Re: [PATCH] [PATCH V4]ARM64: SCS: Add gcc plugin to support Shadow Call Stack

[Nick Desaulniers]

On Fri, Oct 15, 2021 at 11:29 AM Dan Li <ashimida@xxxxxxxxxxxxxxxxx> wrote:
>
>
>
> On 10/15/21 2:44 AM, Nick Desaulniers wrote:
> >   On Wed, Oct 13, 2021 at 4:28 PM Dan Li <ashimida@xxxxxxxxxxxxxxxxx> wrote:
> >> --- a/include/linux/compiler-gcc.h
> >> +++ b/include/linux/compiler-gcc.h
> >> @@ -50,6 +50,10 @@
> >>   #define __latent_entropy __attribute__((latent_entropy))
> >>   #endif
> >>
> >> +#if defined(SHADOW_CALL_STACK_PLUGIN) && !defined(__CHECKER__)
> >> +#define __noscs __attribute__((no_shadow_call_stack))
> >> +#endif
> >
> > Cool this is a nice addition, and something I don't think that clang
> > has.  For any new feature, having a function attribute to disable it
> > at the function granularity is nice, and plays better with LTO than -f
> > group flags.  Though that begs the question: what happens if a __noscs
> > callee is inlined into a non-__noscs caller, or vice versa?
> Thanks Nick,
>
> According to my understanding, all inline optimizations in gcc should
> happen before inserting scs insns (scs and paciasp/autiasp use the
> same insertion point). Therefore, the check for the __noscs attribute
> will also occur after all inlining is completed.
>
> As in the following example:
> - Since __noscs attribute is specified, scs_test1 does not insert scs insns
> - Since normal functions scs_test2/3 uses x30, it needs to insert scs insns
> - Since __noscs attribute is specified, scs_test4 after inlining does not
> need to insert scs insns
>
> __always_inline __noscs void scs_test1(void)
> {
>      asm volatile("mov x1, x1\n\t":::"x30");
> }
>
> //scs insns inserted after function inline
> void scs_test2(void)
> {
>      scs_test1();
> }

That may be surprising to developers.  Perhaps __always_inline on
scs_test1 is distracting this test case, but I suspect it may not make
a difference.  This particular issue comes up time and again with
stack protectors; ie. the callee is marked no stack protector, then
gets inlined into a caller and suddenly gets a stack protector.

>
> __always_inline void scs_test3(void)
> {
>      asm volatile("mov x3, x3\n\t":::"x30");
> }
>
> //no scs insns inserted
> __noscs void scs_test4(void)
> {
>      scs_test3();
> }
>
> ffff800010012900 <scs_test1>:
> ffff800010012900:       a9bf7bfd        stp     x29, x30, [sp, #-16]!
> ffff800010012904:       910003fd        mov     x29, sp
> ffff800010012908:       aa0103e1        mov     x1, x1
> ffff80001001290c:       a8c17bfd        ldp     x29, x30, [sp], #16
> ffff800010012910:       d65f03c0        ret
>
> ffff800010012914 <scs_test2>:
> ffff800010012914:       f800865e        str     x30, [x18], #8
> ffff800010012918:       a9bf7bfd        stp     x29, x30, [sp, #-16]!
> ffff80001001291c:       910003fd        mov     x29, sp
> ffff800010012920:       aa0103e1        mov     x1, x1
> ffff800010012924:       a8c17bfd        ldp     x29, x30, [sp], #16
> ffff800010012928:       f85f8e5e        ldr     x30, [x18, #-8]!
> ffff80001001292c:       d65f03c0        ret
>
> ffff800010012930 <scs_test3>:
> ffff800010012930:       f800865e        str     x30, [x18], #8
> ffff800010012934:       a9bf7bfd        stp     x29, x30, [sp, #-16]!
> ffff800010012938:       910003fd        mov     x29, sp
> ffff80001001293c:       aa0303e3        mov     x3, x3
> ffff800010012940:       a8c17bfd        ldp     x29, x30, [sp], #16
> ffff800010012944:       f85f8e5e        ldr     x30, [x18, #-8]!
> ffff800010012948:       d65f03c0        ret
> ffff80001001294c:       d503201f        nop
>
> ffff800010012950 <scs_test4>:
> ffff800010012950:       a9bf7bfd        stp     x29, x30, [sp, #-16]!
> ffff800010012954:       910003fd        mov     x29, sp
> ffff800010012958:       aa0303e3        mov     x3, x3
> ffff80001001295c:       a8c17bfd        ldp     x29, x30, [sp], #16
> ffff800010012960:       d65f03c0        ret
> > I noticed that __noscs isn't actually applied anywhere in the kernel,
> > yet, at least in this series.  Were there any places necessary that
> > you've found thus far?
> At present, I have not found a function that must use the __noscs
> attribute in the kernel. I have only used this attribute in test cases.


-- 
Thanks,
~Nick Desaulniers