[PATCH 6.1.y] drm/nouveau/svm: drain fault handler before freeing svmm

[Sasha Levin <sashal@kernel.org>]

From: Zhenhao Wan <whi4ed0g@gmail.com>

[ Upstream commit b46ddaf0823405f81e681eb26ec98508f3eb9a07 ]

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 <danisjiang@gmail.com>
Assisted-by: Claude:claude-opus-5
Cc: stable@vger.kernel.org
Signed-off-by: Zhenhao Wan <whi4ed0g@gmail.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patch.msgid.link/20260814-nouveau-svm-svmm-uaf-v2-1-8e6590519f07@gmail.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 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 d0bff13ac758d..75218519c29e0 100644
--- a/drivers/gpu/drm/nouveau/nouveau_svm.c
+++ b/drivers/gpu/drm/nouveau/nouveau_svm.c
@@ -306,6 +306,19 @@ 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].notify.work);
+
 		mutex_lock(&svmm->mutex);
 		svmm->vmm = NULL;
 		mutex_unlock(&svmm->mutex);
-- 
2.53.0