Re: [RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI_CLANG

Peter Zijlstra
2022-05-17T08:05:39Z

On Mon, May 16, 2022 at 03:59:41PM -0700, Kees Cook wrote:

> I'm still not convinced about this, but I'm on the fence.
>
> Cons:
> - FineIBT does callee-based hash verification, which means any
> attacker-constructed memory region just has to have an endbr and nops at
> "shellcode - 9". KCFI would need the region to have the hash at
> "shellcode - 6" and an endbr at "shellcode". However, that hash is well
> known, so it's not much protection.

How would you get the ENDBR there anyway? If you can write code it's
game over.

> - Potential performance hit due to making an additional "call" outside
> the cache lines of both caller and callee.

That was all an effort to shrink and simplify, all this CFI stuff is
massive bloat :/

If we use %eax instead of %r10d for the hash transfer (as per Joao), and
use int3 instead of ud2, then we can shrink the fineibt sequence to:

__cfi_\func:
endbr # 4
xorl $0x12345678, %eax # 5
jz 1f # 2
int3 # 1
\func:
...

Which is 12 bytes, and needs a larger preamble (up from 9 in the current
proposal).

If we do the objtool/linker fixup, such that direct calls/jumps will
*never* hit ENDBR, then we can do something ugly like:

kCFI FineIBT

__cfi_\func: __cfi_\func:
int3 endbr
movl $0x12345678, %rax xorl $0x12345678, %eax
int3 jz 1f
int3 int3
\func:
endbr
__direct_\func: __direct_\func:
... ...

which is 12 bytes on both sides and shrinks the preaamble to 8 bytes
while additionally also supporting kCFI+IBT for those few people that
don't care about speculation based attacks much.

But now it's complicated again and requires significant tools work :/
(also, using int3 isn't ideal).

> Pros:
> - FineIBT can be done without read access to the kernel text, which will
> be nice in the exec-only future.

- Mostly kills SpectreBHB (because it has the hash check *after*
ENDBR).

So were IBT limits speculation to all sites that have ENDBR, you can
still target any of them. With FineIBT you loose all sites that don't
match on hash value, significantly reducing the options.

> I'd kind of like the "dynamic FineIBT conversion" to be a config option,
> at least at first. We could at least do performance comparisons between
> them.

Why would you need a config option for that? Since it is dynamic anyway
a boot option works fine.

Also, regardless of all this, it probably makes sense to add an LTO pass
to remove all unused __cfi_ symbols and endbr instructions, less viable
targets is more better :-)

I've been doing that with objtool for the IBT builds.