[PATCH 2/2] arm64: Clear the stack

[ard.biesheuvel@linaro.org (Ard Biesheuvel)]

On 3 May 2018 at 09:19, Mark Rutland <mark.rutland@arm.com> 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.
>
>> +
>>  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.