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

[Sudeep Holla]

Hi Mostafa,

On Tue, Jul 04, 2023 at 02:18:09PM +0000, Mostafa Saleh wrote:
> Hi Sudeep,
> 
> On Tue, Jul 04, 2023 at 02:41:36PM +0100, Sudeep Holla wrote:
> > 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.
> 
> One of the challenges of BTI is that we need to add explicit BTI instructions
> for assembly code. I checked the code to make sure that nothing was missing,
> but maybe this is not the case.
> Can you please share more about the issue (is ESR a Branch Target Exception,
> call stack...) if possible.

I haven't debugged it any further, just reported it as soon as I bisected it.
Reverting this get back the booting system. I am not sure if anything is going
wrong when the CPU is entering suspend(highly unlikely in normal scenario but
I am not so sure with pKVM trapping these PSCI calls now) or when it is woken
up and resuming back. IIUC this now will happen via kvm_hyp_cpu_resume->
__kvm_hyp_init_cpu->___kvm_hyp_init. 

> Also, is this with CONFIG_ARM_PSCI_CPUIDLE?

Yes, basically the cpus can enter cpu_suspend which IIUC pKVM traps and
handle for the host.

> 
> > Not sure if this is something to do with the fact that pKVM skips to
> > __kvm_handle_stub_hvc in __host_hvc.

Sorry, my bad. I meant pKVM skips calling __kvm_handle_stub_hvc in __host_hvc
and jumps to __host_exit directly. Sorry for that, one wrong "to" changed the
whole meaning.

> __kvm_handle_stub_hvc is called from __host_hvc with "br x5"
> That's why "bti j" was added at the beginning of __kvm_handle_stub_hvc,
> so this should be fine.
>

Yes I saw that and understood that but I wanted to tell the above which went
horribly wrong before.

-- 
Regards,
Sudeep