From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 784A1398F97 for ; Fri, 5 Dec 2025 06:50:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764917403; cv=none; b=VboJwQ6z9mKrZrURVytPup8ZeV+cN0w4CsCbtBYdN7KOes2IbB0MVNnJ0G7q18onfXxrKZGbNo2fwEj/T+QMPEIGIzlgJZqW5ywFu/Taz1R/COW7CG9U0vUT6Y+Kqd7HNaulPAuKZHNyl/ox+TX7kMfMLgk8qTHjXwoX8fIOehQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764917403; c=relaxed/simple; bh=8aDf/vibDSHxCUkdQ9C98faaNXbC8BlaxlUd+loxUFc=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=nigaY7Wl8ZZgnaDdIbsT7WfaxT9oyaF54eDq/GpeyD6V5/s3xMH+6uvVffPycBKd4vT8AOsfeZsAYkDDIaYFe6vmKL4y6h7qTVBQUPc/CXK3KxaCdNIpDswCRiGSyVNjvmMPczcQQ5nO7LlEr8Vmdt5IZ3rfX14zPXVhoQiKINU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=j1wlC/C/; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="j1wlC/C/" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D3D9BC4CEF1; Fri, 5 Dec 2025 06:50:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1764917403; bh=8aDf/vibDSHxCUkdQ9C98faaNXbC8BlaxlUd+loxUFc=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=j1wlC/C/r2z4XDKAk81naayn2F7xuj+JAxD8IRNaFn916/aFnI1qxuqaQ6sbo9gi0 N1Wb4WpgfTAVfGa0AKq4NqnPr33yB29WPlHRJaYngUt3Q6VBLHq5XiZhebKtqS78Nk 9VgeStuzidbhdTtMfX5rUjPXnwOv93GTZnnG941nI3zAMjNe0h8TJSuxCK2OlHV0ES 3hYydzxTPNSzTjMfoAQ5TAtCddqrFR/0BBOX2O4mM4hk9ZIoH9g6r/bFERktqi4lLf k78xt0DLmvZDGxHvdngMHSdbFc7ZrNIqrHUrUgVZtgbGlYDlB80NByz1yhux3qFqD+ g1KyhyuozXUGg== Date: Thu, 4 Dec 2025 22:48:09 -0800 From: Eric Biggers To: Ard Biesheuvel Cc: linux-arm-kernel@lists.infradead.org, linux-crypto@vger.kernel.org, Will Deacon , Catalin Marinas , Kees Cook , Justin Stitt Subject: Re: [PATCH] arm64/simd: Avoid pointless clearing of FP/SIMD buffer Message-ID: <20251205064809.GA26371@sol> References: <20251204162815.522879-2-ardb@kernel.org> Precedence: bulk X-Mailing-List: linux-crypto@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: <20251204162815.522879-2-ardb@kernel.org> On Thu, Dec 04, 2025 at 05:28:15PM +0100, Ard Biesheuvel wrote: > The buffer provided to kernel_neon_begin() is only used if the task is > scheduled out while the FP/SIMD is in use by the kernel, or when such a > section is interrupted by a softirq that also uses the FP/SIMD. > > IOW, this happens rarely, and even if it happened often, there is still > no reason for this buffer to be cleared beforehand, which happens by > default when using a compiler that supports -ftrivial-auto-var-init. > > So mark the buffer as __uninitialized. Given that this is a variable > attribute not a type attribute, this requires that the expression is > tweaked a bit. > > Cc: Will Deacon , > Cc: Catalin Marinas , > Cc: Kees Cook > Cc: Eric Biggers > Cc: Justin Stitt > Signed-off-by: Ard Biesheuvel > --- > arch/arm64/include/asm/simd.h | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > The issue here is that returning a pointer to an automatic variable as > it goes out of scope is slightly dodgy, especially in the context of > __attribute__((cleanup())), on which the scoped guard API relies > heavily. However, in this case it should be safe, given that this > expression is the input to the guarded variable type's constructor. > > It is definitely not pretty, though, so hopefully here is a better way > to attach this. > > diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h > index 0941f6f58a14..825b7fe94003 100644 > --- a/arch/arm64/include/asm/simd.h > +++ b/arch/arm64/include/asm/simd.h > @@ -48,6 +48,7 @@ DEFINE_LOCK_GUARD_1(ksimd, > kernel_neon_begin(_T->lock), > kernel_neon_end(_T->lock)) > > -#define scoped_ksimd() scoped_guard(ksimd, &(struct user_fpsimd_state){}) > +#define scoped_ksimd() \ > + scoped_guard(ksimd, ({ struct user_fpsimd_state __uninitialized s; &s; })) Ick. I should have looked at the generated code more closely. It's actually worse than you describe, because the zeroing is there even without CONFIG_INIT_STACK_ALL_ZERO=y, simply because the user_fpsimd_state struct is declared using a compound literal. I'm afraid that this patch probably isn't a good idea, as it relies on undefined behavior. Before this patch, the user_fpsimd_state is declared using a compound literal, which takes on its enclosing scope, i.e. the 'for' statement generated by scoped_guard(). After this patch, it's in a new inner scope, and the pointer to it escapes from it. Unfortunately I don't think there's any way to solve this while keeping the scoped_ksimd() API as-is. Best I can come up with is to leave it to the callers to allocate the state, and then use scoped_guard() similar to a regular lock: struct user_fpsimd_state __uninitialized fpsimd_state; scoped_guard(ksimd, &fpsimd_state) foo_neon(...) Maybe wrap the state declaration with a macro: DECLARE_FPSIMD_STATE_ONSTACK(fpsimd_state); - Eric