[COMMITTED] libdw/libdw_findcu.c: Fix TOCTOU race condition in __libdw_findcu

[Aaron Merey]

Ensure that dwarf_lock is held before accessing next_tu_offset and
next_cu_offset.

This fixes a TOCTOU bug in __libdw_findcu that causes NULL to be
incorrectly returned.

Signed-off-by: Aaron Merey <amerey@redhat.com>
---
 libdw/libdw_findcu.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/libdw/libdw_findcu.c b/libdw/libdw_findcu.c
index 8805af9b..0e4dcc37 100644
--- a/libdw/libdw_findcu.c
+++ b/libdw/libdw_findcu.c
@@ -240,6 +240,8 @@ struct Dwarf_CU *
 internal_function
 __libdw_findcu (Dwarf *dbg, Dwarf_Off start, bool v4_debug_types)
 {
+  mutex_lock (dbg->dwarf_lock);
+
   search_tree *tree = v4_debug_types ? &dbg->tu_tree : &dbg->cu_tree;
   Dwarf_Off *next_offset
     = v4_debug_types ? &dbg->next_tu_offset : &dbg->next_cu_offset;
@@ -249,9 +251,10 @@ __libdw_findcu (Dwarf *dbg, Dwarf_Off start, bool v4_debug_types)
   struct Dwarf_CU **found = eu_tfind (&fake, tree, findcu_cb);
   struct Dwarf_CU *result = NULL;
   if (found != NULL)
-    return *found;
-
-  mutex_lock (dbg->dwarf_lock);
+    {
+      mutex_unlock (dbg->dwarf_lock);
+      return *found;
+    }
 
   if (start < *next_offset)
     __libdw_seterrno (DWARF_E_INVALID_DWARF);
-- 
2.49.0