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Fri, 13 Sep 2024 15:15:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1726240508; bh=hG0+mr6n6c2AOqquKXji3PgaSzohcst94Wj42JS1S+I=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=BWnylCm4mReU8AS0leB+Zu+H/60Ov75IX+4RcMT07bw/NSlFUtgczPE92soGj0A0q 542x8M8eWvcVYa3w8j1pJ4wodResMwstCU9WRWL+4Bv7klLpSkpILEqOXkK2xTbvjI k7LO3FcSRX+QQPQ85stGEPkaCLs/93PocVVcAqwc97TkLzlGfXUTiDkvoa/W31NhS8 ZTsEo1Zfjv/PTpTlD075NPfKSAhzSd7lUsWT3ZAvYgMWVY06K4V/tyQXnFNxB9zsX0 tss/gEKf+QaNU/wq7UCRsBMZMRdCi4O4eV+W/SoX8gfoYY4bCaUnaRFI3uHHSOXLks 8MndKUMe6veNw== Date: Fri, 13 Sep 2024 16:14:59 +0100 From: Will Deacon To: Joey Gouly 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: <20240913151459.GA24129@willie-the-truck> 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> <20240912124835.GA1220495@e124191.cambridge.arm.com> 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: <20240912124835.GA1220495@e124191.cambridge.arm.com> User-Agent: Mutt/1.10.1 (2018-07-13) On Thu, Sep 12, 2024 at 01:48:35PM +0100, Joey Gouly wrote: > On Thu, Sep 12, 2024 at 11:50:18AM +0100, Will Deacon wrote: > > 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? I think fix + documentation is what we need before 6.12, but you've still got plenty of time after the merge window. Cheers, Will