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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id mm12sm196929pjb.49.2021.03.29.11.41.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 29 Mar 2021 11:41:20 -0700 (PDT) Date: Mon, 29 Mar 2021 11:41:18 -0700 From: Kees Cook To: Thomas Gleixner Cc: Elena Reshetova , x86@kernel.org, Andy Lutomirski , Peter Zijlstra , Catalin Marinas , Will Deacon , Mark Rutland , Alexander Potapenko , Alexander Popov , Ard Biesheuvel , Jann Horn , Vlastimil Babka , David Hildenbrand , Mike Rapoport , Andrew Morton , Jonathan Corbet , Randy Dunlap , kernel-hardening@lists.openwall.com, linux-hardening@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v7 3/6] stack: Optionally randomize kernel stack offset each syscall Message-ID: <202103291139.54AA7CDE@keescook> References: <20210319212835.3928492-1-keescook@chromium.org> <20210319212835.3928492-4-keescook@chromium.org> <87eefzcpc4.ffs@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87eefzcpc4.ffs@nanos.tec.linutronix.de> X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 94FE55001EA0 X-Stat-Signature: zfmsqb9xudkmaw354b7b7tzt5ob84w94 Received-SPF: none (chromium.org>: No applicable sender policy available) receiver=imf01; identity=mailfrom; envelope-from=""; helo=mail-pf1-f179.google.com; client-ip=209.85.210.179 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1617043281-3066 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 Sun, Mar 28, 2021 at 04:42:03PM +0200, Thomas Gleixner wrote: > On Fri, Mar 19 2021 at 14:28, 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 > > VLAs are bad, VLAs to the rescue! :) I'm aware of the irony, but luto's idea really makes things easy. As documented there, though, this has a hard-coded (low) upper bound, so it's not like "regular" VLA use. > > > + * The asm statement is designed to convince the compiler to keep the > > + * allocation around even after "ptr" goes out of scope. > > + */ > > +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); \ > > Not that it matters on x86, but as this has to be called in the > interrupt disabled region of the syscall entry, shouldn't this be a > raw_cpu_read(). The asm-generic version has a preempt_disable/enable > pair around the raw read for native wordsize reads, otherwise a > irqsave/restore pair. > > __this_cpu_read() is fine as well, but that has an sanity check before > the raw read when CONFIG_DEBUG_PREEMPT is on, which is harmless but > also pointless in this case. > > Probably the same for the counterpart this_cpu_write(). Oh! Excellent point. I think this will make a big difference on arm64. I will adjust and test. -- Kees Cook