[PATCH] infiniband: fix a possible use-after-free bug

[Cong Wang <xiyou.wangcong@gmail.com>]

ucma_process_join() will free the new allocated "mc" struct,
if there is any error after that, especially the copy_to_user().

But in parallel, ucma_leave_multicast() could find this "mc"
through idr_find() before ucma_process_join() frees it, since it
is already published.

So "mc" could be used in ucma_leave_multicast() after it is been
allocated and freed in ucma_process_join(), since we don't refcnt
it.

Fix this by separating "publish" from ID allocation, so that we
can get an ID first and publish it later after copy_to_user().

Fixes c8f6a362bf3e ("RDMA/cma: Add multicast communication support")
Reported-by: Noam Rathaus <noamr@beyondsecurity.com>
Cc: Sean Hefty <sean.hefty@intel.com>
Cc: Doug Ledford <dledford@redhat.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: linux-rdma@vger.kernel.org
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
---
 drivers/infiniband/core/ucma.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/infiniband/core/ucma.c b/drivers/infiniband/core/ucma.c
index eab43b17e9cf..ec8fb289621f 100644
--- a/drivers/infiniband/core/ucma.c
+++ b/drivers/infiniband/core/ucma.c
@@ -235,7 +235,7 @@ static struct ucma_multicast* ucma_alloc_multicast(struct ucma_context *ctx)
 		return NULL;
 
 	mutex_lock(&mut);
-	mc->id = idr_alloc(&multicast_idr, mc, 0, 0, GFP_KERNEL);
+	mc->id = idr_alloc(&multicast_idr, NULL, 0, 0, GFP_KERNEL);
 	mutex_unlock(&mut);
 	if (mc->id < 0)
 		goto error;
@@ -1421,6 +1421,10 @@ static ssize_t ucma_process_join(struct ucma_file *file,
 		goto err3;
 	}
 
+	mutex_lock(&mut);
+	idr_replace(&multicast_idr, mc, mc->id);
+	mutex_unlock(&mut);
+
 	mutex_unlock(&file->mut);
 	ucma_put_ctx(ctx);
 	return 0;
-- 
2.13.0