[PATCH] #13594: Avoid race in nscd

[Andreas Jaeger <aj at suse dot de>]

Jeff Law says in bugzilla:
"Based on just reading the code, I wonder if a one thread is mucking up
hst_map_handle.mapped behind the back of nscd_get_mapping.

nscd_get_nl_timestamp doesn't bother to grab the hst_map_handle lock and 
calls
into nscd_get_mapping which could potentially change 
hst_map_handle.mapped to
NO_MAPPING.

If this occurs after another thread had passed the NO_MAPPING check in
nscd_get_map_ref, but hasn't yet hit the atomic_decrement_val in
nscd_get_mapping then it could cause the failure mode reported in this 
report
(and several others across various distros, upstream kde and possibly
elsewhere)."

I agree with his analysis and cleaned up the patch submission (added
copyright header and ChangeLog entry).

This patch has been tested in openSUSE and Fedora.

Ok to commit? Andreas

2012-05-14  Jeff Law  <law@redhat.com>

	[BZ #13594]
	* nscd/nscd_gethst_r.c (__nscd_get_nl_timestamp): Add locking to
	code changing __hst_map_handle.map.

diff --git a/nscd/nscd_gethst_r.c b/nscd/nscd_gethst_r.c
index c1661f8..36c4bb3 100644
--- a/nscd/nscd_gethst_r.c
+++ b/nscd/nscd_gethst_r.c
@@ -1,5 +1,4 @@
-/* Copyright (C) 1998-2005, 2006, 2007, 2008, 2009, 2011
-   Free Software Foundation, Inc.
+/* Copyright (C) 1998-2012  Free Software Foundation, Inc.
    This file is part of the GNU C Library.
    Contributed by Ulrich Drepper <drepper@cygnus.com>, 1998.
 @@ -100,9 +99,27 @@ libc_freeres_fn (hst_map_free)
 uint32_t
 __nscd_get_nl_timestamp (void)
 {
+  uint32_t retval;
   if (__nss_not_use_nscd_hosts != 0)
     return 0;
 +  int cnt = 0;
+  /* __nscd_get_mapping can change hst_map_handle.mapped to NO_MAPPING.
+   However, __nscd_get_mapping assumes the prior value was not NO_MAPPING.
+   Thus we have to acquire the lock to prevent this thread from changing
+   hst_map_handle.mapped to NO_MAPPING while another thread is inside
+    __nscd_get_mapping.  */
+  while (__builtin_expect
+	 (atomic_compare_and_exchange_val_acq (&__hst_map_handle.lock,
+					       1, 0) != 0, 0))
+    {
+      /* XXX Best number of rounds?  */
+      if (__builtin_expect (++cnt > 5, 0))
+	return 0;
+
+      atomic_delay ();
+    }
+
   struct mapped_database *map = __hst_map_handle.mapped;
    if (map == NULL
@@ -112,9 +129,14 @@ __nscd_get_nl_timestamp (void)
     map = __nscd_get_mapping (GETFDHST, "hosts", 
&__hst_map_handle.mapped);
    if (map == NO_MAPPING)
-    return 0;
+    retval =  0;
+  else
+    retval =  map->head->extra_data[NSCD_HST_IDX_CONF_TIMESTAMP];
 -  return map->head->extra_data[NSCD_HST_IDX_CONF_TIMESTAMP];
+  /* Release the lock.  */
+  __hst_map_handle.lock = 0;
+
+  return retval;
 }

--
 Andreas Jaeger aj@{suse.com,opensuse.org} Twitter/Identica: jaegerandi
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