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iommufd: Move vevent memory allocation outside spinlock

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The veventq memory allocation happens inside the spinlock. Given its depth
is decided by the user space, this leaves a vulnerability, where userspace
can allocate large queues to exhaust atomic memory reserves.

Move the allocation outside the spinlock and use GFP_NOWAIT, which can fail
fast under memory pressure without dipping into the GFP_ATOMIC reserves or
direct-reclaiming from the threaded IRQ handler. On allocation failure,
queue the lost_events_header (so userspace learns of the drop) and return
-ENOMEM so the caller learns of the kernel-side memory pressure.

This is intentionally distinct from the queue-overflow path, which also
queues the lost_events_header but returns 0: a full queue is an expected
userspace-pacing condition rather than a kernel error.

A subsequent change will cap the upper bound of the veventq_depth.

Fixes: e36ba5ab80 ("iommufd: Add IOMMUFD_OBJ_VEVENTQ and IOMMUFD_CMD_VEVENTQ_ALLOC")
Link: https://patch.msgid.link/r/5ff36b5d80f7f6299f851be532a5195c1d2f1dae.1779408671.git.nicolinc@nvidia.com
Cc: stable@vger.kernel.org
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
This commit is contained in:
Nicolin Chen 2026-05-21 17:36:33 -07:00 committed by Jason Gunthorpe
parent 85345becfe
commit 47443565d1
1 changed files with 8 additions and 5 deletions
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13
drivers/iommu/iommufd/driver.c
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@ -149,15 +149,18 @@ int iommufd_viommu_report_event(struct iommufd_viommu *viommu,
goto out_unlock_veventqs;
}
spin_lock(&veventq->common.lock);
if (veventq->num_events == veventq->depth) {
/* Pre-allocate to avoid GFP_ATOMIC; use GFP_NOWAIT to avoid sleeping */
vevent = kzalloc_flex(*vevent, event_data, data_len, GFP_NOWAIT);
if (!vevent) {
spin_lock(&veventq->common.lock);
vevent = &veventq->lost_events_header;
rc = -ENOMEM;
goto out_set_header;
}
vevent = kzalloc_flex(*vevent, event_data, data_len, GFP_ATOMIC);
if (!vevent) {
rc = -ENOMEM;
spin_lock(&veventq->common.lock);
if (veventq->num_events == veventq->depth) {
kfree(vevent);
vevent = &veventq->lost_events_header;
goto out_set_header;
}