Re: [PATCH -tip v3 1/2] kcov: Make runtime functions noinstr-compatible

Marco Elver
2020-06-15T14:53:49Z

On Mon, 15 Jun 2020, Peter Zijlstra wrote:

> On Mon, Jun 15, 2020 at 09:53:06AM +0200, Marco Elver wrote:
> >
> > Disabling KCOV for smp_processor_id now moves the crash elsewhere. In
> > the case of KASAN into its 'memcpy' wrapper, called after
> > __this_cpu_read in fixup_bad_iret. This is making me suspicious,
> > because it shouldn't be called from the noinstr functions.
>
> With your .config, objtool complains about exactly that though:
>
> vmlinux.o: warning: objtool: fixup_bad_iret()+0x8e: call to memcpy() leaves .noinstr.text section
>
> The utterly gruesome thing below 'cures' that.

Is __memcpy() generally available? I think that bypasses KASAN and
whatever else.

> > For KCSAN the crash still happens in check_preemption_disabled, in the
> > inlined native_save_fl function (apparently on its 'pushf'). If I turn
> > fixup_bad_iret's __this_cpu_read into a raw_cpu_read (to bypass
> > check_preemption_disabled), no more crash with KCSAN.
>
> vmlinux.o: warning: objtool: debug_smp_processor_id()+0x0: call to __sanitizer_cov_trace_pc() leaves .noinstr.text section
> vmlinux.o: warning: objtool: check_preemption_disabled()+0x1f: call to __sanitizer_cov_trace_pc() leaves .noinstr.text section
> vmlinux.o: warning: objtool: __this_cpu_preempt_check()+0x4: call to __sanitizer_cov_trace_pc() leaves .noinstr.text section
>
> That could be either of those I suppose, did you have the NOP patches
> on? Let me try... those seem to placate objtool at least.
>
> I do see a fair amount of __kasan_check*() crud though:
>
> vmlinux.o: warning: objtool: rcu_nmi_exit()+0x44: call to __kasan_check_read() leaves .noinstr.text section
> vmlinux.o: warning: objtool: rcu_dynticks_eqs_enter()+0x1c: call to __kasan_check_write() leaves .noinstr.text section
> vmlinux.o: warning: objtool: rcu_nmi_enter()+0x46: call to __kasan_check_read() leaves .noinstr.text section
> vmlinux.o: warning: objtool: rcu_dynticks_eqs_exit()+0x21: call to __kasan_check_write() leaves .noinstr.text section
> vmlinux.o: warning: objtool: __rcu_is_watching()+0x1c: call to __kasan_check_read() leaves .noinstr.text section
> vmlinux.o: warning: objtool: debug_locks_off()+0x1b: call to __kasan_check_write() leaves .noinstr.text section
>
> That wasn't supported to happen with the __no_sanitize patches on (which
> I didn't forget). Aah, I think we've lost a bunch of patches.. /me goes
> rummage.
>
> This:
>
> https://lkml.kernel.org/r/20200603114051.896465666@infradead.org
>
> that cures the rcu part of that.
>
> Let me go look at your KCSAN thing now...

I tried to find the stack that is used by the crashing code -- which led
me to entry_stack? So I tried this:

--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -370,7 +370,7 @@ struct x86_hw_tss {
#define IO_BITMAP_OFFSET_INVALID (__KERNEL_TSS_LIMIT + 1)

struct entry_stack {
- unsigned long words[64];
+ unsigned long words[128];
};

struct entry_stack_page {

No more crash. But that's probably not what we want. Just a datapoint.

> ---
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index af75109485c26..031a21fb5a741 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -675,6 +675,14 @@ struct bad_iret_stack {
> struct pt_regs regs;
> };
>
> +void __always_inline __badcpy(void *dst, void *src, int nr)
> +{
> + unsigned long *d = dst, *s = src;
> + nr /= sizeof(unsigned long);
> + while (nr--)
> + *(d++) = *(s++);
> +}
> +

If we can use __memcpy() here, that would probably solve that.

Thanks,
-- Marco