From 7e967fd0b84a843b2475acc67a5a8df138c5f5c0 Mon Sep 17 00:00:00 2001
From: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Date: Tue, 4 Aug 2015 17:13:32 -0600
Subject: [PATCH] IB/ucma: Fix theoretical user triggered use-after-free

Something like this:

CPU A                         CPU B
Acked-by: Sean Hefty <sean.hefty@intel.com>

========================      ================================
ucma_destroy_id()
 wait_for_completion()
                              .. anything
                                ucma_put_ctx()
                                  complete()
 .. continues ...
                              ucma_leave_multicast()
                               mutex_lock(mut)
                                 atomic_inc(ctx->ref)
                               mutex_unlock(mut)
 ucma_free_ctx()
  ucma_cleanup_multicast()
   mutex_lock(mut)
     kfree(mc)
                               rdma_leave_multicast(mc->ctx->cm_id,..

Fix it by latching the ref at 0. Once it goes to 0 mc and ctx cannot
leave the mutex(mut) protection.

The other atomic_inc in ucma_get_ctx is OK because mutex(mut) protects
it from racing with ucma_destroy_id.

Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Acked-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>
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