EVM: Use crypto_memneq() for digest comparisons

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 <[email protected]>
Signed-off-by: Ryan Ware <[email protected]>
Cc: [email protected]
Signed-off-by: Mimi Zohar <[email protected]>
Signed-off-by: James Morris <[email protected]>

diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index f7160253f17faad71c17a86b19833123a2edebd4..e6ea9d4b1de91a8d58e3ccab0fd3cef222ea6fcd 100644 (file)
--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c
@@ -23,6 +23,7 @@
 #include <linux/integrity.h>
 #include <linux/evm.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;