Re: [RFC PATCH 02/12] drivers: hv: hv_common: Allocate Hyper-V output arg page when VSM is enabled

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] hv_apicid_to_vp_index() unconditionally dereferences the unallocated hyperv_pcpu_output_arg pointer in standard/SNP guests.
--

commit 0ee3313e113c9ca7969923f2af5a1ed18ddb25d2
Author: Thara Gopinath <tgopinath@linux.microsoft.com>

drivers: hv: hv_common: Allocate Hyper-V output arg page when VSM is enabled

This commit extends hv_output_page_exists() to return true when 
CONFIG_HYPERV_VSM is enabled. This ensures the per-CPU output argument
page is allocated at boot for VSM hypercalls.

> diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
> index 31256cb22b39e..44f4a10984618 100644
> --- a/drivers/hv/hv_common.c
> +++ b/drivers/hv/hv_common.c
> @@ -259,7 +259,8 @@ static void hv_kmsg_dump_register(void)
>  
>  static inline bool hv_output_page_exists(void)
>  {
> -	return hv_parent_partition() || IS_ENABLED(CONFIG_HYPERV_VTL_MODE);
> +	return hv_parent_partition() || IS_ENABLED(CONFIG_HYPERV_VTL_MODE) ||
> +		IS_ENABLED(CONFIG_HYPERV_VSM);

[Severity: High]
This is a pre-existing issue, but modifying this function highlights a
vulnerability for SEV-SNP guests where VSM and VTL modes are disabled.

When an SEV-SNP guest boots secondary CPUs, hv_smp_prepare_cpus() overrides
the wakeup callback with hv_snp_boot_ap(), which then calls
hv_apicid_to_vp_index() in arch/x86/hyperv/ivm.c.

If CONFIG_HYPERV_VSM and CONFIG_HYPERV_VTL_MODE are both disabled,
hv_output_page_exists() evaluates to false. This means hyperv_pcpu_output_arg
is never allocated during hv_common_init() and remains NULL.

Consequently, arch/x86/hyperv/hv_init.c:hv_apicid_to_vp_index() dereferences
this unallocated pointer:

arch/x86/hyperv/hv_init.c:hv_apicid_to_vp_index() {
        ...
        output = *this_cpu_ptr(hyperv_pcpu_output_arg);
        ...
        status = hv_do_hypercall(HVCALL_GET_VP_ID_FROM_APIC_ID, input, output);
        ...
}

Since this_cpu_ptr(NULL) resolves to the base of the per-CPU area, it
dereferences a garbage pointer. This is then passed to hv_do_hypercall(),
causing the hypervisor to overwrite random physical memory with the hypercall
result, and eventually oopsing the kernel when output[0] is read.

Should hv_output_page_exists() also be extended to account for SEV-SNP
guests that rely on hv_apicid_to_vp_index() during boot?

>  }
>  
>  void __init hv_get_partition_id(void)

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260901165647.3160413-1-tgopinath@linux.microsoft.com?part=2