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infiniband: fix a possible use-after-free bug

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

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: c8f6a36 ("RDMA/cma: Add multicast communication support")
Reported-by: Noam Rathaus <>
Signed-off-by: Cong Wang <>
Signed-off-by: Jason Gunthorpe <>
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congwang authored and jgunthorpe committed Jun 1, 2018
1 parent e4b1672 commit cb2595c1393b4a5211534e6f0a0fbad369e21ad8
Showing with 5 additions and 1 deletion.
  1. +5 −1 drivers/infiniband/core/ucma.c
@@ -235,7 +235,7 @@ static struct ucma_multicast* ucma_alloc_multicast(struct ucma_context *ctx)
return NULL;

mc->id = idr_alloc(&multicast_idr, mc, 0, 0, GFP_KERNEL);
mc->id = idr_alloc(&multicast_idr, NULL, 0, 0, GFP_KERNEL);
if (mc->id < 0)
goto error;
@@ -1421,6 +1421,10 @@ static ssize_t ucma_process_join(struct ucma_file *file,
goto err3;

idr_replace(&multicast_idr, mc, mc->id);

return 0;

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