From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 5F3441E0DD6 for ; Mon, 21 Oct 2024 10:06:33 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1729505195; cv=none; b=Dp1Yos2qemTiG1pcVw1kVQzVZkxiZcfoEQOXDYRvgdyMSMRozR0H0rHMdA2Udfr+S3h/0Z5lyGp/QBEf9jdp2zyEXjFdOujaIaC0jYeq24yXFvFM88YuYbaIZyMHtk5vaBfNa7z3IYQRL2jiVHrsXNByOR3J3PHMhDCyH4TQBNg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1729505195; c=relaxed/simple; bh=5yxIvM/q7hEra3NiLu1dppB10SS3WKaWw2cNVD3BDUI=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=sY0eJn8Jjh0fFkwu0SdXclJ+TcWw8rGx6hPgQbxLraaXQdgV43rcda07ZtvOEg7d6zaWy/Hliq3RjDbzeh7seg0JhL3qG25X+5hr4K6SWB0WYA5B7ohQcqDWZxVDq59VUs2lfw4XvloIV0ccOkCzufODM5J8HNfQgutMnTOBgjo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com 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 72015DA7; Mon, 21 Oct 2024 03:07:02 -0700 (PDT) Received: from [10.57.65.82] (unknown [10.57.65.82]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 72E003F73B; Mon, 21 Oct 2024 03:06:28 -0700 (PDT) Message-ID: <627c1297-8151-43d1-b46b-a962301b18fa@arm.com> Date: Mon, 21 Oct 2024 12:06:25 +0200 Precedence: bulk X-Mailing-List: linux-kselftest@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/5] Improve arm64 pkeys handling in signal delivery To: Dave Martin Cc: linux-arm-kernel@lists.infradead.org, akpm@linux-foundation.org, anshuman.khandual@arm.com, aruna.ramakrishna@oracle.com, broonie@kernel.org, catalin.marinas@arm.com, dave.hansen@linux.intel.com, jeffxu@chromium.org, joey.gouly@arm.com, shuah@kernel.org, will@kernel.org, linux-kselftest@vger.kernel.org, x86@kernel.org, pierre.langlois@arm.com References: <20241017133909.3837547-1-kevin.brodsky@arm.com> Content-Language: en-GB From: Kevin Brodsky In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 17/10/2024 17:48, Dave Martin wrote: > On Thu, Oct 17, 2024 at 02:39:04PM +0100, Kevin Brodsky wrote: >> This series is a follow-up to Joey's Permission Overlay Extension (POE) >> series [1] that recently landed on mainline. The goal is to improve the >> way we handle the register that governs which pkeys/POIndex are >> accessible (POR_EL0) during signal delivery. As things stand, we may >> unexpectedly fail to write the signal frame on the stack because POR_EL0 >> is not reset before the uaccess operations. See patch 3 for more details >> and the main changes this series brings. >> >> A similar series landed recently for x86/MPK [2]; the present series >> aims at aligning arm64 with x86. Worth noting: once the signal frame is >> written, POR_EL0 is still set to POR_EL0_INIT, granting access to pkey 0 >> only. This means that a program that sets up an alternate signal stack >> with a non-zero pkey will need some assembly trampoline to set POR_EL0 >> before invoking the real signal handler, as discussed here [3]. > This feels a bit bogus (though it's anyway orthogonal to this series). I'm not very fond of this either. However I believe this is the correct first step: bring arm64 in line with x86. Removing all restrictions before uaccess and then setting POR_EL0 to POR_EL0_INIT enables userspace to use any pkey for the alternate signal stack without an ABI change, albeit not in a very comfortable way (if the pkey is not 0). > Really, we want some way for userspace to tell the kernel what > permissions to use for the alternate signal stack and signal handlers > using it, and then honour that request consistently (just as we try to > do for the main stack today). > > ss_flags is mostly unused... I wonder whether we could add something in > there? Or add a sigaltstack2()? Yes, this would be sensible as a second step (backwards-compatible extension). Exactly how that API would look like is not trivial though: is the pkey implicitly derived from the pointer provided to sigaltstack()? Is there a need to specify another pkey for code, or do we just assume that the signal handler is only using code with pkey 0? (Not a concern on x86 as MPK doesn't restrict execution.) Would be very interested to hear opinions on this. Kevin