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    SubjectRe: [PATCH 2/2] arm64: Clear the stack
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    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.

    >> +}
    >
    > 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.

    >> 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?

    Thanks a lot!

    Best regards,
    Alexander

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