random: add and use memzero_explicit() for clearing data

commit d4c5efdb97773f59a2b711754ca0953f24516739 upstream.

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>
[bwh: Backported to 3.2:
 - extract_buf() needs to use this for the 'extract' array as well
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/drivers/char/random.c b/drivers/char/random.c
index c244f0e..edf45ae 100644 (file)
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -954,8 +954,8 @@ static void extract_buf(struct entropy_store *r, __u8 *out)
         * pool while mixing, and hash one final time.
         */
        sha_transform(hash.w, extract, workspace);
-       memset(extract, 0, sizeof(extract));
-       memset(workspace, 0, sizeof(workspace));
+       memzero_explicit(extract, sizeof(extract));
+       memzero_explicit(workspace, sizeof(workspace));
 
        /*
         * In case the hash function has some recognizable output
@@ -978,7 +978,7 @@ static void extract_buf(struct entropy_store *r, __u8 *out)
        }
 
        memcpy(out, &hash, EXTRACT_SIZE);
-       memset(&hash, 0, sizeof(hash));
+       memzero_explicit(&hash, sizeof(hash));
 }
 
 static ssize_t extract_entropy(struct entropy_store *r, void *buf,
@@ -1010,7 +1010,7 @@ 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;
 }
@@ -1047,7 +1047,7 @@ 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;
 }

diff --git a/include/linux/string.h b/include/linux/string.h
index e033564..8515a4d 100644 (file)
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -144,5 +144,7 @@ static inline bool strstarts(const char *str, const char *prefix)
 {
        return strncmp(str, prefix, strlen(prefix)) == 0;
 }
+
+void memzero_explicit(void *s, size_t count);
 #endif
 #endif /* _LINUX_STRING_H_ */

diff --git a/lib/string.c b/lib/string.c
index dc4a863..40136f6 100644 (file)
--- a/lib/string.c
+++ b/lib/string.c
@@ -583,6 +583,22 @@ void *memset(void *s, int c, size_t count)
 EXPORT_SYMBOL(memset);
 #endif
 
+/**
+ * memzero_explicit - Fill a region of memory (e.g. sensitive
+ *                   keying data) with 0s.
+ * @s: Pointer to the start of the area.
+ * @count: The size of the area.
+ *
+ * memzero_explicit() doesn't need an arch-specific version as
+ * it just invokes the one of memset() implicitly.
+ */
+void memzero_explicit(void *s, size_t count)
+{
+       memset(s, 0, count);
+       OPTIMIZER_HIDE_VAR(s);
+}
+EXPORT_SYMBOL(memzero_explicit);
+
 #ifndef __HAVE_ARCH_MEMCPY
 /**
  * memcpy - Copy one area of memory to another