Re: [RFC] Circumventing FineIBT Via Entrypoints

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 13/02/2025 8:28 pm, Kees Cook wrote:
> On Thu, Feb 13, 2025 at 01:31:30AM +0000, Andrew Cooper wrote:
>>>> Assuming this is an issue you all feel is worth addressing, I will
>>>> continue working on providing a patch. I'm concerned though that the
>>>> overhead from adding a wrmsr on both syscall entry and exit to
>>>> overwrite and restore the KERNEL_GS_BASE MSR may be quite high, so
>>>> any feedback in regards to the approach or suggestions of alternate
>>>> approaches to patching are welcome :) 
>>> Since the kernel, as far as I understand, uses FineIBT without
>>> backwards control flow protection (in other words, I think we assume
>>> that the kernel stack is trusted?),
>> This is fun indeed.  Linux cannot use supervisor shadow stacks because
>> the mess around NMI re-entrancy (and IST more generally) requires ROP
>> gadgets in order to function safely.  Implementing this with shadow
>> stacks active, while not impossible, is deemed to be prohibitively
>> complicated.
> And just validate my understanding here, this attack is fundamentally
> about FineIBT, not regular CFI (IBT or not), as the validation of target
> addresses is done at indirect call time, yes?

I'm not sure I'd classify it like that.  As a pivot primitive, it works
very widely.

FineIBT (more specifically any hybrid CFI scheme which includes CET-IBT)
relies on hardware to do the course grain violation detection, and some
software hash for fine grain violation detection.

In this case, the requirement for the SYSCALL entrypoint to have an
ENDBR64 instruction means it passes the CET-IBT check (does not yield
#CP), and then lacks the software hash check as well.

i.e. this renders FineIBT (and other hybrid CFI schemes) rather moot,
because one hole is all the attacker needs to win, if they can control a
function pointer / return address.  At which point it's a large overhead
for no security benefit over simple CET-IBT.

The problem is that SYSCALL entry/exit is a toxic operating mode,
because you only have to think about sneezing and another user->kernel
priv-esc appears.

~Andrew