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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id v42sm11945357pgn.6.2020.03.31.07.25.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 31 Mar 2020 07:25:21 -0700 (PDT) Date: Mon, 30 Mar 2020 11:27:19 -0700 From: Kees Cook To: Mark Rutland Cc: Thomas Gleixner , Elena Reshetova , x86@kernel.org, Andy Lutomirski , Peter Zijlstra , Catalin Marinas , Will Deacon , 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 3/5] stack: Optionally randomize kernel stack offset each syscall Message-ID: <202003301122.354B722@keescook> References: <20200324203231.64324-1-keescook@chromium.org> <20200324203231.64324-4-keescook@chromium.org> <20200330112536.GD1309@C02TD0UTHF1T.local> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200330112536.GD1309@C02TD0UTHF1T.local> 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, Mar 30, 2020 at 12:25:36PM +0100, Mark Rutland wrote: > On Tue, Mar 24, 2020 at 01:32:29PM -0700, Kees Cook wrote: > > +/* > > + * Do not use this anywhere else in the kernel. This is used here because > > + * it provides an arch-agnostic way to grow the stack with correct > > + * alignment. Also, since this use is being explicitly masked to a max of > > + * 10 bits, stack-clash style attacks are unlikely. For more details see > > + * "VLAs" in Documentation/process/deprecated.rst > > + */ > > +void *__builtin_alloca(size_t size); > > + > > +#define add_random_kstack_offset() do { \ > > + if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ > > + &randomize_kstack_offset)) { \ > > + u32 offset = this_cpu_read(kstack_offset); \ > > + char *ptr = __builtin_alloca(offset & 0x3FF); \ > > + asm volatile("" : "=m"(*ptr)); \ > > Is this asm() a homebrew OPTIMIZER_HIDE_VAR(*ptr)? If the asm > constraints generate metter code, could we add those as alternative > constraints in OPTIMIZER_HIDE_VAR() ? Er, no, sorry, not the same. I disassembled the wrong binary. :) With asm volatile("" : "=m"(*ptr)) ffffffff810038bc: 48 8d 44 24 0f lea 0xf(%rsp),%rax ffffffff810038c1: 48 83 e0 f0 and $0xfffffffffffffff0,%rax With __asm__ ("" : "=r" (var) : "0" (var)) ffffffff810038bc: 48 8d 54 24 0f lea 0xf(%rsp),%rdx ffffffff810038c1: 48 83 e2 f0 and $0xfffffffffffffff0,%rdx ffffffff810038c5: 0f b6 02 movzbl (%rdx),%eax ffffffff810038c8: 88 02 mov %al,(%rdx) It looks like OPTIMIZER_HIDE_VAR() is basically just: var = var; In the former case, we avoid the write and retain the allocation. So I think don't think OPTIMIZER_HIDE_VAR() should be used here, nor should OPTIMIZER_HIDE_VAR() be changed to remove the "0" (var) bit. -- Kees Cook