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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id x10sm373971pgq.79.2020.04.20.15.34.58 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 20 Apr 2020 15:34:58 -0700 (PDT) Date: Mon, 20 Apr 2020 15:34:57 -0700 From: Kees Cook To: Will Deacon Cc: Thomas Gleixner , Elena Reshetova , x86@kernel.org, Andy Lutomirski , Peter Zijlstra , Catalin Marinas , Mark Rutland , Alexander Potapenko , Ard Biesheuvel , Jann Horn , "Perla, Enrico" , kernel-hardening@lists.openwall.com, linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 5/5] arm64: entry: Enable random_kstack_offset support Message-ID: <202004201529.BB787BB@keescook> References: <20200324203231.64324-1-keescook@chromium.org> <20200324203231.64324-6-keescook@chromium.org> <20200420205458.GC29998@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200420205458.GC29998@willie-the-truck> X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Apr 20, 2020 at 09:54:58PM +0100, Will Deacon wrote: > On Tue, Mar 24, 2020 at 01:32:31PM -0700, Kees Cook wrote: > > + /* > > + * Since the compiler chooses a 4 bit alignment for the stack, > > + * let's save one additional bit (9 total), which gets us up > > + * near 5 bits of entropy. > > + */ > > + choose_random_kstack_offset(get_random_int() & 0x1FF); > > Hmm, this comment doesn't make any sense to me. I mean, I get that 0x1ff > is 9 bits, and that is 4+5 but so what? Er, well, yes. I guess I was just trying to explain why there were 9 bits saved here and to document what I was seeing the compiler actually doing with the values. (And it serves as a comparison to the x86 comment which is explaining similar calculations in the face of x86_64 vs ia32.) Would something like this be better? /* * Since the compiler uses 4 bit alignment for the stack (1 more than * x86_64), let's try to match the 5ish-bit entropy seen in x86_64, * instead of having needlessly lower entropy. As a result, keep the * low 9 bits. */ -- Kees Cook