iommufd/selftest: Check for overflow in IOMMU_TEST_OP_ADD_RESERVED

syzkaller found it could overflow math in the test infrastructure and
cause a WARN_ON by corrupting the reserved interval tree. This only
effects test kernels with CONFIG_IOMMUFD_TEST.

Validate the user input length in the test ioctl.

Fixes: f4b20bb ("iommufd: Add kernel support for testing iommufd")
Link: https://patch.msgid.link/r/0-v1-cd99f6049ba5+51-iommufd_syz_add_resv_jgg@nvidia.com
Reviewed-by: Samiullah Khawaja <skhawaja@google.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Tested-by: Yi Liu <yi.l.liu@intel.com>
Reported-by: syzbot+57fdb0cf6a0c5d1f15a2@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/69368129.a70a0220.38f243.008f.GAE@google.com
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

Author: jgunthorpe

drivers/iommu/iommufd/selftest.c

@@ -1184,14 +1184,20 @@ static int iommufd_test_add_reserved(struct iommufd_ucmd *ucmd,
 				     unsigned int mockpt_id,
 				     unsigned long start, size_t length)
 {
+	unsigned long last;
 	struct iommufd_ioas *ioas;
 	int rc;
 
+	if (!length)
+		return -EINVAL;
+	if (check_add_overflow(start, length - 1, &last))
+		return -EOVERFLOW;
+
 	ioas = iommufd_get_ioas(ucmd->ictx, mockpt_id);
 	if (IS_ERR(ioas))
 		return PTR_ERR(ioas);
 	down_write(&ioas->iopt.iova_rwsem);
-	rc = iopt_reserve_iova(&ioas->iopt, start, start + length - 1, NULL);
+	rc = iopt_reserve_iova(&ioas->iopt, start, last, NULL);
 	up_write(&ioas->iopt.iova_rwsem);
 	iommufd_put_object(ucmd->ictx, &ioas->obj);
 	return rc;