[2.28 COMMITTED] rtld: Check __libc_enable_secure before honoring LD_PREFER_MAP_32BIT_EXEC (CVE-2019-19126) [BZ #25204]

[2.28 COMMITTED] rtld: Check __libc_enable_secure before honoring LD_PREFER_MAP_32BIT_EXEC (CVE-2019-19126) [BZ #25204]

[Florian Weimer]

From: Marcin Kościelnicki <mwk@0x04.net>

The problem was introduced in glibc 2.23, in commit
b9eb92ab05204df772eb4929eccd018637c9f3e9
("Add Prefer_MAP_32BIT_EXEC to map executable pages with MAP_32BIT").

(cherry picked from commit d5dfad4326fc683c813df1e37bbf5cf920591c8e)

diff --git a/NEWS b/NEWS
index f249ff690c..2a97ce5dac 100644
--- a/NEWS
+++ b/NEWS
@@ -69,6 +69,7 @@ The following bugs are resolved with this release:
   [24228] old x86 applications that use legacy libio crash on exit
   [24476] dlfcn: Guard __dlerror_main_freeres with __libc_once_get (once)
   [24744] io: Remove the copy_file_range emulation.
+  [25204] Ignore LD_PREFER_MAP_32BIT_EXEC for SUID programs
 
 Security related changes:
 
@@ -97,6 +98,13 @@ Security related changes:
   CVE-2019-9169: Attempted case-insensitive regular-expression match
   via proceed_next_node in posix/regexec.c leads to heap-based buffer
   over-read.  Reported by Hongxu Chen.
+
+  CVE-2019-19126: ld.so failed to ignore the LD_PREFER_MAP_32BIT_EXEC
+  environment variable during program execution after a security
+  transition, allowing local attackers to restrict the possible mapping
+  addresses for loaded libraries and thus bypass ASLR for a setuid
+  program.  Reported by Marcin Kościelnicki.
+
 \f
 Version 2.28
 
diff --git a/sysdeps/unix/sysv/linux/x86_64/64/dl-librecon.h b/sysdeps/unix/sysv/linux/x86_64/64/dl-librecon.h
index 194369174d..ac694c032e 100644
--- a/sysdeps/unix/sysv/linux/x86_64/64/dl-librecon.h
+++ b/sysdeps/unix/sysv/linux/x86_64/64/dl-librecon.h
@@ -31,7 +31,8 @@
    environment variable, LD_PREFER_MAP_32BIT_EXEC.  */
 #define EXTRA_LD_ENVVARS \
   case 21:								  \
-    if (memcmp (envline, "PREFER_MAP_32BIT_EXEC", 21) == 0)		  \
+    if (!__libc_enable_secure						  \
+	&& memcmp (envline, "PREFER_MAP_32BIT_EXEC", 21) == 0)		  \
       GLRO(dl_x86_cpu_features).feature[index_arch_Prefer_MAP_32BIT_EXEC] \
 	|= bit_arch_Prefer_MAP_32BIT_EXEC;				  \
     break;