EVM: Use crypto_memneq() for digest comparisons

commit 613317bd212c585c20796c10afe5daaa95d4b0a1 upstream.

This patch fixes vulnerability CVE-2016-2085.  The problem exists
because the vm_verify_hmac() function includes a use of memcmp().
Unfortunately, this allows timing side channel attacks; specifically
a MAC forgery complexity drop from 2^128 to 2^12.  This patch changes
the memcmp() to the cryptographically safe crypto_memneq().

Reported-by: Xiaofei Rex Guo <xiaofei.rex.guo@intel.com>
Signed-off-by: Ryan Ware <ware@linux.intel.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>
Cc: Moritz Muehlenhoff <jmm@inutil.org>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index 41f32259437c9a1324cd1139ad86ec48c77a5f6f..c9db5ab63ad2ac2437f518eb0d3132c80855665d 100644 (file)
--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c
@@ -24,6 +24,7 @@
 #include <linux/evm.h>
 #include <linux/magic.h>
 #include <crypto/hash.h>
+#include <crypto/algapi.h>
 #include "evm.h"
 
 int evm_initialized;
@@ -148,7 +149,7 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
                                   xattr_value_len, calc.digest);
                if (rc)
                        break;
-               rc = memcmp(xattr_data->digest, calc.digest,
+               rc = crypto_memneq(xattr_data->digest, calc.digest,
                            sizeof(calc.digest));
                if (rc)
                        rc = -EINVAL;