Re: [RFC] Circumventing FineIBT Via Entrypoints

[Rudolf Marek <r.marek@assembler.cz>]

Hi,

Dne 13. 02. 25 v 3:42 Andrew Cooper napsal(a):
> The SYSCALL behaviour TLDR is:
> 
>      %rcx = %rip
>      %r11 = %eflags
>      %cs = fixed attr
>      %ss = fixed attr
>      %rip = MSR_LSTAR
> 
> which means that %rcx (old rip) is the only piece of state which
> userspace can't feasibly forge (and therefore could distinguish a
> SYSCALL from user vs kernel mode), yet if we're talking about a JOP
> chain to get here, then %rcx is under attacker control too.

The SYSCALL instruction also provides means to create "incoherent" state of the processor selectors
where the value of selector do not match pre-loaded values in the descriptor caches.

Would it work to have KERNEL_CS as last entry in the GDT table? Therefore executing SYSCALL would set the CS as usual,
but the numeric value of SS selector would be larger than GDT limit?

That would mean that "impossible" selector is loaded into SS if we came from usermode,
but operation with stack would still work as the descriptor caches will be sane.
The "impossible" selector value can be fixed by loading SS with NULL which is cheap.

The check in hotpath could maybe use VERR %SS which would fail because of GDT limit is reached. The VERR with mem operand
does not use any GPR!

Or simply check for "impossible" selector would work if we misuse zeros in high32 of R11 (usermode rflags) maybe like:

entry:
endbr64
rol $32, %r11
movw %ss, %r11w
cmpw $IMPOSSIBLE_SEL, %r11w
jnz panic
; load null to SS, fix R11 and pretend above never happened

If attacker would execute SYSCALL in the kernel, likely we could check the %RCX if it is OK or not?

Bit variation to this "theme" would be to have SYSCALL SS GDT entry still in the GDT but set as "not present".
Another brainstorm idea would be to misuse RFLAGS.ID and clear it in MSR FMASK but run kernel or most of it with RFLAGS.ID set.
I don't know what is the threat model you are trying to fix.

Lets fight x86 insanity with yet another x86 insanity - I think it is fair.

I hope above helps or at least I will learn why not if I overseen something obvious!

I tried to CC all the lists. I'm not subscribed.

Thanks,
Rudolf

PS: I'm leaving as an exercise to a reader NMI and #MC handling!