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

[Vitaly Kuznetsov]

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/

-- 
Vitaly