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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id BB3B2C433F5 for ; Wed, 4 May 2022 16:41:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1353548AbiEDQpQ (ORCPT ); Wed, 4 May 2022 12:45:16 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46286 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S245372AbiEDQpO (ORCPT ); Wed, 4 May 2022 12:45:14 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 36464220E8 for ; Wed, 4 May 2022 09:41:38 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id C6DE06173B for ; Wed, 4 May 2022 16:41:37 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id AED16C385A5; Wed, 4 May 2022 16:41:35 +0000 (UTC) Date: Wed, 4 May 2022 17:41:32 +0100 From: Catalin Marinas To: Mark Rutland Cc: linux-arm-kernel@lists.infradead.org, akpm@linux-foundation.org, alex.popov@linux.com, keescook@chromium.org, linux-kernel@vger.kernel.org, luto@kernel.org, will@kernel.org Subject: Re: [PATCH v2 01/13] arm64: stackleak: fix current_top_of_stack() Message-ID: References: <20220427173128.2603085-1-mark.rutland@arm.com> <20220427173128.2603085-2-mark.rutland@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220427173128.2603085-2-mark.rutland@arm.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Apr 27, 2022 at 06:31:16PM +0100, Mark Rutland wrote: > Due to some historical confusion, arm64's current_top_of_stack() isn't > what the stackleak code expects. This could in theory result in a number > of problems, and practically results in an unnecessary performance hit. > We can avoid this by aligning the arm64 implementation with the x86 > implementation. > > The arm64 implementation of current_top_of_stack() was added > specifically for stackleak in commit: > > 0b3e336601b82c6a ("arm64: Add support for STACKLEAK gcc plugin") > > This was intended to be equivalent to the x86 implementation, but the > implementation, semantics, and performance characteristics differ > wildly: > > * On x86, current_top_of_stack() returns the top of the current task's > task stack, regardless of which stack is in active use. > > The implementation accesses a percpu variable which the x86 entry code > maintains, and returns the location immediately above the pt_regs on > the task stack (above which x86 has some padding). > > * On arm64 current_top_of_stack() returns the top of the stack in active > use (i.e. the one which is currently being used). > > The implementation checks the SP against a number of > potentially-accessible stacks, and will BUG() if no stack is found. > > The core stackleak_erase() code determines the upper bound of stack to > erase with: > > | if (on_thread_stack()) > | boundary = current_stack_pointer; > | else > | boundary = current_top_of_stack(); > > On arm64 stackleak_erase() is always called on a task stack, and > on_thread_stack() should always be true. On x86, stackleak_erase() is > mostly called on a trampoline stack, and is sometimes called on a task > stack. > > Currently, this results in a lot of unnecessary code being generated for > arm64 for the impossible !on_thread_stack() case. Some of this is > inlined, bloating stackleak_erase(), while portions of this are left > out-of-line and permitted to be instrumented (which would be a > functional problem if that code were reachable). > > As a first step towards improving this, this patch aligns arm64's > implementation of current_top_of_stack() with x86's, always returning > the top of the current task's stack. With GCC 11.1.0 this results in the > bulk of the unnecessary code being removed, including all of the > out-of-line instrumentable code. > > While I don't believe there's a functional problem in practice I've > marked this as a fix since the semantic was clearly wrong, the fix > itself is simple, and other code might rely upon this in future. > > Fixes: 0b3e336601b82c6a ("arm64: Add support for STACKLEAK gcc plugin") > Signed-off-by: Mark Rutland > Cc: Alexander Popov > Cc: Andrew Morton > Cc: Andy Lutomirski > Cc: Catalin Marinas > Cc: Kees Cook > Cc: Will Deacon I thought this was queued already but I couldn't find it in -next. So: Acked-by: Catalin Marinas