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SubjectRe: [PATCH] x86/ibt: Implement FineIBT
On Tue, Oct 18, 2022 at 11:09:13AM -0700, Kees Cook wrote:

> > +#ifdef CONFIG_FINEIBT
> > +/*
> > + * kCFI FineIBT
> > + *
> > + * __cfi_\func: __cfi_\func:
> > + * movl $0x12345678,%eax endbr64 // 4
>
> kCFI emits endbr64 here first too ...
>
> > + * nop subl $0x12345678,%r10d // 7
> > + * nop jz 1f // 2
> > + * nop ud2 // 2
> > + * nop 1: nop // 1
> > + * nop
> > + * nop
> > + * nop
> > + * nop
> > + * nop
> > + * nop
> > + * nop

It does not; it does emit ENDBR at the start of the regular symbol
though:

0000000000001040 <__cfi_yield>:
1040: b8 0c 67 40 a5 mov $0xa540670c,%eax
1045: 90 nop
1046: 90 nop
1047: 90 nop
1048: 90 nop
1049: 90 nop
104a: 90 nop
104b: 90 nop
104c: 90 nop
104d: 90 nop
104e: 90 nop
104f: 90 nop

0000000000001050 <yield>:
1050: f3 0f 1e fa endbr64
1054: e8 00 00 00 00 call 1059 <yield+0x9> 1055: R_X86_64_PLT32 __fentry__-0x4
1059: 65 48 8b 05 00 00 00 00 mov %gs:0x0(%rip),%rax # 1061 <yield+0x11> 105d: R_X86_64_PC32 pcpu_hot-0x4
1061: 31 c9 xor %ecx,%ecx
1063: 87 48 18 xchg %ecx,0x18(%rax)
1066: e9 00 00 00 00 jmp 106b <yield+0x1b> 1067: R_X86_64_PLT32 .text+0xc08c
106b: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)

Not doing that is an option...


> Tangent: why are these nop instead of 0xcc? These bytes aren't executed
> ever are they?

Because that's what the compiler gets us through -fpatchable-function-entry.


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