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Re: [PATCH v2] xen/vcpu: ensure no write when runstate handler is NULL
- To: Roger Pau Monne <roger@xxxxxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 26 Oct 2026 14:06:48 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Mon, 26 Oct 2026 13:06:55 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 23.10.2026 18:17, Roger Pau Monne wrote:
> --- a/xen/arch/x86/x86_64/domain.c
> +++ b/xen/arch/x86/x86_64/domain.c
> @@ -58,6 +58,11 @@ compat_vcpu_op(int cmd, unsigned int vcpuid,
> XEN_GUEST_HANDLE_PARAM(void) arg)
> rc = 0;
> guest_from_compat_handle(v->runstate_guest.compat, area.addr.h);
>
> + /* NULL handle is deregistering the address. */
> + if ( compat_handle_is_null(area.addr.h) )
> + break;
> +
> + /* Update the newly-registered runstate area, best-effort. */
> if ( v == current )
> {
> XLAT_vcpu_runstate_info(&info, &v->runstate);
While likely unnecessary in this x86-only handling, ...
> --- a/xen/common/domain.c
> +++ b/xen/common/domain.c
> @@ -2330,6 +2330,11 @@ long common_vcpu_op(int cmd, struct vcpu *v,
> XEN_GUEST_HANDLE_PARAM(void) arg)
> rc = 0;
> runstate_guest(v) = area.addr.h;
>
> + /* NULL handle is deregistering the address. */
> + if ( guest_handle_is_null(area.addr.h) )
> + break;
> +
> + /* Update the newly-registered runstate area, best-effort. */
> if ( v == current )
> {
> __copy_to_guest(runstate_guest(v), &v->runstate, 1);
... if we make null an official "de-register", then imo the earlier
guest_handle_okay() also wants bypassing in the null case. An arch
could - in principle - reject a null handle.
Jan
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