[PATCH 6.12.y] iommufd: Fix event queue release use-after-free

[Sasha Levin <sashal@kernel.org>]

From: Nirmoy Das <nirmoyd@nvidia.com>

[ Upstream commit 440ca8a4b69f40088065b9e05ef11373292a254e ]

iommufd_eventq_fops_release() drops the file's users reference before
reading eventq->ictx. Once that reference is dropped, IOMMU_DESTROY in
another thread can remove and free the event queue, so the later load
accesses freed memory.

Read ictx while the users reference still pins the event queue, then
drop the reference and put the saved context. The context reference
taken by iommufd_eventq_init() keeps ictx valid until the put. Fault and
vEVENTQ file descriptors share this release path.

Fixes: 07838f7fd529 ("iommufd: Add iommufd fault object")
Cc: stable@vger.kernel.org
Assisted-by: LLM, Specula
Signed-off-by: Nirmoy Das <nirmoyd@nvidia.com>
Link: https://patch.msgid.link/20261014133632.3440297-1-nirmoyd@nvidia.com
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
[ sashal: Adapted eventq identifiers to fault identifiers in the existing fault release callback. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/iommu/iommufd/fault.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/iommufd/fault.c b/drivers/iommu/iommufd/fault.c
index bdaecb6e1f7a3..6e918eb0ffbf0 100644
--- a/drivers/iommu/iommufd/fault.c
+++ b/drivers/iommu/iommufd/fault.c
@@ -373,9 +373,11 @@ static __poll_t iommufd_fault_fops_poll(struct file *filep,
 static int iommufd_fault_fops_release(struct inode *inode, struct file *filep)
 {
 	struct iommufd_fault *fault = filep->private_data;
+	/* fault may be freed once the users reference is dropped. */
+	struct iommufd_ctx *ictx = fault->ictx;
 
 	refcount_dec(&fault->obj.users);
-	iommufd_ctx_put(fault->ictx);
+	iommufd_ctx_put(ictx);
 	return 0;
 }
 
-- 
2.53.0