linux-kernel - Re: arm64 spectre-bhb backports break boot on stable kernels <= v5.4

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Message-ID: <20220408162139.GB28108@willie-the-truck>
Date:   Fri, 8 Apr 2022 17:21:40 +0100
From:   Will Deacon <will@...nel.org>
To:     James Morse <james.morse@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        gregkh@...uxfoundation.org, maz@...nel.org,
        catalin.marinas@....com, mark.rutland@....com
Subject: Re: arm64 spectre-bhb backports break boot on stable kernels <= v5.4

Hi James,

On Fri, Apr 08, 2022 at 05:08:00PM +0100, James Morse wrote:
> 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).

Yup! FWIW, the hardening options are enabled in Android (GKI), but this was
reported to us externally by somebody using a custom config.

> > 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.

Makes sense. Another option is to check for ARM64_HARDEN_BRANCH_PREDICTOR in
spectre_bhb_enable_mitigation(), but then I think the KVM code would need
to query the mitigation state rather than just the cap.

> > 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;
>         }

That'll work, but will sysfs report that BHB is mitigated even if
!ARM64_HARDEN_BRANCH_PREDICTOR?

> --------------------%<--------------------
> 
> 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.

Cheers. I know 4.19 is busted too, but I didn't check 4.14.

Will