[2.24] Ignore and remove LD_HWCAP_MASK for AT_SECURE programs (bug #21209)

[Florian Weimer]

The LD_HWCAP_MASK environment variable may alter the selection of
function variants for some architectures.  For AT_SECURE process it
means that if an outdated routine has a bug that would otherwise not
affect newer platforms by default, LD_HWCAP_MASK will allow that bug
to be exploited.

To be on the safe side, ignore and disable LD_HWCAP_MASK for setuid
binaries.

	[BZ #21209]
	* elf/rtld.c (process_envvars): Ignore LD_HWCAP_MASK for
	AT_SECURE processes.
	* sysdeps/generic/unsecvars.h: Add LD_HWCAP_MASK.

(cherry picked from commit 1c1243b6fc33c029488add276e56570a07803bfd)

2017-03-07  Siddhesh Poyarekar  <siddhesh@sourceware.org>

	[BZ #21209]
	* elf/rtld.c (process_envvars): Ignore LD_HWCAP_MASK for
	AT_SECURE processes.
	* sysdeps/generic/unsecvars.h: Add LD_HWCAP_MASK.

diff --git a/NEWS b/NEWS
index d42af91..c4c082b 100644
--- a/NEWS
+++ b/NEWS
@@ -23,6 +23,7 @@ Security related changes:
 
 The following bugs are resolved with this release:
 
+  [21209] Ignore and remove LD_HWCAP_MASK for AT_SECURE programs
   [21289] Fix symbol redirect for fts_set
   [21386] Assertion in fork for distinct parent PID is incorrect
   [21624] Unsafe alloca allows local attackers to alias stack and heap (CVE-2017-1000366)
diff --git a/elf/rtld.c b/elf/rtld.c
index 302bb63..8f56d6e 100644
--- a/elf/rtld.c
+++ b/elf/rtld.c
@@ -2553,7 +2553,8 @@ process_envvars (enum mode *modep)
 
 	case 10:
 	  /* Mask for the important hardware capabilities.  */
-	  if (memcmp (envline, "HWCAP_MASK", 10) == 0)
+	  if (!__libc_enable_secure
+	      && memcmp (envline, "HWCAP_MASK", 10) == 0)
 	    GLRO(dl_hwcap_mask) = __strtoul_internal (&envline[11], NULL,
 						      0, 0);
 	  break;
diff --git a/sysdeps/generic/unsecvars.h b/sysdeps/generic/unsecvars.h
index 3e56538..ac57bd5 100644
--- a/sysdeps/generic/unsecvars.h
+++ b/sysdeps/generic/unsecvars.h
@@ -10,6 +10,7 @@
   "LD_DEBUG\0"								      \
   "LD_DEBUG_OUTPUT\0"							      \
   "LD_DYNAMIC_WEAK\0"							      \
+  "LD_HWCAP_MASK\0"							      \
   "LD_LIBRARY_PATH\0"							      \
   "LD_ORIGIN_PATH\0"							      \
   "LD_PRELOAD\0"							      \