Re: [PATCH 2/2] arm64: Clear the stack

From: Ard Biesheuvel
Date: Thu May 03 2018 - 07:38:03 EST

On 3 May 2018 at 09:19, Mark Rutland <mark.rutland@xxxxxxx> wrote:
> 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@xxxxxxxxxx>
>> ---
>> 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?
>

I think the AAPCS permits the compiler to allocate the stack space for
outgoing variables (i.e., args 9 and beyond or other argument types
that require passing via the stack) at a smaller scope than the entire
function, although GCC does appear to allocate it at the beginning
(based on some quick experiments)


>> +
>> + 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.