[PATCH v4] crypto: riscv/poly1305 - import OpenSSL/CRYPTOGAMS implementation

[Andy Polyakov]

>>>>> +.globl   poly1305_init
>>>>> +.type    poly1305_init,\@function
>>>>> +poly1305_init:
>>>>> +#ifdef   __riscv_zicfilp
>>>>> + lpad    0
>>>>> +#endif
>>>>
>>>> But they appear to be unnecessary.
>>>
>>> They are better be there if Control Flow Integrity is on. It's the same deal
>>> as with endbranch instruction on Intel and hint #34 on ARM. It's possible
>>> that the kernel never engages CFI for itself, in which case all the
>>> mentioned instructions are executed as nop-s. But note that here they are
>>> compiled conditionally, so that if you don't compile the kernel with
>>> -march=..._zicfilp_..., then they won't be there.
>>
>> There appears to be no kernel-mode support for Zicfilp yet.  This would be the
>> very first occurrence of the lpad instruction in the kernel source.
> 
> Of course, if the kernel actually ends up calling these functions
> indirectly at some point, lpad alone isn't sufficient,

The exception condition is specified as "tag != x7 *and* tag != 0." In 
other words lpad 0 provides a coarse-grained, but a protection 
nevertheless. As effective as the endbranch on Intel. Since a 
"fit-multiple-use-cases" assembly module, such as cryptogams' ones, 
can't make specific assumptions about the caller, lpad 0 is a working 
fall-back.

> we would need
> to use SYM_TYPED_FUNC_START to emit CFI type information for them.

I'm confused. My understanding is that SYM_TYPED_FUNC_START is about 
clang -fsanitize=kcfi, which is a different mechanism. Well, you might 
be referring to future developments...

> Also, if the kernel decides to use type-based landing pad labels for
> finer-grained CFI, "lpad 0" isn't going to work anyway.

It would, it simply won't be fine-graned.

> Perhaps it
> would make sense to just drop the lpad instruction in kernel builds
> for now to avoid confusion?

Let's say I go for

#ifdef __KERNEL__
SYM_TYPED_FUNC_START(poly1305_init, ...)
#else
.globl	poly1305_init
.type	poly1305_init,\@function
poly1305_init:
# ifdef	__riscv_zicfilp
	lpad	0
# endif
#endif

Would it be sufficient to #include <linux/cfi_types.h>?

Cheers.