Messages in this thread Patch in this message | ![/](/images/icornerl.gif) | | Date | Tue, 4 Jul 2023 16:27:04 +0000 | From | Mostafa Saleh <> | Subject | Re: [PATCH v3] KVM: arm64: Use BTI for nvhe |
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Hi Sudeep,
On Tue, Jul 04, 2023 at 03:33:39PM +0100, Sudeep Holla wrote: > 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.
Thanks a lot for the information.
I checked this now, and I believe I found an issue. I see that __kvm_hyp_init_cpu calls kvm_host_psci_cpu_entry indirectly and there is no BTI there. I think this is the only C function that needs special handling.
Can you please check if this solves the issue?
diff --git a/arch/arm64/kvm/hyp/nvhe/host.S b/arch/arm64/kvm/hyp/nvhe/host.S index c87c63133e10..7df63f364c3c 100644 --- a/arch/arm64/kvm/hyp/nvhe/host.S +++ b/arch/arm64/kvm/hyp/nvhe/host.S @@ -297,3 +297,8 @@ SYM_CODE_START(__kvm_hyp_host_forward_smc) ret SYM_CODE_END(__kvm_hyp_host_forward_smc) + +SYM_CODE_START(kvm_host_psci_cpu_entry) + bti j + b __kvm_host_psci_cpu_entry +SYM_CODE_END(kvm_host_psci_cpu_entry) diff --git a/arch/arm64/kvm/hyp/nvhe/psci-relay.c b/arch/arm64/kvm/hyp/nvhe/psci-relay.c index 08508783ec3d..24543d2a3490 100644 --- a/arch/arm64/kvm/hyp/nvhe/psci-relay.c +++ b/arch/arm64/kvm/hyp/nvhe/psci-relay.c @@ -200,7 +200,7 @@ static int psci_system_suspend(u64 func_id, struct kvm_cpu_context *host_ctxt) __hyp_pa(init_params), 0); } -asmlinkage void __noreturn kvm_host_psci_cpu_entry(bool is_cpu_on) +asmlinkage void __noreturn __kvm_host_psci_cpu_entry(bool is_cpu_on) { struct psci_boot_args *boot_args; struct kvm_cpu_context *host_ctxt;
> > 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.
My current setup has no CONFIG_ARM_PSCI_CPUIDLE?, I will try to find something I can test with.
> > > > > 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.
I don't see an issue in this, as this path has no indirect branches.
> > __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. >
Thanks, Mostafa
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