arm64 spectre-bhb backports break boot on stable kernels <= v5.4

[James Morse]

Hi Will,

On 08/04/2022 13:00, Will Deacon wrote:
> Booting stable kernels <= v5.4 on arm64 with CONFIG_HARDEN_BRANCH_PREDICTOR=n
> results in a NULL pointer dereference during boot due to kvm_get_hyp_vector()
> dereferencing a NULL pointer from arm64_get_bp_hardening_data():
> 
> [    2.384444] Internal error: Oops: 96000004 [#1] PREEMPT SMP
> [    2.384461] pstate: 20400085 (nzCv daIf +PAN -UAO)
> [    2.384472] pc : cpu_hyp_reinit+0x114/0x30c
> [    2.384476] lr : cpu_hyp_reinit+0x80/0x30c
> [    2.384477] sp : ffff800010013e20
> [    2.384479] x29: ffff800010013e20 x28: ffff0000df033f00
> [    2.384484] x27: ffff800011caa000 x26: ffff0000de58e000
> [    2.384487] x25: ffff800011e57000 x24: ffff800011caad90
> [    2.384490] x23: ffff800011c778e0 x22: ffff800011a56000
> [    2.384494] x21: ffff800011a56000 x20: 0000b471121a6000
> [    2.384497] x19: 00000000de588000 x18: 0000000000000000
> [    2.384500] x17: 0000000000000000 x16: 0000000000000400
> [    2.384503] x15: 0000000000001000 x14: 00000000000007f2
> [    2.384507] x13: 0000000000000004 x12: 0000000000000002
> [    2.384510] x11: 0000000000000000 x10: 00000000121a6000
> [    2.384513] x9 : ffff800011c77fd8 x8 : 0000000000000010
> [    2.384516] x7 : 4f422effff306b64 x6 : 0000008080000000
> [    2.384519] x5 : 0000008080000000 x4 : ffff800011fabc94
> [    2.384522] x3 : ffff800011f5bde0 x2 : 0000000000000001
> [    2.384526] x1 : 00000000121a0000 x0 : ffff8000118f962d
> [    2.384529] Call trace:
> [    2.384533]  cpu_hyp_reinit+0x114/0x30c
> [    2.384537]  _kvm_arch_hardware_enable+0x30/0x54
> [    2.384541]  flush_smp_call_function_queue+0xe4/0x154
> [    2.384544]  generic_smp_call_function_single_interrupt+0x10/0x18
> [    2.384549]  ipi_handler+0x170/0x2b0
> [    2.384555]  handle_percpu_devid_fasteoi_ipi+0x120/0x1cc
> [    2.384560]  __handle_domain_irq+0x9c/0xf4
> [    2.384563]  gic_handle_irq+0x6c/0xe4
> [    2.384566]  el1_irq+0xf0/0x1c0
> [    2.384570]  arch_cpu_idle+0x28/0x44
> [    2.384574]  do_idle+0x100/0x2a8
> [    2.384577]  cpu_startup_entry+0x20/0x24
> [    2.384581]  secondary_start_kernel+0x1b0/0x1cc
> [    2.384589] Code: b9469d08 7100011f 540003ad 52800208 (f9400108)
> [    2.384600] ---[ end trace 266d08dbf96ff143 ]---
> [    2.385171] Kernel panic - not syncing: Fatal exception in interrupt

Yikes!

Interesting to know that stuff behind CONFIG_EXPERT has someone who cares about it.
(I was going to propose dropping the Kconfig option after a while).


> The faulting load is the access to 'data->template_start', for which the
> compiler emits the hilarious:
> 
> 	mov	w8, #0x10
> 	ldr	x8, [x8]

This is nasal-daemon territory, so I guess the compiler can do whatever it likes.


> I can bodge this as below (untested), but it's pretty grotty.

I wanted to keep the detection code even if the feature is disabled so the sysfs reporting
is always correct.


> Please can you take a look?

Ugh, arm64_get_bp_hardening_data() returns NULL with that Kconfig setup.


For v5.4, this fixes it for me:
--------------------%<--------------------
diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
index 78d110667c0c..ffe0aad96b17 100644
--- a/arch/arm64/include/asm/kvm_mmu.h
+++ b/arch/arm64/include/asm/kvm_mmu.h
@@ -479,7 +479,8 @@ static inline void *kvm_get_hyp_vector(void)
        int slot = -1;

        if ((cpus_have_const_cap(ARM64_HARDEN_BRANCH_PREDICTOR) ||
-            cpus_have_const_cap(ARM64_SPECTRE_BHB)) && data->template_start) {
+            cpus_have_const_cap(ARM64_SPECTRE_BHB)) &&
+           data && data->template_start) {
                vect = kern_hyp_va(kvm_ksym_ref(__bp_harden_hyp_vecs_start));
                slot = data->hyp_vectors_slot;
        }

--------------------%<--------------------

I'll check the other versions and post patches to the stable list. Earlier stable
backports grew a dependency between these features as it wasn't possible to unpick the
dependencies.


Thanks,

James