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 801D8C4321E for ; Sun, 8 May 2022 19:04:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240651AbiEHTIY (ORCPT ); Sun, 8 May 2022 15:08:24 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59882 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237753AbiEHR2e (ORCPT ); Sun, 8 May 2022 13:28:34 -0400 Received: from mail-ed1-f50.google.com (mail-ed1-f50.google.com [209.85.208.50]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4F879DED0 for ; Sun, 8 May 2022 10:24:43 -0700 (PDT) Received: by mail-ed1-f50.google.com with SMTP id ba17so13848788edb.5 for ; Sun, 08 May 2022 10:24:43 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:message-id:date:mime-version:user-agent:reply-to :subject:content-language:to:cc:references:from:in-reply-to :content-transfer-encoding; bh=wqd1pfOpFBNim3pwEebs2UZOYdL6NZjjBjoV+dIe6iY=; b=Fa7QmJcNo9SbT+IHpWFUIRIB7pilgr4sqMl/rPj/8sa13GAJqZBvJG9GreTTzTPIeF pn1bId3gtRSSVKeShFNhmACtM2Wje1WDxpiAAoH40mQ1zGCsMa4R3y9YyIQciMA6HaPe 7uMxltjyniEsTBRCfqcN8+Q0GwGNABCT9TM6X8WujSYuXiZN1jER0SqMGHxNeobdFmp3 VqNHYJNhNw9yjS2OZJqFqrjuhtm/IcpliGZslXH4SaStId0c8oWJuPNh+9BdJXMbdJWU YH4ddX2JnDgCqVFBU6Y6xFaEvjUD1tJ8YzvFGfdMtXsJElj6uqd3wzXquEUdqypOJ5Kx t/WQ== X-Gm-Message-State: AOAM531lSrJBBhNuLlIUi2GSWuM1QfYMTOPDv8PGwjjIgI/K4cBjPP5t 8AMJEEJVoYAfcA5Uo130Xy0= X-Google-Smtp-Source: ABdhPJyo/rfuIQKNUAXvvILf5E3b882Jcte6XFXtzxTuyZm+ayghbOJg1YOFA1rXbKlUpGy3fKe2Nw== X-Received: by 2002:aa7:ca15:0:b0:428:3259:984a with SMTP id y21-20020aa7ca15000000b004283259984amr13826192eds.59.1652030681735; Sun, 08 May 2022 10:24:41 -0700 (PDT) Received: from [10.9.0.34] ([46.166.128.205]) by smtp.gmail.com with ESMTPSA id qs24-20020a170906459800b006f3ef214e66sm4143967ejc.204.2022.05.08.10.24.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 08 May 2022 10:24:41 -0700 (PDT) Message-ID: <3d65baac-93b6-7f21-1bf6-9b17d1fce843@linux.com> Date: Sun, 8 May 2022 20:24:38 +0300 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.7.0 Reply-To: alex.popov@linux.com Subject: Re: [PATCH v2 01/13] arm64: stackleak: fix current_top_of_stack() Content-Language: en-US To: Mark Rutland , linux-arm-kernel@lists.infradead.org Cc: akpm@linux-foundation.org, catalin.marinas@arm.com, keescook@chromium.org, linux-kernel@vger.kernel.org, luto@kernel.org, will@kernel.org References: <20220427173128.2603085-1-mark.rutland@arm.com> <20220427173128.2603085-2-mark.rutland@arm.com> From: Alexander Popov In-Reply-To: <20220427173128.2603085-2-mark.rutland@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Mark! On 27.04.2022 20:31, 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. As I could understand, for arm64, calling stackleak_erase() not from the thread stack would bring troubles because current_top_of_stack() would return an unexpected address from a foreign stack. Is this correct? But this bug doesn't happen because arm64 always calls stackleak_erase() from the current thread stack. Right? > 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). Sorry, I didn't understand this part about instrumentation. Could you elaborate please? > 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 > --- > arch/arm64/include/asm/processor.h | 10 ++++------ > 1 file changed, 4 insertions(+), 6 deletions(-) > > diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h > index 73e38d9a540ce..6b1a12c23fe77 100644 > --- a/arch/arm64/include/asm/processor.h > +++ b/arch/arm64/include/asm/processor.h > @@ -381,12 +381,10 @@ long get_tagged_addr_ctrl(struct task_struct *task); > * of header definitions for the use of task_stack_page. > */ > > -#define current_top_of_stack() \ > -({ \ > - struct stack_info _info; \ > - BUG_ON(!on_accessible_stack(current, current_stack_pointer, 1, &_info)); \ > - _info.high; \ > -}) > +/* > + * The top of the current task's task stack > + */ > +#define current_top_of_stack() ((unsigned long)current->stack + THREAD_SIZE) > #define on_thread_stack() (on_task_stack(current, current_stack_pointer, 1, NULL)) > > #endif /* __ASSEMBLY__ */