Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation

[Thomas Gleixner]

On Mon, Jul 18 2022 at 15:55, Sami Tolvanen wrote:
> On Mon, Jul 18, 2022 at 3:47 PM Joao Moreira <joao@xxxxxxxxxxxxxxxxxx> wrote:
>> Thus the hash will be 6 bytes before the function entry point. Then we
>> can get the compiler to emit a padding area before the __cfi_\func
>> snippet and, during boot, if the CPU needs the call depth tracking
>> mitigation, we:
>> - move the __cfi_func into the padding area
>> - patch the call depth tracking snippet ahead of it (overwriting the old
>> __cfi_\func:)
>> - fix the cmpl offset in the caller
>>
>> func_whatever:
>>         ...
>>         cmpl $0x\hash, -FIXED_OFFSET(%rax)
>>         je 1f
>>         ud2
>> 1:
>>         call *%rax
>>         ...
>
> The problem with this is that the cmpl instruction contains the full
> type hash, which means that any instruction that's FIXED_OFFSET from
> the cmpl is a valid indirect call target as far as KCFI is concerned.
> -6 was chosen specifically to make the ud2 the only possible target.

But that's an implementation detail, right? Whatever we put in between
will still be a fixed offset, no? It's a different offset, but that's
what patching can deal with.

Thanks,

        tglx