Re: [PATCH v3] KVM: arm64: Use BTI for nvhe

[Sudeep Holla]

On Tue, May 30, 2023 at 03:08:45PM +0000, Mostafa Saleh wrote:
> CONFIG_ARM64_BTI_KERNEL compiles the kernel to support ARMv8.5-BTI.
> However, the nvhe code doesn't make use of it as it doesn't map any
> pages with Guarded Page(GP) bit.
> 
> kvm pgtable code is modified to map executable pages with GP bit
> if BTI is enabled for the kernel.
> 
> At hyp init, SCTLR_EL2.BT is set to 1 to match EL1 configuration
> (SCTLR_EL1.BT1) set in bti_enable().
> 
> One difference between kernel and nvhe code, is that the kernel maps
> .text with GP while nvhe maps all the executable pages, this makes
> nvhe code need to deal with special initialization code coming from
> other executable sections (.idmap.text).
> For this we need to add bti instruction at the beginning of
> __kvm_handle_stub_hvc as it can be called by  __host_hvc through
> branch instruction(br) and unlike SYM_FUNC_START, SYM_CODE_START
> doesn’t add bti instruction at the beginning, and it can’t be modified
> to add it as it is used with vector tables.
> Another solution which is more intrusive is to convert
> __kvm_handle_stub_hvc to a function and inject “bti jc” instead of
> “bti c” in SYM_FUNC_START
>

I was chasing a bug in linux-next yesterday with protected nVHE(pKVM) and
cpuidle enabled. The system fails to boot. I just bisected the issue to this
patch and also saw this patch landed in the linus tree yesterday/today.
Not sure if this is something to do with the fact that pKVM skips to
__kvm_handle_stub_hvc in __host_hvc.

Let me know if you want be to try something.

-- 
Regards,
Sudeep