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From: =?UTF-8?q?Marcin=20Ko=C5=9Bcielnicki?= <mwk@0x04.net>
Date: Thu, 21 Nov 2019 00:20:15 +0100
Subject: [PATCH] rtld: Check __libc_enable_secure before honoring
 LD_PREFER_MAP_32BIT_EXEC (CVE-2019-19126) [BZ #25204]

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)
---
 NEWS                                            | 10 ++++++++++
 sysdeps/unix/sysv/linux/x86_64/64/dl-librecon.h |  3 ++-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/NEWS b/NEWS
index ee9ed4de5a3f..ea2dd3c68721 100644
--- a/NEWS
+++ b/NEWS
@@ -5,6 +5,16 @@ See the end for copying conditions.
 Please send GNU C library bug reports via <https://sourceware.org/bugzilla/>
 using `glibc' in the "product" field.
 
+Version 2.30.1
+
+Security related changes:
+
+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 Koƛcielnicki.
+
 Version 2.30
 
 Major new features:
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 975cbe2950e2..df2cdfdb6b32 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;