Re: [PATCH 00/10] KVM: arm64: Add support for hypervisor kCFI

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Hi Marc,

On Thu, Mar 14, 2024 at 10:40:47PM +0000, Marc Zyngier wrote:
> Hi Pierre-Clément,
> On Thu, 14 Mar 2024 20:23:00 +0000,
> Pierre-Clément Tosi <ptosi@xxxxxxxxxx> wrote:
> > 
> > CONFIG_CFI_CLANG ("kernel Control Flow Integrity") makes the compiler inject
> > runtime type checks before any indirect function call. On AArch64, it generates
> > a BRK instruction to be executed on type mismatch and encodes the indices of the
> > registers holding the branch target and expected type in the immediate of the
> > instruction. As a result, a synchronous exception gets triggered on kCFI failure
> > and the fault handler can retrieve the immediate (and indices) from ESR_ELx.
> > 
> > This feature has been supported at EL1 ("host") since it was introduced by
> > b26e484b8bb3 ("arm64: Add CFI error handling"), where cfi_handler() decodes
> > ESR_EL1, giving informative panic messages such as
> > 
> >   [   21.885179] CFI failure at lkdtm_indirect_call+0x2c/0x44 [lkdtm]
> >   (target: lkdtm_increment_int+0x0/0x1c [lkdtm]; expected type: 0x7e0c52a)
> >   [   21.886593] Internal error: Oops - CFI: 0 [#1] PREEMPT SMP
> > 
> > However, it is not or only partially supported at EL2: in nVHE (or pKVM),
> > CONFIG_CFI_CLANG gets filtered out at build time, preventing the compiler from
> > injecting the checks. In VHE (or hVHE), EL2 code gets compiled with the checks
> Are you sure about hVHE? hVHE is essentially the nVHE object running
> with a slightly different HCR_EL2 configuration. So if you don't have
> the checks in the nVHE code, you don't have them for hVHE either.

No, I am not and my assumption that hVHE was running the VHE hyp code was wrong.
FYI, these patches were tested in VHE, nVHE, and pKVM (with NVHE_EL2_DEBUG set
and unset) but not in hVHE (clearly!). Thanks for pointing this out.

> Or am I missing something obvious?
> Thanks,
> 	M.
> -- 
> Without deviation from the norm, progress is not possible.


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