Re: [PATCH v4 14/18] mshv: Order pt_vp_array publish against irqfd assertion path

[Anirudh Rayabharam <anirudh@anirudhrb.com>]

On Thu, May 07, 2026 at 03:44:16PM +0000, Stanislav Kinsburskii wrote:
> mshv_partition_ioctl_create_vp() initialises a VP struct (allocations,
> mutex_init, init_waitqueue_head, page mappings) and then publishes the
> pointer into partition->pt_vp_array.  Several ISR paths read this array
> locklessly: the intercept ISR, the two scheduler ISRs, and
> mshv_try_assert_irq_fast() on the irqfd fast path.
> 
> Of these, only mshv_try_assert_irq_fast() can structurally race the
> publish.  It runs from an eventfd waker without holding pt_mutex, and
> MSHV_IRQFD does not require the target lapic_apic_id (== vp_index) to
> refer to an existing VP at registration time.  A user can therefore
> register an irqfd targeting a yet-to-be-created VP, then trigger
> mshv_try_assert_irq_fast() concurrently with MSHV_CREATE_VP for the
> same index.  On weakly-ordered architectures the reader can observe a
> non-NULL pointer in pt_vp_array before the initialising stores to the
> VP struct become visible, leading to use of partially-initialised
> fields (e.g. vp_register_page).
> 
> The other ISR readers cannot reach this race: the hypervisor will not
> generate intercept or scheduler messages for a VP that has never been
> told to run, and the user can only call MSHV_RUN_VP on the VP fd
> returned by MSHV_CREATE_VP, which by construction is returned after
> the publish.  Leave those readers as plain loads.
> 
> Use smp_store_release() in mshv_partition_ioctl_create_vp() to publish
> the pointer, and pair it with smp_load_acquire() in
> mshv_try_assert_irq_fast().  On x86 these compile to plain accesses
> under TSO; on ARM64 they emit one-instruction acquire/release barriers,
> acceptable on this fast path.
> 
> The destroy-side path (destroy_partition() clearing pt_vp_array[i] to
> NULL after kfree(vp)) has a separate ordering and lifetime concern
> that is out of scope here.
> 
> Fixes: 621191d709b14 ("Drivers: hv: Introduce mshv_root module to expose /dev/mshv to VMMs")
> Signed-off-by: Stanislav Kinsburskii <skinsburskii@linux.microsoft.com>
> ---
>  drivers/hv/mshv_eventfd.c   |    9 ++++++++-
>  drivers/hv/mshv_root_main.c |    8 +++++++-
>  2 files changed, 15 insertions(+), 2 deletions(-)

Reviewed-by: Anirudh Rayabharam (Microsoft) <anirudh@anirudhrb.com>