Re: [PATCH 1/2] x86/hyperv: Fix NULL deref in set_hv_tscchange_cb() if Hyper-V setup fails

[Wei Liu]

On Fri, Oct 29, 2021 at 11:14:59AM +0200, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@xxxxxxxxxx> writes:
> 
> > Check for re-enlightenment support and for a valid hv_vp_index array
> > prior to derefencing hv_vp_index when setting Hyper-V's TSC change
> > callback.  If Hyper-V setup failed in hyperv_init(), e.g. because of a
> > bad VMM config that doesn't advertise the HYPERCALL MSR, the kernel will
> > still report that it's running under Hyper-V, but will have silently
> > disabled nearly all functionality.
> >
> >   BUG: kernel NULL pointer dereference, address: 0000000000000010
> >   #PF: supervisor read access in kernel mode
> >   #PF: error_code(0x0000) - not-present page
> >   PGD 0 P4D 0
> >   Oops: 0000 [#1] SMP
> >   CPU: 4 PID: 1 Comm: swapper/0 Not tainted 5.15.0-rc2+ #75
> >   Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
> >   RIP: 0010:set_hv_tscchange_cb+0x15/0xa0
> >   Code: <8b> 04 82 8b 15 12 17 85 01 48 c1 e0 20 48 0d ee 00 01 00 f6 c6 08
> >   ...
> >   Call Trace:
> >    kvm_arch_init+0x17c/0x280
> >    kvm_init+0x31/0x330
> >    vmx_init+0xba/0x13a
> >    do_one_initcall+0x41/0x1c0
> >    kernel_init_freeable+0x1f2/0x23b
> >    kernel_init+0x16/0x120
> >    ret_from_fork+0x22/0x30
> >
> > Fixes: 93286261de1b ("x86/hyperv: Reenlightenment notifications support")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Cc: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > ---
> >  arch/x86/hyperv/hv_init.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> > index 708a2712a516..6cc845c026d4 100644
> > --- a/arch/x86/hyperv/hv_init.c
> > +++ b/arch/x86/hyperv/hv_init.c
> > @@ -139,7 +139,7 @@ void set_hv_tscchange_cb(void (*cb)(void))
> >  	struct hv_reenlightenment_control re_ctrl = {
> >  		.vector = HYPERV_REENLIGHTENMENT_VECTOR,
> >  		.enabled = 1,
> > -		.target_vp = hv_vp_index[smp_processor_id()]
> > +		.target_vp = -1,
> >  	};
> >  	struct hv_tsc_emulation_control emu_ctrl = {.enabled = 1};
> >  
> > @@ -148,6 +148,11 @@ void set_hv_tscchange_cb(void (*cb)(void))
> >  		return;
> >  	}
> >  
> > +	if (!hv_vp_index)
> > +		return;
> > +
> > +	re_ctrl.target_vp = hv_vp_index[smp_processor_id()];
> > +
> >  	hv_reenlightenment_cb = cb;
> >  
> >  	/* Make sure callback is registered before we write to MSRs */
> 
> The patch looks good, however, it needs to be applied on top of the
> already merged:
> 
> https://lore.kernel.org/linux-hyperv/20211012155005.1613352-1-vkuznets@xxxxxxxxxx/

Sean, are you going to rebase?

Wei.