Re: [PATCH v5 00/15] x86: Add support for Clang CFI

[Ard Biesheuvel]

On Wed, 27 Oct 2021 at 16:03, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Wed, Oct 27, 2021 at 03:30:11PM +0200, Ard Biesheuvel wrote:
>
> > As far as I can tell from playing around with Clang, the stubs can
> > actually be executed directly,
>
> I had just finished reading the clang docs which suggest as much and was
> about to try what the compiler actually generates.
>
> > they just jumps to the actual function.
> > The compiler simply generates a jump table for each prototype that
> > appears in the code as the target of an indirect jump, and checks
> > whether the target appears in the list.
> >
> > E.g., the code below
> >
> > void foo(void) {}
> > void bar(int) {}
> > void baz(int) {}
> > void (* volatile fn1)(void) = foo;
> > void (* volatile fn2)(int) = bar;
> >
> > int main(int argc, char *argv[])
> > {
> >   fn1();
> >   fn2 = baz;
> >   fn2(-1);
> > }
> >
> > produces
> >
> > 0000000000400594 <foo.cfi>:
> >   400594: d65f03c0 ret
> >
> > 0000000000400598 <bar.cfi>:
> >   400598: d65f03c0 ret
> >
> > 000000000040059c <baz.cfi>:
> >   40059c: d65f03c0 ret
>
> Right, so these are the actual functions ^.
>
> > 00000000004005a0 <main>:
> >   4005a0: a9bf7bfd stp x29, x30, [sp, #-16]!
> >
> > // First indirect call
> >   4005a4: b0000088 adrp x8, 411000 <__libc_start_main@GLIBC_2.17>
> >   4005a8: f9401508 ldr x8, [x8, #40]
> >   4005ac: 90000009 adrp x9, 400000 <__abi_tag-0x278>
> >   4005b0: 91182129 add x9, x9, #0x608
> >   4005b4: 910003fd mov x29, sp
> >   4005b8: eb09011f cmp x8, x9
> >   4005bc: 54000241 b.ne 400604 <main+0x64>  // b.any
> >   4005c0: d63f0100 blr x8
>
> That's impenetrable to me, sorry.
>

This loads the value of fn1 in x8, and takes the address of the jump
table in x9. Since it is only one entry long, it does a simple compare
to check whether x8 appears in the jump table, and branches to the BRK
at the end if they are different.

> > // Assignment of fn2
> >   4005c4: 90000009 adrp x9, 400000 <__abi_tag-0x278>
> >   4005c8: b0000088 adrp x8, 411000 <__libc_start_main@GLIBC_2.17>
> >   4005cc: 91184129 add x9, x9, #0x610
> >   4005d0: f9001909 str x9, [x8, #48]
>
> I'm struggling here, x9 points to the branch at 400610, but then what?
>
> x8 is in .data somewhere?
>

This takes the address of the jump table entry of 'baz' in x9, and
stores it in fn2 whose address is taken in x8.


> > // Second indirect call
> >   4005d4: f9401908 ldr x8, [x8, #48]
> >   4005d8: 90000009 adrp x9, 400000 <__abi_tag-0x278>
> >   4005dc: 91183129 add x9, x9, #0x60c
> >   4005e0: cb090109 sub x9, x8, x9
> >   4005e4: 93c90929 ror x9, x9, #2
> >   4005e8: f100053f cmp x9, #0x1
> >   4005ec: 540000c8 b.hi 400604 <main+0x64>  // b.pmore
> >   4005f0: 12800000 mov w0, #0xffffffff            // #-1
> >   4005f4: d63f0100 blr x8
> >
> >
> >   4005f8: 2a1f03e0 mov w0, wzr
> >   4005fc: a8c17bfd ldp x29, x30, [sp], #16
> >   400600: d65f03c0 ret
> >   400604: d4200020 brk #0x1
>
>
> > 0000000000400608 <__typeid__ZTSFvvE_global_addr>:
> >   400608: 17ffffe3 b 400594 <foo.cfi>
> >
> > 000000000040060c <__typeid__ZTSFviE_global_addr>:
> >   40060c: 17ffffe3 b 400598 <bar.cfi>
> >   400610: 17ffffe3 b 40059c <baz.cfi>
>
> And these are the stubs per type.
>
> > So it looks like taking the address is fine, although not optimal due
> > to the additional jump.
>
> Right.
>

... although it does seem that function_nocfi() doesn't actually work
as expected, given that we want the address of <func>.cfi and not the
address of the stub.

> > We could fudge around that by checking the
> > opcode at the target of the call, or token paste ".cfi" after the
> > symbol name in the static_call_update() macro, but it doesn't like
> > like anything is terminally broken tbh.
>
> Agreed, since the jump table entries are actually executable it 'works'.
>
> I really don't like that extra jump though, so I think I really do want
> that nocfi_ptr() thing. And going by:
>
>   https://clang.llvm.org/docs/ControlFlowIntegrityDesign.html#forward-edge-cfi-for-indirect-function-calls
>
> and the above, that might be possible (on x86) with something like:
>
> /*
>  * Turns a Clang CFI jump-table entry into an actual function pointer.
>  * These jump-table entries are simply jmp.d32 instruction with their
>  * relative offset pointing to the actual function, therefore decode the
>  * instruction to find the real function.
>  */
> static __always_inline void *nocfi_ptr(void *func)
> {
>         union text_poke_insn insn = *(union text_poke_insn *)func;
>
>         return func + sizeof(insn) + insn.disp;
> }
>
> But really, that wants to be a compiler intrinsic.

Agreed. We could easily do something similar on arm64, but I'd prefer
to avoid that too.