Messages in this thread | | | Date | Thu, 3 May 2018 08:19:18 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH 2/2] arm64: Clear the stack |
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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 <labbott@redhat.com> > --- > 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.
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