RDMA/hfi1: Fix use-after-free bug for mm struct

Under certain conditions, such as MPI_Abort, the hfi1 cleanup code may
represent the last reference held on the task mm.
hfi1_mmu_rb_unregister() then drops the last reference and the mm is freed
before the final use in hfi1_release_user_pages().  A new task may
allocate the mm structure while it is still being used, resulting in
problems. One manifestation is corruption of the mmap_sem counter leading
to a hang in down_write().  Another is corruption of an mm struct that is
in use by another task.

Fixes: 3d2a9d64 ("IB/hfi1: Ensure correct mm is used at all times")
Link: https://lore.kernel.org/r/20220408133523.122165.72975.stgit@awfm-01.cornelisnetworks.com
Cc: <stable@vger.kernel.org>
Signed-off-by: Douglas Miller <doug.miller@cornelisnetworks.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/hw/hfi1/mmu_rb.c

@@ -80,6 +80,9 @@ void hfi1_mmu_rb_unregister(struct mmu_rb_handler *handler)
 	unsigned long flags;
 	struct list_head del_list;
 
+	/* Prevent freeing of mm until we are completely finished. */
+	mmgrab(handler->mn.mm);
+
 	/* Unregister first so we don't get any more notifications. */
 	mmu_notifier_unregister(&handler->mn, handler->mn.mm);
 
@@ -102,6 +105,9 @@ void hfi1_mmu_rb_unregister(struct mmu_rb_handler *handler)
 
 	do_remove(handler, &del_list);
 
+	/* Now the mm may be freed. */
+	mmdrop(handler->mn.mm);
+
 	kfree(handler);
 }