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

[Mostafa Saleh]

Hi Marc and Oliver,

I was double checking that nothing else was missed.

I found there is another problem for hw that has BTI and is affected
by specterv3a.

"br'' instructions are generated at runtime for the vector
table(__bp_harden_hyp_vecs).
These branches would land on vectors in __kvm_hyp_vector at offset 8.

As all the macros are defined with valid_vect/invalid_vect, it is
sufficient to add "bti j"
there at the correct offset.

I am not sure if such hardware exists. I tested this with a stubbed
"has_spectre_v3a" which
confirms the issue and the fix.

Please let me know if this fix suitable, I can include it with the other fix in
"[PATCH] KVM: arm64: Add missing BTI instruction in kvm_host_psci_cpu_entry"

diff --git a/arch/arm64/kvm/hyp/hyp-entry.S b/arch/arm64/kvm/hyp/hyp-entry.S
index 8f3f93fa119e..175c030379e3 100644
--- a/arch/arm64/kvm/hyp/hyp-entry.S
+++ b/arch/arm64/kvm/hyp/hyp-entry.S
@@ -154,6 +154,12 @@ SYM_CODE_END(\label)
  esb
  stp x0, x1, [sp, #-16]!
 662:
+ /*
+ * Specter vectors __bp_harden_hyp_vecs generate br instructions at runtime
+ * that jump at offset 8 at __kvm_hyp_vector.
+ * As hyp .text is guarded section, it needs bti j.
+ */
+ bti j
  b \target

 check_preamble_length 661b, 662b
@@ -165,6 +171,8 @@ check_preamble_length 661b, 662b
  nop
  stp x0, x1, [sp, #-16]!
 662:
+ /* Check valid_vect */
+ bti j
  b \target

 check_preamble_length 661b, 662b

--
Thanks,
Mostafa




On Wed, Jul 5, 2023 at 4:56 PM Mostafa Saleh <smostafa@xxxxxxxxxx> wrote:
>
> Hi Sudeep,
>
> On Tue, Jul 04, 2023 at 08:25:29PM +0100, Sudeep Holla wrote:
> > On Tue, Jul 04, 2023 at 04:27:04PM +0000, Mostafa Saleh wrote:
> > > 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.
> > >
> >
> > So it is in the wake up path. Thanks for the description, now I understand
> > the issue and fix better.
> >
> > > Can you please check if this solves the issue?
> > >
> >
> > Yes, the below patch fixed the issue. Feel free to add when you post the
> > formal patch.
> >
> > Reported-and-Tested-by: Sudeep Holla <sudeep.holla@xxxxxxx>
>
> Thanks for testing the patch, I will post it on the list.
>
> > > 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.
> > >
> >
> > No worries, I can help until you find one.
> >
> >
> > > > >
> > > > > > 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.
> > >
> >
> > Understood.
>
> Thanks,
> Mostafa
>