From: Stanislav Kinsburskii Date: Thu, 23 Jul 2026 00:28:11 +0000 (+0000) Subject: mshv: Order pt_vp_array publish against irqfd assertion path X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=b098dc869219c15dc49bf9cf63fb5fc1481d3373;p=thirdparty%2Flinux.git mshv: Order pt_vp_array publish against irqfd assertion path 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 Reviewed-by: Anirudh Rayabharam (Microsoft) Signed-off-by: Wei Liu --- diff --git a/drivers/hv/mshv_eventfd.c b/drivers/hv/mshv_eventfd.c index 047e5bd43238..06aef99c8298 100644 --- a/drivers/hv/mshv_eventfd.c +++ b/drivers/hv/mshv_eventfd.c @@ -169,7 +169,14 @@ static int mshv_try_assert_irq_fast(struct mshv_irqfd *irqfd) return -EOPNOTSUPP; #endif - vp = partition->pt_vp_array[irq->lapic_apic_id]; + /* + * Pairs with smp_store_release() in mshv_partition_ioctl_create_vp(). + * MSHV_IRQFD does not require the target lapic_apic_id to refer to an + * existing VP, so this read can race a concurrent VP creation; the + * acquire ensures that a non-NULL pointer implies the VP's + * initialising stores are visible. + */ + vp = smp_load_acquire(&partition->pt_vp_array[irq->lapic_apic_id]); if (!vp->vp_register_page) return -EOPNOTSUPP; diff --git a/drivers/hv/mshv_root_main.c b/drivers/hv/mshv_root_main.c index 644f9b10cbba..8a15448e2ace 100644 --- a/drivers/hv/mshv_root_main.c +++ b/drivers/hv/mshv_root_main.c @@ -1157,7 +1157,13 @@ mshv_partition_ioctl_create_vp(struct mshv_partition *partition, /* already exclusive with the partition mutex for all ioctls */ partition->pt_vp_count++; - partition->pt_vp_array[args.vp_index] = vp; + /* + * Pairs with smp_load_acquire() in mshv_try_assert_irq_fast(), which + * can run concurrently from an irqfd waker without holding pt_mutex. + * The release ensures the VP's initialising stores are visible to any + * reader that observes a non-NULL pointer in pt_vp_array. + */ + smp_store_release(&partition->pt_vp_array[args.vp_index], vp); goto out;