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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id c1-20020a056a00008100b0050dc7628202sm270483pfj.220.2022.05.10.20.00.38 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 10 May 2022 20:00:39 -0700 (PDT) Date: Tue, 10 May 2022 20:00:38 -0700 From: Kees Cook To: Mark Rutland Cc: Alexander Popov , linux-arm-kernel@lists.infradead.org, akpm@linux-foundation.org, catalin.marinas@arm.com, linux-kernel@vger.kernel.org, luto@kernel.org, will@kernel.org Subject: Re: [PATCH v2 03/13] stackleak: remove redundant check Message-ID: <202205101958.2A33DE20@keescook> References: <20220427173128.2603085-1-mark.rutland@arm.com> <20220427173128.2603085-4-mark.rutland@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, May 10, 2022 at 12:46:48PM +0100, Mark Rutland wrote: > On Sun, May 08, 2022 at 09:17:01PM +0300, Alexander Popov wrote: > > On 27.04.2022 20:31, Mark Rutland wrote: > > > In __stackleak_erase() we check that the `erase_low` value derived from > > > `current->lowest_stack` is above the lowest legitimate stack pointer > > > value, but this is already enforced by stackleak_track_stack() when > > > recording the lowest stack value. > > > > > > Remove the redundant check. > > > > > > There should be no functional change as a result of this patch. > > > > Mark, I can't agree here. I think this check is important. > > The performance profit from dropping it is less than the confidence decrease :) > > > > With this check, if the 'lowest_stack' value is corrupted, stackleak doesn't > > overwrite some wrong kernel memory, but simply clears the whole thread > > stack, which is safe behavior. > > If you feel strongly about it, I can restore the check, but I struggle to > believe that it's worthwhile. The `lowest_stack` value lives in the > task_struct, and if you have the power to corrupt that you have the power to do > much more interesting things. > > If we do restore it, I'd like to add a big fat comment explaining the > rationale (i.e. that it only matter if someone could corrupt > `current->lowest_stack`, as otherwise that's guarnateed to be within bounds). Yeah, let's restore it and add the comment. While I do agree it's likely that such an corruption would likely mean an attacker had significant control over kernel memory already, it is not uncommon that an attack only has a limited index from a given address, etc. Or some manipulation is possible via weird gadgets, etc. It's unlikely, but not impossible, and a bounds-check for that value is cheap compared to the rest of the work happening. :) -Kees -- Kees Cook