Re: [PATCH v2 1/2] x86/hyperv: Fix NULL deref in set_hv_tscchange_cb() if Hyper-V setup fails

[Wei Liu <wei.liu@kernel.org>]

On Fri, Nov 05, 2021 at 11:16:14AM +0100, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@google.com> writes:
> 
> > Check 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(), 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@vger.kernel.org
> > Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> >  arch/x86/hyperv/hv_init.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> > index 24f4a06ac46a..7d252a58fbe4 100644
> > --- a/arch/x86/hyperv/hv_init.c
> > +++ b/arch/x86/hyperv/hv_init.c
> > @@ -177,6 +177,9 @@ void set_hv_tscchange_cb(void (*cb)(void))
> >  		return;
> >  	}
> >  
> > +	if (!hv_vp_index)
> > +		return;
> > +
> 
> Arguably, we could've merged this with 'if (!hv_reenlightenment_available())' 
> above to get a message printed:
> 
> diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> index 24f4a06ac46a..4a2a091c2f0e 100644
> --- a/arch/x86/hyperv/hv_init.c
> +++ b/arch/x86/hyperv/hv_init.c
> @@ -172,7 +172,7 @@ void set_hv_tscchange_cb(void (*cb)(void))
>         };
>         struct hv_tsc_emulation_control emu_ctrl = {.enabled = 1};
>  
> -       if (!hv_reenlightenment_available()) {
> +       if (!hv_reenlightenment_available() || !hv_vp_index) {
>                 pr_warn("Hyper-V: reenlightenment support is unavailable\n");
>                 return;
>         }
> 
> just to have an indication that something is off.
> 
> >  	hv_reenlightenment_cb = cb;
> >  
> >  	/* Make sure callback is registered before we write to MSRs */
> 
> With or without the change,
> 
> Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

Thanks Sean and Vitaly. Both patches look good to me. They will be taken
via hyperv-fixes after v5.16-rc1 is cut.

Wei.

> 
> -- 
> Vitaly
>