IB/uverbs: Get rid of null_obj_type

If the method fails after calling rdma_explicit_destroy (eg if
copy_to_user faults) then it will trigger a kernel oops:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
PGD 800000000548d067 P4D 800000000548d067 PUD 54a0067 PMD 0
SMP PTI
CPU: 0 PID: 359 Comm: ibv_rc_pingpong Not tainted 4.18.0-rc1+ #28
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.7.5-0-ge51488c-20140602_164612-nilsson.home.kraxel.org 04/01/2014
RIP: 0010:          (null)
Code: Bad RIP value.
RSP: 0018:ffffc900001a3bf0 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff88000603bd00 RCX: 0000000000000003
RDX: 0000000000000001 RSI: 0000000000000001 RDI: ffff88000603bd00
RBP: 0000000000000001 R08: ffffc900001a3cf8 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffffc900001a3cf0
R13: 0000000000000000 R14: ffffc900001a3cf0 R15: 0000000000000000
FS:  00007fb00dda8700(0000) GS:ffff880007c00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffd6 CR3: 000000000548e004 CR4: 00000000003606b0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 ? rdma_lookup_put_uobject+0x22/0x50 [ib_uverbs]
 ? uverbs_finalize_object+0x3b/0x60 [ib_uverbs]
 ? uverbs_finalize_attrs+0x128/0x140 [ib_uverbs]
 ? ib_uverbs_cmd_verbs+0x698/0x7c0 [ib_uverbs]
 ? find_held_lock+0x2d/0x90
 ? __might_fault+0x39/0x90
 ? ib_uverbs_ioctl+0x111/0x1f0 [ib_uverbs]
 ? do_vfs_ioctl+0xa0/0x6d0
 ? trace_hardirqs_on_caller+0xed/0x180
 ? _raw_spin_unlock_irq+0x24/0x40
 ? syscall_trace_enter+0x138/0x1d0
 ? ksys_ioctl+0x35/0x60
 ? __x64_sys_ioctl+0x11/0x20
 ? do_syscall_64+0x5b/0x1c0
 ? entry_SYSCALL_64_after_hwframe+0x49/0xbe

This is because the type was replaced with the null_type during explicit
destroy that cannot complete the destruction.

One of the side effects of replacing the type is to make the object
handle totally unreachable - so no other command could attempt to use
it, even though it remains on the uboject list.

We can get the same end result by just fully destroying the object inside
rdma_explicit_destroy and leaving the caller the residual kref for the
uobj with no attached HW object, and no presence in the ubojects list.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>

drivers/infiniband/core/rdma_core.c

@@ -439,12 +439,17 @@ static int __must_check _rdma_remove_commit_uobject(struct ib_uobject *uobj,
 	struct ib_uverbs_file *ufile = uobj->ufile;
 	int ret;
 
+	if (!uobj->object)
+		return 0;
+
 	ret = uobj->type->type_class->remove_commit(uobj, why);
 	if (ib_is_destroy_retryable(ret, why, uobj)) {
 		/* We couldn't remove the object, so just unlock the uobject */
 		atomic_set(&uobj->usecnt, 0);
 		uobj->type->type_class->lookup_put(uobj, true);
 	} else {
+		uobj->object = NULL;
+
 		mutex_lock(&ufile->uobjects_lock);
 		list_del(&uobj->list);
 		mutex_unlock(&ufile->uobjects_lock);
@@ -459,35 +464,13 @@ static int __must_check _rdma_remove_commit_uobject(struct ib_uobject *uobj,
 int __must_check rdma_remove_commit_uobject(struct ib_uobject *uobj)
 {
 	int ret;
-	struct ib_uverbs_file *ufile = uobj->ufile;
 
-	/* put the ref count we took at lookup_get */
-	uverbs_uobject_put(uobj);
-	/* Cleanup is running. Calling this should have been impossible */
-	if (!down_read_trylock(&ufile->cleanup_rwsem)) {
-		WARN(true, "ib_uverbs: Cleanup is running while removing an uobject\n");
-		return 0;
-	}
-	assert_uverbs_usecnt(uobj, true);
-	ret = _rdma_remove_commit_uobject(uobj, RDMA_REMOVE_DESTROY);
-
-	up_read(&ufile->cleanup_rwsem);
+	ret = rdma_explicit_destroy(uobj);
+	/* Pairs with the lookup_get done by the caller */
+	rdma_lookup_put_uobject(uobj, true);
 	return ret;
 }
 
-static int null_obj_type_class_remove_commit(struct ib_uobject *uobj,
-					     enum rdma_remove_reason why)
-{
-	return 0;
-}
-
-static const struct uverbs_obj_type null_obj_type = {
-	.type_class = &((const struct uverbs_obj_type_class){
-			.remove_commit = null_obj_type_class_remove_commit,
-			/* be cautious */
-			.needs_kfree_rcu = true}),
-};
-
 int rdma_explicit_destroy(struct ib_uobject *uobject)
 {
 	int ret;
@@ -499,14 +482,8 @@ int rdma_explicit_destroy(struct ib_uobject *uobject)
 		return 0;
 	}
 	assert_uverbs_usecnt(uobject, true);
-	ret = uobject->type->type_class->remove_commit(uobject,
-						       RDMA_REMOVE_DESTROY);
-	if (ret)
-		goto out;
-
-	uobject->type = &null_obj_type;
+	ret = _rdma_remove_commit_uobject(uobject, RDMA_REMOVE_DESTROY);
 
-out:
 	up_read(&ufile->cleanup_rwsem);
 	return ret;
 }