random: add and use memzero_explicit() for clearing data

zatimend has reported that in his environment (3.16/gcc4.8.3/corei7)
memset() calls which clear out sensitive data in extract_{buf,entropy,
entropy_user}() in random driver are being optimized away by gcc.

Add a helper memzero_explicit() (similarly as explicit_bzero() variants)
that can be used in such cases where a variable with sensitive data is
being cleared out in the end. Other use cases might also be in crypto
code. [ I have put this into lib/string.c though, as it's always built-in
and doesn't need any dependencies then. ]

Fixes kernel bugzilla: 82041

Reported-by: zatimend@hotmail.co.uk
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org

1 files changed, 4 insertions, 4 deletions

diff --git a/drivers/char/random.c b/drivers/char/random.c
index c18d41db83d8..8c86a95203a0 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1106,7 +1106,7 @@ static void extract_buf(struct entropy_store *r, __u8 *out)
         __mix_pool_bytes(r, hash.w, sizeof(hash.w));
         spin_unlock_irqrestore(&r->lock, flags);
 
-        memset(workspace, 0, sizeof(workspace));
+        memzero_explicit(workspace, sizeof(workspace));
 
         /*
          * In case the hash function has some recognizable output
@@ -1118,7 +1118,7 @@ static void extract_buf(struct entropy_store *r, __u8 *out)
         hash.w[2] ^= rol32(hash.w[2], 16);
 
         memcpy(out, &hash, EXTRACT_SIZE);
-        memset(&hash, 0, sizeof(hash));
+        memzero_explicit(&hash, sizeof(hash));
 }
 
 /*
@@ -1175,7 +1175,7 @@ static ssize_t extract_entropy(struct entropy_store *r, void *buf,
         }
 
         /* Wipe data just returned from memory */
-        memset(tmp, 0, sizeof(tmp));
+        memzero_explicit(tmp, sizeof(tmp));
 
         return ret;
 }
@@ -1218,7 +1218,7 @@ static ssize_t extract_entropy_user(struct entropy_store *r, void __user *buf,
         }
 
         /* Wipe data just returned from memory */
-        memset(tmp, 0, sizeof(tmp));
+        memzero_explicit(tmp, sizeof(tmp));
 
         return ret;
 }