Re: [PATCH net] xfrm6: fix inet6_dev refcount underflow problem

There are race conditions that may lead to inet6_dev refcount underflow
in xfrm6_dst_destroy() and rt6_uncached_list_flush_dev().

One of the refcount underflow bugs is shown below:
        (cpu 1)                 |       (cpu 2)
xfrm6_dst_destroy()             |
  ...                           |
  in6_dev_put()                 |
                                |  rt6_uncached_list_flush_dev()
  ...                           |    ...
                                |    in6_dev_put()
  rt6_uncached_list_del()       |    ...
  ...                           |

xfrm6_dst_destroy() calls rt6_uncached_list_del() after in6_dev_put(),
so rt6_uncached_list_flush_dev() has a chance to call in6_dev_put()
again for the same inet6_dev.

Fix it by moving in6_dev_put() after rt6_uncached_list_del() in
xfrm6_dst_destroy().

Fixes: 510c321b5571 ("xfrm: reuse uncached_list to track xdsts")
Signed-off-by: Zhang Changzhong <redacted>
---
 net/ipv6/xfrm6_policy.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv6/xfrm6_policy.c b/net/ipv6/xfrm6_policy.c
index 41a680c..42fb6996 100644
--- a/net/ipv6/xfrm6_policy.c
+++ b/net/ipv6/xfrm6_policy.c

@@ -117,10 +117,10 @@ static void xfrm6_dst_destroy(struct dst_entry *dst)
 {
        struct xfrm_dst *xdst = (struct xfrm_dst *)dst;

-       if (likely(xdst->u.rt6.rt6i_idev))
-               in6_dev_put(xdst->u.rt6.rt6i_idev);
        dst_destroy_metrics_generic(dst);
        rt6_uncached_list_del(&xdst->u.rt6);
+       if (likely(xdst->u.rt6.rt6i_idev))
+               in6_dev_put(xdst->u.rt6.rt6i_idev);
        xfrm_dst_destroy(xdst);
 }

--

2.9.5