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cbfe2b24a1
Fix a potential race between decrementing an LPI's reference count and
evicting that structure from the LPI xarray.
LPI structures are maintained in the VGIC LPI xarray (dist->lpi_xa).
When the reference count of an LPI structure drops to zero,
vgic_release_lpi_locked() removes the structure from the xarray and
frees it under the xarray lock.
However, the release of an LPI can race with a concurrent LPI
re-registration with the same INTID via vgic_add_lpi() on another CPU,
since the reference count drop and the xarray eviction are not performed
in a single atomic step. This can happen e.g. if the guest issues a
DISCARD while the LPI is still referenced from a vCPU's active-pending
list (ap_list), and the same INTID is re-mapped via MAPTI.
Particularly, vgic_release_lpi_locked() is called from two distinct
paths: direct release via vgic_put_irq(), and deferred release via
vgic_release_deleted_lpis(). During direct release, the issue can result
in deleting a newly registered LPI from the xarray:
CPU0 (Releasing LPI) CPU1 (Adding new LPI)
==================== =====================
vgic_put_irq()
__vgic_put_irq()
refcount_dec_and_test()
vgic_add_lpi()
xa_lock_irqsave()
old_irq = xa_load(.., intid)
vgic_try_get_irq_ref(old_irq) == false
new IRQ inserted --> __xa_store(.., intid, ..)
xa_unlock_irqrestore()
xa_lock_irqsave();
vgic_release_lpi_locked()
__xa_erase(.., irq->intid) <-- BUG: new IRQ is erased
kfree_rcu(old_irq)
During the deferred release path, the old IRQ can be leaked:
CPU0 (Releasing LPI) CPU1 (Adding new LPI)
==================== =====================
vgic_put_irq_norelease()
__vgic_put_irq()
refcount_dec_and_test()
irq->pending_release = true
vgic_add_lpi()
xa_lock_irqsave()
old_irq = xa_load(.., intid)
vgic_try_get_irq_ref(oldirq) == false
BUG: old IRQ overwritten --> __xa_store(.., intid, ..)
xa_unlock_irqrestore()
vgic_release_deleted_lpis()
xa_lock_irqsave()
xa_for_each() { .. } <-- old IRQ with pending_release = true
is gone, so it cannot be released
To fix the direct release path, move the reference count drop inside
the xarray lock, making sure that vgic_add_lpi() never encounters the
to-be-released LPI.
In the deferred release path, the refcount drop must happen under a raw
spinlock, so the xarray lock cannot be grabbed, and the same solution
does not work. Instead, update vgic_add_lpi(), so that if it evicts
an LPI from the xarray, it takes on the responsibility of freeing it.
Consequently, an LPI may now be freed concurrently after a deferred
release drops the refcount, so accessing the pending_release field is no
longer safe from use-after-free. Delete all uses of the flag, and update
vgic_release_deleted_lpis() to identify orphaned LPIs purely based on
their refcount.
Reported-by: Claude:claude-opus-4-6
Fixes:
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|---|---|---|
| .. | ||
| acpi | ||
| asm-generic | ||
| clocksource | ||
| crypto | ||
| cxl | ||
| drm | ||
| dt-bindings | ||
| hyperv | ||
| keys | ||
| kunit | ||
| kvm | ||
| linux | ||
| math-emu | ||
| media | ||
| memory | ||
| misc | ||
| net | ||
| pcmcia | ||
| ras | ||
| rdma | ||
| rv | ||
| scsi | ||
| soc | ||
| sound | ||
| target | ||
| trace | ||
| uapi | ||
| ufs | ||
| vdso | ||
| video | ||
| xen | ||
| Kbuild | ||