From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751354AbeECHTd (ORCPT ); Thu, 3 May 2018 03:19:33 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:37650 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751279AbeECHT0 (ORCPT ); Thu, 3 May 2018 03:19:26 -0400 Date: Thu, 3 May 2018 08:19:18 +0100 From: Mark Rutland To: Laura Abbott Cc: Alexander Popov , Kees Cook , Ard Biesheuvel , kernel-hardening@lists.openwall.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/2] arm64: Clear the stack Message-ID: <20180503071917.xm2xvgagvzkworay@salmiak> References: <20180502203326.9491-1-labbott@redhat.com> <20180502203326.9491-3-labbott@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180502203326.9491-3-labbott@redhat.com> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Laura, On Wed, May 02, 2018 at 01:33:26PM -0700, Laura Abbott wrote: > > Implementation of stackleak based heavily on the x86 version > > Signed-off-by: Laura Abbott > --- > Now written in C instead of a bunch of assembly. This looks neat! I have a few minor comments below. > diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile > index bf825f38d206..0ceea613c65b 100644 > --- a/arch/arm64/kernel/Makefile > +++ b/arch/arm64/kernel/Makefile > @@ -55,6 +55,9 @@ arm64-reloc-test-y := reloc_test_core.o reloc_test_syms.o > arm64-obj-$(CONFIG_CRASH_DUMP) += crash_dump.o > arm64-obj-$(CONFIG_ARM_SDE_INTERFACE) += sdei.o > > +arm64-obj-$(CONFIG_GCC_PLUGIN_STACKLEAK) += erase.o > +KASAN_SANITIZE_erase.o := n I suspect we want to avoid the full set of instrumentation suspects here, e.g. GKOV, KASAN, UBSAN, and KCOV. > + > obj-y += $(arm64-obj-y) vdso/ probes/ > obj-m += $(arm64-obj-m) > head-y := head.o > diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S > index ec2ee720e33e..3144f1ebdc18 100644 > --- a/arch/arm64/kernel/entry.S > +++ b/arch/arm64/kernel/entry.S > @@ -401,6 +401,11 @@ tsk .req x28 // current thread_info > > .text > > + .macro ERASE_KSTACK > +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK > + bl erase_kstack > +#endif > + .endm Nit: The rest of our asm macros are lower-case -- can we stick to that here? > /* > * Exception vectors. > */ > @@ -906,6 +911,7 @@ ret_to_user: > cbnz x2, work_pending > finish_ret_to_user: > enable_step_tsk x1, x2 > + ERASE_KSTACK > kernel_exit 0 > ENDPROC(ret_to_user) I believe we also need this in ret_fast_syscall. [...] > +asmlinkage void erase_kstack(void) > +{ > + unsigned long p = current->thread.lowest_stack; > + unsigned long boundary = p & ~(THREAD_SIZE - 1); > + unsigned long poison = 0; > + const unsigned long check_depth = STACKLEAK_POISON_CHECK_DEPTH / > + sizeof(unsigned long); > + > + /* > + * Let's search for the poison value in the stack. > + * Start from the lowest_stack and go to the bottom. > + */ > + while (p > boundary && poison <= check_depth) { > + if (*(unsigned long *)p == STACKLEAK_POISON) > + poison++; > + else > + poison = 0; > + > + p -= sizeof(unsigned long); > + } > + > + /* > + * One long int at the bottom of the thread stack is reserved and > + * should not be poisoned (see CONFIG_SCHED_STACK_END_CHECK). > + */ > + if (p == boundary) > + p += sizeof(unsigned long); I wonder if end_of_stack() should be taught about CONFIG_SCHED_STACK_END_CHECK, given that's supposed to return the last *usable* long on the stack, and we don't account for this elsewhere. If we did, then IIUC we could do: unsigned long boundary = (unsigned long)end_of_stack(current); ... at the start of the function, and not have to worry about this explicitly. > + > +#ifdef CONFIG_STACKLEAK_METRICS > + current->thread.prev_lowest_stack = p; > +#endif > + > + /* > + * So let's write the poison value to the kernel stack. > + * Start from the address in p and move up till the new boundary. > + */ > + boundary = current_stack_pointer; I worry a little that the compiler can move the SP during a function's lifetime, but maybe that's only the case when there are VLAs, or something like that? > + > + BUG_ON(boundary - p >= THREAD_SIZE); > + > + while (p < boundary) { > + *(unsigned long *)p = STACKLEAK_POISON; > + p += sizeof(unsigned long); > + } > + > + /* Reset the lowest_stack value for the next syscall */ > + current->thread.lowest_stack = current_stack_pointer; > +} Once this function returns, its data is left on the stack. Is that not a problem? No strong feelings either way, but it might be worth mentioning in the commit message. > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c > index f08a2ed9db0d..156fa0a0da19 100644 > --- a/arch/arm64/kernel/process.c > +++ b/arch/arm64/kernel/process.c > @@ -364,6 +364,9 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start, > p->thread.cpu_context.pc = (unsigned long)ret_from_fork; > p->thread.cpu_context.sp = (unsigned long)childregs; > > +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK > + p->thread.lowest_stack = (unsigned long)task_stack_page(p); Nit: end_of_stack(p) would be slightly better semantically, even though currently equivalent to task_stack_page(p). [...] > +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK > +void __used check_alloca(unsigned long size) > +{ > + unsigned long sp, stack_left; > + > + sp = current_stack_pointer; > + > + stack_left = sp & (THREAD_SIZE - 1); > + BUG_ON(stack_left < 256 || size >= stack_left - 256); > +} Is this arbitrary, or is there something special about 256? Even if this is arbitrary, can we give it some mnemonic? > +EXPORT_SYMBOL(check_alloca); > +#endif > diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile > index a34e9290a699..25dd2a14560d 100644 > --- a/drivers/firmware/efi/libstub/Makefile > +++ b/drivers/firmware/efi/libstub/Makefile > @@ -20,7 +20,8 @@ cflags-$(CONFIG_EFI_ARMSTUB) += -I$(srctree)/scripts/dtc/libfdt > KBUILD_CFLAGS := $(cflags-y) -DDISABLE_BRANCH_PROFILING \ > -D__NO_FORTIFY \ > $(call cc-option,-ffreestanding) \ > - $(call cc-option,-fno-stack-protector) > + $(call cc-option,-fno-stack-protector) \ > + $(DISABLE_STACKLEAK_PLUGIN) > > GCOV_PROFILE := n > KASAN_SANITIZE := n I believe we'll also need to do this for the KVM hyp code in arch/arm64/kvm/hyp/. Thanks, Mark. From mboxrd@z Thu Jan 1 00:00:00 1970 From: mark.rutland@arm.com (Mark Rutland) Date: Thu, 3 May 2018 08:19:18 +0100 Subject: [PATCH 2/2] arm64: Clear the stack In-Reply-To: <20180502203326.9491-3-labbott@redhat.com> References: <20180502203326.9491-1-labbott@redhat.com> <20180502203326.9491-3-labbott@redhat.com> Message-ID: <20180503071917.xm2xvgagvzkworay@salmiak> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org Hi Laura, On Wed, May 02, 2018 at 01:33:26PM -0700, Laura Abbott wrote: > > Implementation of stackleak based heavily on the x86 version > > Signed-off-by: Laura Abbott > --- > Now written in C instead of a bunch of assembly. This looks neat! I have a few minor comments below. > diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile > index bf825f38d206..0ceea613c65b 100644 > --- a/arch/arm64/kernel/Makefile > +++ b/arch/arm64/kernel/Makefile > @@ -55,6 +55,9 @@ arm64-reloc-test-y := reloc_test_core.o reloc_test_syms.o > arm64-obj-$(CONFIG_CRASH_DUMP) += crash_dump.o > arm64-obj-$(CONFIG_ARM_SDE_INTERFACE) += sdei.o > > +arm64-obj-$(CONFIG_GCC_PLUGIN_STACKLEAK) += erase.o > +KASAN_SANITIZE_erase.o := n I suspect we want to avoid the full set of instrumentation suspects here, e.g. GKOV, KASAN, UBSAN, and KCOV. > + > obj-y += $(arm64-obj-y) vdso/ probes/ > obj-m += $(arm64-obj-m) > head-y := head.o > diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S > index ec2ee720e33e..3144f1ebdc18 100644 > --- a/arch/arm64/kernel/entry.S > +++ b/arch/arm64/kernel/entry.S > @@ -401,6 +401,11 @@ tsk .req x28 // current thread_info > > .text > > + .macro ERASE_KSTACK > +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK > + bl erase_kstack > +#endif > + .endm Nit: The rest of our asm macros are lower-case -- can we stick to that here? > /* > * Exception vectors. > */ > @@ -906,6 +911,7 @@ ret_to_user: > cbnz x2, work_pending > finish_ret_to_user: > enable_step_tsk x1, x2 > + ERASE_KSTACK > kernel_exit 0 > ENDPROC(ret_to_user) I believe we also need this in ret_fast_syscall. [...] > +asmlinkage void erase_kstack(void) > +{ > + unsigned long p = current->thread.lowest_stack; > + unsigned long boundary = p & ~(THREAD_SIZE - 1); > + unsigned long poison = 0; > + const unsigned long check_depth = STACKLEAK_POISON_CHECK_DEPTH / > + sizeof(unsigned long); > + > + /* > + * Let's search for the poison value in the stack. > + * Start from the lowest_stack and go to the bottom. > + */ > + while (p > boundary && poison <= check_depth) { > + if (*(unsigned long *)p == STACKLEAK_POISON) > + poison++; > + else > + poison = 0; > + > + p -= sizeof(unsigned long); > + } > + > + /* > + * One long int at the bottom of the thread stack is reserved and > + * should not be poisoned (see CONFIG_SCHED_STACK_END_CHECK). > + */ > + if (p == boundary) > + p += sizeof(unsigned long); I wonder if end_of_stack() should be taught about CONFIG_SCHED_STACK_END_CHECK, given that's supposed to return the last *usable* long on the stack, and we don't account for this elsewhere. If we did, then IIUC we could do: unsigned long boundary = (unsigned long)end_of_stack(current); ... at the start of the function, and not have to worry about this explicitly. > + > +#ifdef CONFIG_STACKLEAK_METRICS > + current->thread.prev_lowest_stack = p; > +#endif > + > + /* > + * So let's write the poison value to the kernel stack. > + * Start from the address in p and move up till the new boundary. > + */ > + boundary = current_stack_pointer; I worry a little that the compiler can move the SP during a function's lifetime, but maybe that's only the case when there are VLAs, or something like that? > + > + BUG_ON(boundary - p >= THREAD_SIZE); > + > + while (p < boundary) { > + *(unsigned long *)p = STACKLEAK_POISON; > + p += sizeof(unsigned long); > + } > + > + /* Reset the lowest_stack value for the next syscall */ > + current->thread.lowest_stack = current_stack_pointer; > +} Once this function returns, its data is left on the stack. Is that not a problem? No strong feelings either way, but it might be worth mentioning in the commit message. > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c > index f08a2ed9db0d..156fa0a0da19 100644 > --- a/arch/arm64/kernel/process.c > +++ b/arch/arm64/kernel/process.c > @@ -364,6 +364,9 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start, > p->thread.cpu_context.pc = (unsigned long)ret_from_fork; > p->thread.cpu_context.sp = (unsigned long)childregs; > > +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK > + p->thread.lowest_stack = (unsigned long)task_stack_page(p); Nit: end_of_stack(p) would be slightly better semantically, even though currently equivalent to task_stack_page(p). [...] > +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK > +void __used check_alloca(unsigned long size) > +{ > + unsigned long sp, stack_left; > + > + sp = current_stack_pointer; > + > + stack_left = sp & (THREAD_SIZE - 1); > + BUG_ON(stack_left < 256 || size >= stack_left - 256); > +} Is this arbitrary, or is there something special about 256? Even if this is arbitrary, can we give it some mnemonic? > +EXPORT_SYMBOL(check_alloca); > +#endif > diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile > index a34e9290a699..25dd2a14560d 100644 > --- a/drivers/firmware/efi/libstub/Makefile > +++ b/drivers/firmware/efi/libstub/Makefile > @@ -20,7 +20,8 @@ cflags-$(CONFIG_EFI_ARMSTUB) += -I$(srctree)/scripts/dtc/libfdt > KBUILD_CFLAGS := $(cflags-y) -DDISABLE_BRANCH_PROFILING \ > -D__NO_FORTIFY \ > $(call cc-option,-ffreestanding) \ > - $(call cc-option,-fno-stack-protector) > + $(call cc-option,-fno-stack-protector) \ > + $(DISABLE_STACKLEAK_PLUGIN) > > GCOV_PROFILE := n > KASAN_SANITIZE := n I believe we'll also need to do this for the KVM hyp code in arch/arm64/kvm/hyp/. Thanks, Mark.