Mitigate a flush+reload cache attack on RSA secret exponents.

* mpi/mpi-pow.c (mpi_powm): Always perform the mpi_mul for exponents
hold in secure memory.
--

The attack is described in a paper to be pusblished at eprint.iacr.org:

Flush+Reload: a High Resolution, Low Noise, L3 Cache Side-Channel
Attack by Yuval Yarom and Katrina Falkner. 18 July 2013.

  Flush+Reload is a cache side-channel attack that monitors access to
  data in shared pages. In this paper we demonstrate how to use the
  attack to extract private encryption keys from GnuPG.  The high
  resolution and low noise of the Flush+Reload attack enables a spy
  program to recover over 98% of the bits of the private key in a
  single decryption or signing round. Unlike previous attacks, the
  attack targets the last level L3 cache. Consequently, the spy
  program and the victim do not need to share the execution core of
  the CPU. The attack is not limited to a traditional OS and can be
  used in a virtualised environment, where it can attack programs
  executing in a different VM.

Signed-off-by: Werner Koch <[email protected]>

2 files changed, 21 insertions, 3 deletions

diff --git a/NEWS b/NEWS
index 110de6d03..b669539b2 100644
--- a/NEWS
+++ b/NEWS
@@ -1,6 +1,15 @@
 Noteworthy changes in version 1.4.14 (unreleased)
 -------------------------------------------------
 
+    * Mitigate the Yarom/Falkner flush+reload side-channel attack on
+      RSA secret keys.
+
+    * Fixed IDEA for big-endian CPUs
+
+    * Improved the diagnostics for failed keyserver lockups.
+
+    * Minor bug and portability fixes.
+
 
 Noteworthy changes in version 1.4.13 (2012-12-20)
 -------------------------------------------------
diff --git a/mpi/mpi-pow.c b/mpi/mpi-pow.c
index beb01f95a..2253ea4d3 100644
--- a/mpi/mpi-pow.c
+++ b/mpi/mpi-pow.c
@@ -1,5 +1,6 @@
 /* mpi-pow.c  -  MPI functions
- *	Copyright (C) 1994, 1996, 1998, 2000 Free Software Foundation, Inc.
+ * Copyright (C) 1994, 1996, 1998, 2000 Free Software Foundation, Inc.
+ * Copyright (C) 2013 Werner Koch
  *
  * This file is part of GnuPG.
  *
@@ -209,7 +210,14 @@ mpi_powm( MPI res, MPI base, MPI exponent, MPI mod)
 		tp = rp; rp = xp; xp = tp;
 		rsize = xsize;
 
-		if( (mpi_limb_signed_t)e < 0 ) {
+                /* To mitigate the Yarom/Falkner flush+reload cache
+                 * side-channel attack on the RSA secret exponent, we
+                 * do the multiplication regardless of the value of
+                 * the high-bit of E.  But to avoid this performance
+                 * penalty we do it only if the exponent has been
+                 * stored in secure memory and we can thus assume it
+                 * is a secret exponent.  */
+                if (esec || (mpi_limb_signed_t)e < 0) {
 		    /*mpihelp_mul( xp, rp, rsize, bp, bsize );*/
 		    if( bsize < KARATSUBA_THRESHOLD ) {
 			mpihelp_mul( xp, rp, rsize, bp, bsize );
@@ -224,7 +232,8 @@ mpi_powm( MPI res, MPI base, MPI exponent, MPI mod)
 			mpihelp_divrem(xp + msize, 0, xp, xsize, mp, msize);
 			xsize = msize;
 		    }
-
+                }
+		if ((mpi_limb_signed_t)e < 0) {
 		    tp = rp; rp = xp; xp = tp;
 		    rsize = xsize;
 		}