey.gouly@arm.com> <425b8f8c-b6b5-422a-b5f4-41dd2d1ae3bb@arm.com> <6c8ad091-a56b-41ba-b403-2e3c2e578100@intel.com> <20240912105017.GA22788@willie-the-truck> X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Post: List-Subscribe: , , List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240912105017.GA22788@willie-the-truck> On Thu, Sep 12, 2024 at 11:50:18AM +0100, Will Deacon wrote: > Hi Dave, > > On Wed, Sep 11, 2024 at 08:33:54AM -0700, Dave Hansen wrote: > > On 9/11/24 08:01, Kevin Brodsky wrote: > > > On 22/08/2024 17:10, Joey Gouly wrote: > > >> @@ -371,6 +382,9 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args) > > >> if (system_supports_tpidr2()) > > >> p->thread.tpidr2_el0 = read_sysreg_s(SYS_TPIDR2_EL0); > > >> > > >> + if (system_supports_poe()) > > >> + p->thread.por_el0 = read_sysreg_s(SYS_POR_EL0); > > > Here we are only reloading POR_EL0's value if the target is a user > > > thread. However, as this series stands, POR_EL0 is also relevant to > > > kthreads, because any uaccess or GUP done from a kthread will also be > > > checked against POR_EL0. This is especially important in cases like the > > > io_uring kthread, which accesses the memory of the user process that > > > spawned it. To prevent such a kthread from inheriting a stale value of > > > POR_EL0, it seems that we should reload POR_EL0's value in all cases > > > (user and kernel thread). > > > > The problem with this is trying to figure out which POR_EL0 to use. The > > kthread could have been spawned ages ago and might not have a POR_EL0 > > which is very different from the current value of any of the threads in > > the process right now. > > > > There's also no great way for a kthread to reach out and grab an updated > > value. It's all completely inherently racy. > > > > > Other approaches could also be considered (e.g. resetting POR_EL0 to > > > unrestricted when creating a kthread), see my reply on v4 [1]. > > > > I kinda think this is the only way to go. It's the only sensible, > > predictable way. I _think_ it's what x86 will end up doing with PKRU, > > but there's been enough churn there that I'd need to go double check > > what happens in practice. > > I agree. > > > Either way, it would be nice to get an io_uring test in here that > > actually spawns kthreads: > > > > tools/testing/selftests/mm/protection_keys.c > > It would be good to update Documentation/core-api/protection-keys.rst > as well, since the example with read() raises more questions than it > answers! > > Kevin, Joey -- I've got this series queued in arm64 as-is, so perhaps > you could send some patches on top so we can iron this out in time for > 6.12? I'll also be at LPC next week if you're about. I found the code in arch/x86 that does this, I must have missed this previously. arch/x86/kernel/process.c: int copy_thread() /* Kernel thread ? */ if (unlikely(p->flags & PF_KTHREAD)) { p->thread.pkru = pkru_get_init_value(); memset(childregs, 0, sizeof(struct pt_regs)); kthread_frame_init(frame, args->fn, args->fn_arg); return 0; } I can send a similar patch for arm64. I have no idea how to write io_uring code, so looking for examples I can work with to get a test written. Might just send the arm64 fix first, if that's fine? Thanks, Joey