Re: [PATCH v2 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 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@xxxxxxxxxxxxxxx
> Cc: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  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@xxxxxxxxxx>

-- 
Vitaly