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[84.3.50.134]) by smtp.gmail.com with ESMTPSA id w18sm5273350ejn.23.2021.03.20.04.58.22 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 20 Mar 2021 04:58:23 -0700 (PDT) Sender: Ingo Molnar Date: Sat, 20 Mar 2021 12:58:20 +0100 From: Ingo Molnar To: Kees Cook Cc: Thomas Gleixner , 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, Linus Torvalds , Peter Zijlstra , Borislav Petkov Subject: Re: [PATCH v7 4/6] x86/entry: Enable random_kstack_offset support Message-ID: <20210320115820.GA4151166@gmail.com> References: <20210319212835.3928492-1-keescook@chromium.org> <20210319212835.3928492-5-keescook@chromium.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210319212835.3928492-5-keescook@chromium.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Kees Cook wrote: > Allow for a randomized stack offset on a per-syscall basis, with roughly > 5-6 bits of entropy, depending on compiler and word size. Since the > method of offsetting uses macros, this cannot live in the common entry > code (the stack offset needs to be retained for the life of the syscall, > which means it needs to happen at the actual entry point). > __visible noinstr void do_syscall_64(unsigned long nr, struct pt_regs *regs) > { > + add_random_kstack_offset(); > nr = syscall_enter_from_user_mode(regs, nr); > @@ -83,6 +84,7 @@ __visible noinstr void do_int80_syscall_32(struct pt_regs *regs) > { > unsigned int nr = syscall_32_enter(regs); > > + add_random_kstack_offset(); > unsigned int nr = syscall_32_enter(regs); > int res; > > + add_random_kstack_offset(); > @@ -70,6 +71,13 @@ static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs, > */ > current_thread_info()->status &= ~(TS_COMPAT | TS_I386_REGS_POKED); > #endif > + > + /* > + * x86_64 stack alignment means 3 bits are ignored, so keep > + * the top 5 bits. x86_32 needs only 2 bits of alignment, so > + * the top 6 bits will be used. > + */ > + choose_random_kstack_offset(rdtsc() & 0xFF); > } 1) Wondering why the calculation of the kstack offset (which happens in every syscall) is separated from the entry-time logic and happens during return to user-space? The two methods: +#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); \ + u8 *ptr = __builtin_alloca(offset & 0x3FF); \ + asm volatile("" : "=m"(*ptr) :: "memory"); \ + } \ +} while (0) + +#define choose_random_kstack_offset(rand) do { \ + if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ + &randomize_kstack_offset)) { \ + u32 offset = this_cpu_read(kstack_offset); \ + offset ^= (rand); \ + this_cpu_write(kstack_offset, offset); \ + } \ +} while (0) choose_random_kstack_offset() basically calculates the offset and stores it in a percpu variable (mixing it with the previous offset value), add_random_kstack_offset() uses it in an alloca() dynamic stack allocation. Wouldn't it be (slightly) lower combined overhead to just do it in a single step? There would be duplication along the 3 syscall entry points, but this should be marginal as this looks small, and the entry points would probably be cache-hot. 2) Another detail I noticed: add_random_kstack_offset() limits the offset to 0x3ff, or 1k - 10 bits. But the RDTSC mask is 0xff, 8 bits: + /* + * x86_64 stack alignment means 3 bits are ignored, so keep + * the top 5 bits. x86_32 needs only 2 bits of alignment, so + * the top 6 bits will be used. + */ + choose_random_kstack_offset(rdtsc() & 0xFF); alloca() itself works in byte units and will round the allocation to 8 bytes on x86-64, to 4 bytes on x86-32, this is what the 'ignored bits' reference in the comment is to, right? Why is there a 0x3ff mask for the alloca() call and a 0xff mask to the RDTSC randomizing value? Shouldn't the two be synced up? Or was the intention to shift the RDTSC value to the left by 3 bits? 3) Finally, kstack_offset is a percpu variable: #ifdef CONFIG_HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET ... DEFINE_PER_CPU(u32, kstack_offset); This is inherited across tasks on scheduling, and new syscalls will mix in new RDTSC values to continue to randomize the offset. Wouldn't it make sense to further mix values into this across context switching boundaries? A really inexpensive way would be to take the kernel stack value and mix it into the offset, and maybe even the randomized t->stack_canary value? This would further isolate the syscall kernel stack offsets of separate execution contexts from each other, should an attacker find a way to estimate or influence likely RDTSC values. Thanks, Ingo