Re: [PATCH] x86/kcfi: Require FRED for FineIBT

[Andrew Cooper]

On 14/02/2025 9:54 pm, Kees Cook wrote:
> On Fri, Feb 14, 2025 at 07:39:20PM +0000, Andrew Cooper wrote:
>> On 14/02/2025 7:22 pm, Kees Cook wrote:
>>> diff --git a/arch/x86/include/asm/cfi.h b/arch/x86/include/asm/cfi.h
>>> index ef5e0a698253..dfa2ba4cceca 100644
>>> --- a/arch/x86/include/asm/cfi.h
>>> +++ b/arch/x86/include/asm/cfi.h
>>> @@ -93,7 +93,7 @@
>>>   *
>>>   */
>>>  enum cfi_mode {
>>> -	CFI_AUTO,	/* FineIBT if hardware has IBT, otherwise kCFI */
>>> +	CFI_AUTO,	/* FineIBT if hardware has IBT, FRED, and XOM */
>> You discuss XOM in the commit message, but there's no check ...
>>
>>>  	CFI_OFF,	/* Taditional / IBT depending on .config */
>>>  	CFI_KCFI,	/* Optionally CALL_PADDING, IBT, RETPOLINE */
>>>  	CFI_FINEIBT,	/* see arch/x86/kernel/alternative.c */
>>> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
>>> index 97422292b609..acc12a6efc18 100644
>>> --- a/arch/x86/kernel/alternative.c
>>> +++ b/arch/x86/kernel/alternative.c
>>> @@ -1323,7 +1323,9 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
>>>  
>>>  	if (cfi_mode == CFI_AUTO) {
>>>  		cfi_mode = CFI_KCFI;
>>> -		if (HAS_KERNEL_IBT && cpu_feature_enabled(X86_FEATURE_IBT))
>>> +		/* FineIBT requires IBT and will only be safe with FRED */
>>> +		if (HAS_KERNEL_IBT && cpu_feature_enabled(X86_FEATURE_IBT) &&
>>> +		    cpu_feature_enabled(X86_FEATURE_FRED))
>> ... here.
>>
>> Is this meant to be "/* TODO: wire up XOM */" or is that accounted for
>> somewhere else?
> Yeah, I wasn't sure how to best capture that in here. XOM doesn't exist
> yet for x86... I could add a TODO like that?

I get the feeling that the PKS series would have an easier time starting
with XOM (even if it hard-codes pkey1=xom and avoids the allocator in
the short term, seeing as Linux does have a good grasp of where it's
executable pages are, and how they're accessed) rather than trying to do
general page hiding.  The capability is in 3 generations of Intel CPU now.

>
>> Also, while I hate to come back and contradict myself from earlier...
>>
>> Architecturally, FineIBT without FRED seems to be no improvement over
>> simple IBT.  (I'd love to find some way of hardening the entrypoints,
>> but I can't see a robust way of doing so.)
> If you're just looking at IBT, yes. But kCFI (with or without IBT,
> but without FineIBT) will do hash checking at the call site, which
> should make it impossible to reach the entrypoints from an indirect call
> in the first place, as they have no hash preceding them.
>
>> However, micro-architecturally, FineIBT is still far better than simple
>> IBT for speculation issue, seeing as Intel keep on staunchly refusing to
>> turn off the indirect predictors by default like AMD do.
>>
>> A security conscious user ought to be using FineIBT for this, given a
>> choice, even if it's not perfect in other regards.
> A security conscious user should use kCFI without FineIBT. :) But I
> think we might be thinking about different elements of security. I am
> focusing on control flow, and I think you're considering speculation?

True.  The security realist knows they're dammed either way, and gets a
stiff drink instead.

~Andrew