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SubjectRe: [PATCH 2/2] arm64: Clear the stack
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On 05/03/2018 10:33 AM, Alexander Popov wrote:
> Hello Mark and Laura,
>
> Let me join the discussion. Mark, thanks for your feedback!
>
> On 03.05.2018 10:19, Mark Rutland 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@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.
>
> I've disabled KASAN instrumentation for that file on x86 because erase_kstack()
> intentionally writes to the stack and causes KASAN false positive reports.
>
> But I didn't see any conflicts with other types of instrumentation that you
> mentioned.
>
>>> +
>>> 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.
>
> I would be afraid to change the meaning of end_of_stack()... Currently it
> considers that magic long as usable (include/linux/sched/task_stack.h):
>
> #define task_stack_end_corrupted(task) \
> (*(end_of_stack(task)) != STACK_END_MAGIC)
>
>
>> 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.
>
> I should mention that erase_kstack() can be called from x86 trampoline stack.
> That's why the boundary is calculated from the lowest_stack.
>
>>> +
>>> +#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?
>
> Oh, I don't know.
>
> However, erase_kstack() doesn't call anything except simple inline functions.
> And as I see from its disasm on x86, the local variables reside in registers.
>
>>> +
>>> + 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;
>
> Laura, that might be wrong and introduce huge performance impact.
>
> I think, lowest_stack should be reset similarly to the original version.
>

Sorry, I'm not understanding here. What's the performance impact and
what do you mean by original version?

>>> +}
>>
>> 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.
>
> I managed to bypass that with "register" specifier. Although it doesn't give an
> absolute guarantee.
>

I guess I was assuming gcc would be smart enough not to spill stuff
on the stack. I also intentionally removed the register keyword
since it wasn't clear gcc does much with it on a modern system? I
could be completely off base here though so please correct me if
I'm wrong. It probably is worth documenting what we are assuming about
the compiler here.


>>> 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).
>
> Thanks, I agree, I'll fix it in v12.
>
>> [...]
>>
>>> +#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?
>
> It's just a reasonable number. We can introduce a macro for it.
>
>>> +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/.
>
> Could you please give more details on that? Why STACKLEAK breaks it?
>

For reference, I originally added this for the efistub because
it would not compile. I did compile this against my Fedora tree
which has KVM enabled.

> Thanks a lot!
>
> Best regards,
> Alexander
>

Thanks,
Laura

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