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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id e6-20020a63aa06000000b00380c8bed5a6sm1650747pgf.46.2022.03.09.12.16.47 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 09 Mar 2022 12:16:48 -0800 (PST) Date: Wed, 9 Mar 2022 12:16:47 -0800 From: Kees Cook To: Dan Li Cc: akpm@linux-foundation.org, arnd@arndb.de, catalin.marinas@arm.com, gregkh@linuxfoundation.org, linux@roeck-us.net, luc.vanoostenryck@gmail.com, elver@google.com, mark.rutland@arm.com, masahiroy@kernel.org, ojeda@kernel.org, nathan@kernel.org, npiggin@gmail.com, ndesaulniers@google.com, samitolvanen@google.com, shuah@kernel.org, tglx@linutronix.de, will@kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, llvm@lists.linux.dev, linux-hardening@vger.kernel.org Subject: Re: [PATCH v3 2/2] lkdtm: Add Shadow Call Stack tests Message-ID: <202203091211.4F00F560@keescook> References: <20220303073340.86008-1-ashimida@linux.alibaba.com> <20220303074339.86337-1-ashimida@linux.alibaba.com> <202203031010.0A492D114@keescook> <202203031105.A1B4CAE6@keescook> <92a767c4-09e1-8783-2581-9848bb72890d@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <92a767c4-09e1-8783-2581-9848bb72890d@linux.alibaba.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Mar 07, 2022 at 07:16:36AM -0800, Dan Li wrote: > The following code seems to work fine under clang/gcc, x86_64/aarch64 > (also tested in lkdtm_CFI_BACKWARD_SHADOW): > > #include > > static __attribute__((noinline)) > void set_return_addr(unsigned long *expected, unsigned long *addr) > { > /* Use of volatile is to make sure final write isn't seen as a dead store. */ > unsigned long * volatile *ret_addr = (unsigned long **)__builtin_frame_address(0) + 1; > > /* Make sure we've found the right place on the stack before writing it. */ > if(*ret_addr == expected) > *ret_addr = (addr); > } > > static volatile int force_label; > > int main(void) > { > void *array[] = {0, &&normal, &&redirected}; > > if (force_label) { > /* Call it with a NULL to avoid parameters being treated as constants in -02. */ > set_return_addr(NULL, NULL); > goto * array[force_label]; > } Hah! I like that. :) I had a weird switch statement that was working for me; this is cleaner. > > do { > > set_return_addr(&&normal, &&redirected); > > normal: > printf("I should be skipped\n"); > break; > > redirected: > printf("Redirected\n"); > > } while (0); > > return 0; > } > > But currently it still crashes when I try to enable > "-mbranch-protection=pac-ret+leaf+bti". > > Because the address of "&&redirected" is not encrypted under pac, > the autiasp check will fail when set_return_addr returns, and > eventually cause the function to crash when it returns to "&&redirected" > ("&&redirected" as a reserved label always seems to start with a bti j > insn). Strictly speaking, this is entirely correct. :) > For lkdtm, if we're going to handle both cases in one function, maybe > it would be better to turn off the -mbranch-protection=pac-ret+leaf+bti > and maybe also turn off -O2 options for the function :) If we can apply a function attribute to turn off pac for the "does this work without protections", that should be sufficient. -- Kees Cook