Re: [PATCH] x86/hyperv: fix kexec crash due to VP assist page corruption

[Anirudh Rayabharam <anirudh@anirudhrb.com>]

On Mon, Aug 26, 2024 at 02:36:44PM +0200, Vitaly Kuznetsov wrote:
> Anirudh Rayabharam <anirudh@anirudhrb.com> writes:
> 
> > From: Anirudh Rayabharam (Microsoft) <anirudh@anirudhrb.com>
> >
> > 9636be85cc5b ("x86/hyperv: Fix hyperv_pcpu_input_arg handling when CPUs go
> > online/offline") introduces a new cpuhp state for hyperv initialization.
> >
> > cpuhp_setup_state() returns the state number if state is CPUHP_AP_ONLINE_DYN
> > or CPUHP_BP_PREPARE_DYN and 0 for all other states. For the hyperv case,
> > since a new cpuhp state was introduced it would return 0. However,
> > in hv_machine_shutdown(), the cpuhp_remove_state() call is conditioned upon
> > "hyperv_init_cpuhp > 0". This will never be true and so hv_cpu_die() won't be
> > called on all CPUs. This means the VP assist page won't be reset. When the
> > kexec kernel tries to setup the VP assist page again, the hypervisor corrupts
> > the memory region of the old VP assist page causing a panic in case the kexec
> > kernel is using that memory elsewhere. This was originally fixed in dfe94d4086e4
> > ("x86/hyperv: Fix kexec panic/hang issues").
> >
> > Set hyperv_init_cpuhp to CPUHP_AP_HYPERV_ONLINE upon successful setup so that
> > the hyperv cpuhp state is removed correctly on kexec and the necessary cleanup
> > takes place.
> >
> > Cc: stable@vger.kernel.org
> > Fixes: 9636be85cc5b ("x86/hyperv: Fix hyperv_pcpu_input_arg handling when CPUs go online/offline")
> > Signed-off-by: Anirudh Rayabharam (Microsoft) <anirudh@anirudhrb.com>
> > ---
> >  arch/x86/hyperv/hv_init.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> > index 17a71e92a343..81d1981a75d1 100644
> > --- a/arch/x86/hyperv/hv_init.c
> > +++ b/arch/x86/hyperv/hv_init.c
> > @@ -607,7 +607,7 @@ void __init hyperv_init(void)
> >  
> >  	register_syscore_ops(&hv_syscore_ops);
> >  
> > -	hyperv_init_cpuhp = cpuhp;
> > +	hyperv_init_cpuhp = CPUHP_AP_HYPERV_ONLINE;
> 
> Do we really need 'hyperv_init_cpuhp' at all? I.e. post-change (which
> LGTM btw), I can only see one usage in hv_machine_shutdown():
> 
>    if (kexec_in_progress && hyperv_init_cpuhp > 0)
>            cpuhp_remove_state(hyperv_init_cpuhp);
> 
> and I'm wondering if the 'hyperv_init_cpuhp' check is really
> needed. This only case where this check would fail is if we're crashing
> in between ms_hyperv_init_platform() and hyperv_init() afaiu. Does it

Or if we fail to setup the cpuhp state for some reason but don't
actually crash and then later do a kexec?

I guess I was just trying to be extra safe and make sure we have
actually setup the cpuhp state before calling cpuhp_remove_state()
for it. However, looking elsewhere in the kernel code I don't
see anybody doing this for custom states...

> hurt if we try cpuhp_remove_state() anyway?

cpuhp_invoke_callback() would trigger a WARNING if we try to remove a
cpuhp state that was never setup.

184         if (cpuhp_step_empty(bringup, step)) {
185                 WARN_ON_ONCE(1);
186                 return 0;
187         }

Thanks,
Anirudh

> 
> >  
> >  	if (cpuid_ebx(HYPERV_CPUID_FEATURES) & HV_ACCESS_PARTITION_ID)
> >  		hv_get_partition_id();
> > @@ -637,7 +637,7 @@ void __init hyperv_init(void)
> >  clean_guest_os_id:
> >  	wrmsrl(HV_X64_MSR_GUEST_OS_ID, 0);
> >  	hv_ivm_msr_write(HV_X64_MSR_GUEST_OS_ID, 0);
> > -	cpuhp_remove_state(cpuhp);
> > +	cpuhp_remove_state(CPUHP_AP_HYPERV_ONLINE);
> >  free_ghcb_page:
> >  	free_percpu(hv_ghcb_pg);
> >  free_vp_assist_page:
> 
> -- 
> Vitaly
>