GHSA-cjwf-5vh9-w48g

Описание

In the Linux kernel, the following vulnerability has been resolved:

iommu/vt-d: Fix potential lockup if qi_submit_sync called with 0 count

If qi_submit_sync() is invoked with 0 invalidation descriptors (for instance, for DMA draining purposes), we can run into a bug where a submitting thread fails to detect the completion of invalidation_wait. Subsequently, this led to a soft lockup. Currently, there is no impact by this bug on the existing users because no callers are submitting invalidations with 0 descriptors. This fix will enable future users (such as DMA drain) calling qi_submit_sync() with 0 count.

Suppose thread T1 invokes qi_submit_sync() with non-zero descriptors, while concurrently, thread T2 calls qi_submit_sync() with zero descriptors. Both threads then enter a while loop, waiting for their respective descriptors to complete. T1 detects its completion (i.e., T1's invalidation_wait status changes to QI_DONE by HW) and proceeds to call reclaim_free_desc() to reclaim all descriptors, potentially including adjacent ones of other threads that are also marked as QI_DONE.

During this time, while T2 is waiting to acquire the qi->q_lock, the IOMMU hardware may complete the invalidation for T2, setting its status to QI_DONE. However, if T1's execution of reclaim_free_desc() frees T2's invalidation_wait descriptor and changes its status to QI_FREE, T2 will not observe the QI_DONE status for its invalidation_wait and will indefinitely remain stuck.

This soft lockup does not occur when only non-zero descriptors are submitted.In such cases, invalidation descriptors are interspersed among wait descriptors with the status QI_IN_USE, acting as barriers. These barriers prevent the reclaim code from mistakenly freeing descriptors belonging to other submitters.

Considered the following example timeline: T1 T2

Where: ID = invalidation descriptor WD = wait descriptor

• Written by hardware

The root of the problem is that the descriptor status QI_DONE flag is used for two conflicting purposes:

1. signal a descriptor is ready for reclaim (to be freed)
2. signal by the hardware that a wait descriptor is complete

The solution (in this patch) is state separation by using QI_FREE flag for #1.

Once a thread's invalidation descriptors are complete, their status would be set to QI_FREE. The reclaim_free_desc() function would then only free descriptors marked as QI_FREE instead of those marked as QI_DONE. This change ensures that T2 (from the previous example) will correctly observe the completion of its invalidation_wait (marked as QI_DONE).