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 EDC19495E5; Fri, 21 Jun 2024 11:08:49 +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=1718968132; cv=none; b=of0F/bUxJ0GzS7Zx3zs5QAzttKYvm053EQ6/sJG1h7sTvuU3Y9NsOjlRxvuVhxZWrWijUAHh+xKxLQosrDY3WZtEIUbmFOMCVpvfQ+RQAij5lzSi7IYLf8xZNNQ8Rddpv0Pt1YXWSaTJuw8pPl7RDWrUP4LdlNRVsf+NDwnT5Yc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1718968132; c=relaxed/simple; bh=e9KZC2wJrvvAHjEOaRFoTQrElwaohbuI1Ox7UdSgKos=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=laFpNEFIi0y31Ixb4DHSHvXKsBA803eKw2TU5UXs0sZGjWm4Q94aWAe2q/aG/kQSkdLIUnxVGuPL6R7Klehrg9TICjgUFxl6h3yNQeOpylc8ZEAjnAUWNO0WgtiejluBqGRs8342+HsBbkXFk7MTMR4/Vwo2V+ZNbD0WPAojzAA= 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 11B64DA7; Fri, 21 Jun 2024 04:09:14 -0700 (PDT) Received: from J2N7QTR9R3 (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 73E0B3F6A8; Fri, 21 Jun 2024 04:08:45 -0700 (PDT) Date: Fri, 21 Jun 2024 12:08:42 +0100 From: Mark Rutland To: Kees Cook Cc: "liuyuntao (F)" , Arnd Bergmann , Catalin Marinas , Will Deacon , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Christian Borntraeger , Sven Schnelle , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , "Gustavo A. R. Silva" , Paul Walmsley , Palmer Dabbelt , Albert Ou , Leonardo Bras , Claudio Imbrenda , Pawan Gupta , linux-kernel@vger.kernel.org, x86@kernel.org, linux-arm-kernel@lists.infradead.org, linux-s390@vger.kernel.org, linux-hardening@vger.kernel.org, linux-riscv@lists.infradead.org Subject: Re: [PATCH] randomize_kstack: Remove non-functional per-arch entropy filtering Message-ID: References: <20240619214711.work.953-kees@kernel.org> <98381dbf-f14e-4b6c-8c96-fb6b97ed46e1@huawei.com> <202406201127.17CE526F0@keescook> Precedence: bulk X-Mailing-List: linux-s390@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <202406201127.17CE526F0@keescook> On Thu, Jun 20, 2024 at 11:34:22AM -0700, Kees Cook wrote: > On Thu, Jun 20, 2024 at 11:47:58AM +0800, liuyuntao (F) wrote: > > > > > > On 2024/6/20 5:47, Kees Cook wrote: > > > An unintended consequence of commit 9c573cd31343 ("randomize_kstack: > > > Improve entropy diffusion") was that the per-architecture entropy size > > > filtering reduced how many bits were being added to the mix, rather than > > > how many bits were being used during the offsetting. All architectures > > > fell back to the existing default of 0x3FF (10 bits), which will consume > > > at most 1KiB of stack space. It seems that this is working just fine, > > > so let's avoid the confusion and update everything to use the default. > > > > > > > My original intent was indeed to do this, but I regret that not being more > > explicit in the commit log.. > > > > Additionally, I've tested the stack entropy by applying the following patch, > > the result was `Bits of stack entropy: 7` on arm64, too. It does not seem to > > affect the entropy value, maybe removing it is OK, or there may be some > > nuances of your intentions that I've overlooked. > > > > --- a/include/linux/randomize_kstack.h > > +++ b/include/linux/randomize_kstack.h > > @@ -79,9 +79,7 @@ DECLARE_PER_CPU(u32, kstack_offset); > > #define choose_random_kstack_offset(rand) do { \ > > if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ > > &randomize_kstack_offset)) { \ > > - u32 offset = raw_cpu_read(kstack_offset); \ > > - offset = ror32(offset, 5) ^ (rand); \ > > - raw_cpu_write(kstack_offset, offset); \ > > + raw_cpu_write(kstack_offset, rand); \ > > } \ > > } while (0) > > #else /* CONFIG_RANDOMIZE_KSTACK_OFFSET */ > > I blame the multiple applications of the word "entropy" in this feature. :) > > So, there's both: > > - "how many bits CAN be randomized?" (i.e. within what range can all > possible stack offsets be?) > > and > > - "is the randomization predictable?" (i.e. is the distribution of > selected positions with the above range evenly distributed?) > > Commit 9c573cd31343 ("randomize_kstack: Improve entropy diffusion") was > trying to improve the latter, but accidentally also grew the former. > This patch is just trying to clean all this up now. > > Thanks for testing! And I'm curious as to why arm64's stack offset > entropy is 7 for you when we're expecting it to be 6. Anyway, that's not > a problem I don't think. Just a greater offset range than expected. Hmm.... I think this is due to the way the compiler aligns the stack in alloca(); it rounds up the value of KSTACK_OFFSET_MAX(offset) and ends up spilling over an additional bit (e.g. 0x3f1 to 0x3ff round up to 0x400). Looking at v6.10-rc4 defconfig + CONFIG_RANDOMIZE_STACKOFFSET=y, the disassembly for arm64's invoke_syscall() looks like: // offset = raw_cpu_read(kstack_offset) mov x4, sp adrp x0, kstack_offset mrs x5, tpidr_el1 add x0, x0, #:lo12:kstack_offset ldr w0, [x0, x5] // offset = KSTACK_OFFSET_MAX(offset) and x0, x0, #0x3ff // alloca(offset) add x0, x0, #0xf and x0, x0, #0x7f0 sub sp, x4, x0 ... which in C would be: offset = raw_cpu_read(kstack_offset) offset &= 0x3ff; // [0x0, 0x3ff] offset += 0xf; // [0xf, 0x40e] offset &= 0x7f0; // [0x0, ... so when *all* bits [3:0] are 0, they'll have no impact, and when *any* of bits [3:0] are 1 they'll trigger a carry into bit 4, which could ripple all the way up and spill into bit 10. I have no idea whether that's important. Kees, does that introduce a bias, and if so do we need to care? If I change the mask to discard the low bits: #define KSTACK_OFFSET_MAX(x) ((x) & 0x3F0) ... then the assembly avoids the rounding: mov x4, sp adrp x0, 0 mrs x5, tpidr_el1 add x0, x0, #:lo12:kstack_offset ldr w0, [x0, x5] and x0, x0, #0x3f0 sub sp, x4, x0 Mark.