[glibc/ibm/2.28/master] rtld: Check __libc_enable_secure before honoring LD_PREFER_MAP_32BIT_EXEC (CVE-2019-19126) [BZ #2520

[glibc/ibm/2.28/master] rtld: Check __libc_enable_secure before honoring LD_PREFER_MAP_32BIT_EXEC (CVE-2019-19126) [BZ #2520

[Tulio Magno Quites Machado Filho]

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=7966ce07e89fa4ccc8fdba00d4439fc652862462

commit 7966ce07e89fa4ccc8fdba00d4439fc652862462
Author: Marcin Kościelnicki <mwk@0x04.net>
Date:   Thu Nov 21 00:20:15 2019 +0100

    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)

Diff:
---
 NEWS                                            | 8 ++++++++
 sysdeps/unix/sysv/linux/x86_64/64/dl-librecon.h | 3 ++-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/NEWS b/NEWS
index f249ff6..2a97ce5 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 1943691..ac694c0 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;