linux-kernel - Re: [PATCH] x86/ibt: Implement FineIBT

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Date:   Tue, 18 Oct 2022 22:09:23 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     x86@...nel.org, Sami Tolvanen <samitolvanen@...gle.com>,
        Joao Moreira <joao@...rdrivepizza.com>,
        linux-kernel@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [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.