[glibc/release/2.24/master] rtld: Check __libc_enable_secure before honoring LD_PREFER_MAP_32BIT_EXEC (CVE-2019-19126) [BZ #2520

[glibc/release/2.24/master] rtld: Check __libc_enable_secure before honoring LD_PREFER_MAP_32BIT_EXEC (CVE-2019-19126) [BZ #2520

[Florian Weimer]

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=a7b9e4cb8ec069a4030b081ed7c40a65e0825f47

commit a7b9e4cb8ec069a4030b081ed7c40a65e0825f47
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                                            | 7 +++++++
 sysdeps/unix/sysv/linux/x86_64/64/dl-librecon.h | 3 ++-
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/NEWS b/NEWS
index fa49c52..d84fdae 100644
--- a/NEWS
+++ b/NEWS
@@ -74,6 +74,12 @@ 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:
 
   [20116] nptl: use after free in pthread_create
@@ -98,6 +104,7 @@ 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 2.24
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 1c67050..513735a 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;