[newlib-cygwin] Cygwin: fix guard checking for current user's AuthZ context

[Corinna Vinschen]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=e206c39bb6bf91808dbcd2cfbd572525825982cd

commit e206c39bb6bf91808dbcd2cfbd572525825982cd
Author: Corinna Vinschen <cori...@vinschen.de>
Date:   Thu Apr 12 09:43:12 2018 +0200

    Cygwin: fix guard checking for current user's AuthZ context
    
    Signed-off-by: Corinna Vinschen <cori...@vinschen.de>

Diff:
---
 winsup/cygwin/sec_helper.cc | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/winsup/cygwin/sec_helper.cc b/winsup/cygwin/sec_helper.cc
index 3478360..a1d8cba 100644
--- a/winsup/cygwin/sec_helper.cc
+++ b/winsup/cygwin/sec_helper.cc
@@ -821,12 +821,16 @@ authz_ctx::get_user_attribute (mode_t *attribute, 
PSECURITY_DESCRIPTOR psd,
   if (RtlEqualSid (user_sid, cygheap->user.sid ())
       && !cygheap->user.issetuid ())
     {
+      /* Avoid lock in default case. */
       if (!user_ctx_hdl)
        {
          authz_guard.acquire ();
-         if (!AuthzInitializeContextFromToken (0, hProcToken, authz, NULL,
-                                               authz_dummy_luid, NULL,
-                                               &user_ctx_hdl))
+         /* Check user_ctx_hdl again under lock to avoid overwriting
+            user_ctx_hdl if it has already been initialized. */
+         if (!user_ctx_hdl
+             && !AuthzInitializeContextFromToken (0, hProcToken, authz, NULL,
+                                                  authz_dummy_luid, NULL,
+                                                  &user_ctx_hdl))
            debug_printf ("AuthzInitializeContextFromToken, %E");
          authz_guard.release ();
        }