[PATCH 3.2 065/164] random: add and use memzero_explicit() for clearing data

[Ben Hutchings]

3.2.65-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Daniel Borkmann <dborkman@xxxxxxxxxx>

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@xxxxxxxxxxxxx
Signed-off-by: Daniel Borkmann <dborkman@xxxxxxxxxx>
Acked-by: Hannes Frederic Sowa <hannes@xxxxxxxxxxxxxxxxxxx>
Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
[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@xxxxxxxxxxxxxxx>
---
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -954,8 +954,8 @@ static void extract_buf(struct entropy_s
 	 * 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_s
 	}
 
 	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 en
 	}
 
 	/* 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(stru
 	}
 
 	/* Wipe data just returned from memory */
-	memset(tmp, 0, sizeof(tmp));
+	memzero_explicit(tmp, sizeof(tmp));
 
 	return ret;
 }
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -144,5 +144,7 @@ static inline bool strstarts(const char
 {
 	return strncmp(str, prefix, strlen(prefix)) == 0;
 }
+
+void memzero_explicit(void *s, size_t count);
 #endif
 #endif /* _LINUX_STRING_H_ */
--- a/lib/string.c
+++ b/lib/string.c
@@ -583,6 +583,22 @@ void *memset(void *s, int c, size_t coun
 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

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