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 9F3C5198A2A; Fri, 16 Aug 2024 11:09:09 +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=1723806549; cv=none; b=jpWFsIxNp1EVeBZVfnmVtTbyhwtN16PYvkyCXzFHN4wFN0m+Xjxkl8V+z+fiUVx95EWoWp0CMBpZc0MQyj3zQKrGeqqux1KtN9/rwA5QcM7gMYMcm1H9C+3Rd0IJVK9xBz08z0+RletBLcnkcv6ZwqnlcA5haMXqcrxu+f04Kt4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1723806549; c=relaxed/simple; bh=FbLmrmMC1NtCf5/c0O+HnpWKmt75tXsuzNE9sTnkVfc=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=ZoOhM5itibErt6YVB5mv66wk0jZn/E0DSI89NG3XZ/zYb+i8xSUqgO9aMFkqobwcbYc6gz4l5vCHCSa3N2cKi5VaQ0x3Gr9rFi5C0RwazeeUVvv6sltIlYkRQDLTxP7qwQm7VrI4qiQqUg+8r9hXfWsGrRzdOmcIiYphhGK2gns= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 Received: by smtp.kernel.org (Postfix) with ESMTPSA id CB36CC32782; Fri, 16 Aug 2024 11:09:03 +0000 (UTC) Date: Fri, 16 Aug 2024 12:09:01 +0100 From: Catalin Marinas To: Mark Brown Cc: Will Deacon , Jonathan Corbet , Andrew Morton , Marc Zyngier , Oliver Upton , James Morse , Suzuki K Poulose , Arnd Bergmann , Oleg Nesterov , Eric Biederman , Shuah Khan , "Rick P. Edgecombe" , Deepak Gupta , Ard Biesheuvel , Szabolcs Nagy , Kees Cook , "H.J. Lu" , Paul Walmsley , Palmer Dabbelt , Albert Ou , Florian Weimer , Christian Brauner , Thiago Jung Bauermann , Ross Burton , linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org, kvmarm@lists.linux.dev, linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org Subject: Re: [PATCH v10 05/40] arm64/gcs: Document the ABI for Guarded Control Stacks Message-ID: References: <20240801-arm64-gcs-v10-0-699e2bd2190b@kernel.org> <20240801-arm64-gcs-v10-5-699e2bd2190b@kernel.org> Precedence: bulk X-Mailing-List: linux-fsdevel@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: <20240801-arm64-gcs-v10-5-699e2bd2190b@kernel.org> On Thu, Aug 01, 2024 at 01:06:32PM +0100, Mark Brown wrote: > +1. General > +----------- [...] > +* EL0 GCS entries with bit 63 set are reserved for use, one such use is defined Maybe "reserved for specific uses". The proposed sentenced feels like it's missing something. > + below for signals and should be ignored when parsing the stack if not > + understood. [...] > +3. Allocation of Guarded Control Stacks > +---------------------------------------- > + > +* When GCS is enabled for a thread a new Guarded Control Stack will be > + allocated for it of size RLIMIT_STACK or 2 gigabytes, whichever is > + smaller. > + > +* When a new thread is created by a thread which has GCS enabled then a > + new Guarded Control Stack will be allocated for the new thread with > + half the size of the standard stack. Is the half size still the case? It also seems a bit inconsistent to have RLIMIT_STACK when GCS is enabled and half the stack size when a new thread is created. [...] > +* When a thread is freed the Guarded Control Stack initially allocated for > + that thread will be freed. Note carefully that if the stack has been > + switched this may not be the stack currently in use by the thread. Is this true for shadow stacks explicitly allocated by the user with map_shadow_stack()? > +4. Signal handling > +-------------------- > + > +* A new signal frame record gcs_context encodes the current GCS mode and > + pointer for the interrupted context on signal delivery. This will always > + be present on systems that support GCS. > + > +* The record contains a flag field which reports the current GCS configuration > + for the interrupted context as PR_GET_SHADOW_STACK_STATUS would. > + > +* The signal handler is run with the same GCS configuration as the interrupted > + context. > + > +* When GCS is enabled for the interrupted thread a signal handling specific > + GCS cap token will be written to the GCS, this is an architectural GCS cap > + token with bit 63 set and the token type (bits 0..11) all clear. The > + GCSPR_EL0 reported in the signal frame will point to this cap token. > + > +* The signal handler will use the same GCS as the interrupted context. I assume this is true even with sigaltstack. Not easy to have alternative shadow stack without additional ABI. -- Catalin