[PATCH v2] drm/nouveau/svm: drain fault handler before freeing svmm

The SVM fault handler nouveau_svm_fault() looks up each fault's
nouveau_svmm under svm->mutex, caches it in the fault array, then drops
svm->mutex and dereferences the svmm across blocking faults
(mmu_interval_notifier_insert(), hmm_range_fault()).

nouveau_svmm is not refcounted. On DRM file close nouveau_svmm_fini() ->
mmu_notifier_put() frees it asynchronously via call_srcu() without
waiting for the handler, and mmget_not_zero() pins the mm, not the svmm.
flush_work() on the fault buffer otherwise runs only at device removal
and suspend, never on close, so a fault handled concurrently with a close
can dereference a freed svmm.

The instance is already unlinked by nouveau_svmm_part() earlier in the
same close, so no new fault can resolve to it. Drain the handler in
nouveau_svmm_fini() before the free, guarding the flush with a NULL check
on drm->svm: on device teardown nouveau_svm_fini() runs first and frees
drm->svm (NULL) after blocking the notify and flushing the buffer, so the
handler is already drained; on per-client close drm->svm is alive and the
flush runs.

Fixes: eeaf06ac1a55 ("drm/nouveau/svm: initial support for shared virtual memory")
Reported-by: Yuhao Jiang <[email protected]>
Assisted-by: Claude:claude-opus-5
Cc: [email protected]
Signed-off-by: Zhenhao Wan <[email protected]>
---
Changes in v2:
- Fix a NULL deref introduced in v1: guard flush_work() with a NULL
  check on drm->svm.  On device teardown nouveau_svm_fini() frees
  drm->svm before the per-client svmm teardown, so nouveau_svmm_fini()
  would deref NULL (reported by Sashiko AI review); the handler is
  already drained there, so there is nothing left to flush.
- Tighten the commit message and the code comment; no functional change.
- Link to v1: https://patch.msgid.link/[email protected]

To: Lyude Paul <[email protected]>
To: Danilo Krummrich <[email protected]>
To: Maarten Lankhorst <[email protected]>
To: Maxime Ripard <[email protected]>
To: Thomas Zimmermann <[email protected]>
To: David Airlie <[email protected]>
To: Simona Vetter <[email protected]>
To: Ben Skeggs <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
---
 drivers/gpu/drm/nouveau/nouveau_svm.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/drivers/gpu/drm/nouveau/nouveau_svm.c b/drivers/gpu/drm/nouveau/nouveau_svm.c
index dcc92131488e..a87406d3da30 100644
--- a/drivers/gpu/drm/nouveau/nouveau_svm.c
+++ b/drivers/gpu/drm/nouveau/nouveau_svm.c
@@ -304,6 +304,19 @@ nouveau_svmm_fini(struct nouveau_svmm **psvmm)
 {
 	struct nouveau_svmm *svmm = *psvmm;
 	if (svmm) {
+		struct nouveau_svm *svm = svmm->vmm->cli->drm->svm;
+
+		/* The fault handler caches svmm pointers under svm->mutex and
+		 * dereferences them after dropping it, across blocking faults.
+		 * The instance is already unlinked (nouveau_svmm_part), so drain
+		 * the handler before the free to release any in-flight reference.
+		 *
+		 * On device teardown nouveau_svm_fini() ran first and freed
+		 * drm->svm (NULL) after draining the buffer; nothing to flush.
+		 */
+		if (svm)
+			flush_work(&svm->buffer[0].work);
+
 		mutex_lock(&svmm->mutex);
 		svmm->vmm = NULL;
 		mutex_unlock(&svmm->mutex);

---
base-commit: db2ddb87143519e20a95aa36c60b36107b736a58
change-id: 20260813-nouveau-svm-svmm-uaf-bbcf36265aaa

Best regards,
--  
Zhenhao Wan <[email protected]>