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

[2.25 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 0ca6b440db..a9f0248f04 100644
--- a/NEWS
+++ b/NEWS
@@ -66,6 +66,12 @@ Security related changes:
   memcmp gave the wrong result since it treated the size argument as
   zero.  Reported by H.J. Lu.
 
+  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.
+
 The following bugs are resolved with this release:
 
   [20257] sunrpc: clntudp_call does not enforce timeout when receiving data
@@ -103,6 +109,7 @@ The following bugs are resolved with this release:
   [24027] malloc: Integer overflow in realloc
   [24097] Can't use 64-bit register for size_t in assembly codes for x32 (CVE-2019-6488)
   [24155] x32 memcmp can treat positive length as 0 (if sign bit in RDX is set) (CVE-2019-7309)
+  [25204] Ignore LD_PREFER_MAP_32BIT_EXEC for SUID programs
 
 \f
 Version 2.25
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 8d474d0d04..37f0b14adb 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;