rxrpc: Fix net namespace cleanup [Linux 5.1]

rxrpc: Fix net namespace cleanup [Linux 5.1]

This Linux kernel change "rxrpc: Fix net namespace cleanup" is included in the Linux 5.1 release. This change is authored by David Howells <dhowells [at] redhat.com> on Tue Apr 30 08:34:08 2019 +0100. The commit for this change in Linux stable tree is b130234 (patch).

rxrpc: Fix net namespace cleanup

In rxrpc_destroy_all_calls(), there are two phases: (1) make sure the
->calls list is empty, emitting error messages if not, and (2) wait for the
RCU cleanup to happen on outstanding calls (ie. ->nr_calls becomes 0).

To avoid taking the call_lock, the function prechecks ->calls and if empty,
it returns to avoid taking the lock - this is wrong, however: it still
needs to go and do the second phase and wait for ->nr_calls to become 0.

Without this, the rxrpc_net struct may get deallocated before we get to the
RCU cleanup for the last calls.  This can lead to:

  Slab corruption (Not tainted): kmalloc-16k start=ffff88802b178000, len=16384
  050: 6b 6b 6b 6b 6b 6b 6b 6b 61 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkakkkkkkk

Note the "61" at offset 0x58.  This corresponds to the ->nr_calls member of
struct rxrpc_net (which is >9k in size, and thus allocated out of the 16k
slab).

Fix this by flipping the condition on the if-statement, putting the locked
section inside the if-body and dropping the return from there.  The
function will then always go on to wait for the RCU cleanup on outstanding
calls.

Fixes: 2baec2c3f854 ("rxrpc: Support network namespacing")
Signed-off-by: David Howells <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

There are 32 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 net/rxrpc/call_object.c | 32 ++++++++++++++++----------------
 1 file changed, 16 insertions(+), 16 deletions(-)

diff --git a/net/rxrpc/call_object.c b/net/rxrpc/call_object.c
index 8aa2937..fe96881 100644
--- a/net/rxrpc/call_object.c
+++ b/net/rxrpc/call_object.c
@@ -604,30 +604,30 @@ void rxrpc_destroy_all_calls(struct rxrpc_net *rxnet)

    _enter("");

-   if (list_empty(&rxnet->calls))
-       return;
+   if (!list_empty(&rxnet->calls)) {
+       write_lock(&rxnet->call_lock);

-   write_lock(&rxnet->call_lock);
+       while (!list_empty(&rxnet->calls)) {
+           call = list_entry(rxnet->calls.next,
+                     struct rxrpc_call, link);
+           _debug("Zapping call %p", call);

-   while (!list_empty(&rxnet->calls)) {
-       call = list_entry(rxnet->calls.next, struct rxrpc_call, link);
-       _debug("Zapping call %p", call);
+           rxrpc_see_call(call);
+           list_del_init(&call->link);

-       rxrpc_see_call(call);
-       list_del_init(&call->link);
+           pr_err("Call %p still in use (%d,%s,%lx,%lx)!\n",
+                  call, atomic_read(&call->usage),
+                  rxrpc_call_states[call->state],
+                  call->flags, call->events);

-       pr_err("Call %p still in use (%d,%s,%lx,%lx)!\n",
-              call, atomic_read(&call->usage),
-              rxrpc_call_states[call->state],
-              call->flags, call->events);
+           write_unlock(&rxnet->call_lock);
+           cond_resched();
+           write_lock(&rxnet->call_lock);
+       }

        write_unlock(&rxnet->call_lock);
-       cond_resched();
-       write_lock(&rxnet->call_lock);
    }

-   write_unlock(&rxnet->call_lock);
-
    atomic_dec(&rxnet->nr_calls);
    wait_var_event(&rxnet->nr_calls, !atomic_read(&rxnet->nr_calls));
 }

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