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Subject[PATCH 3.10 029/123] random: add and use memzero_explicit() for clearing data
Date
3.10-stable review patch.  If anyone has any objections, please let me know.

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

From: Daniel Borkmann <dborkman@redhat.com>

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/char/random.c | 10 +++++-----
include/linux/string.h | 5 +++--
lib/string.c | 16 ++++++++++++++++
3 files changed, 24 insertions(+), 7 deletions(-)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -933,8 +933,8 @@ 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
@@ -957,7 +957,7 @@ 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,
@@ -1005,7 +1005,7 @@ 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;
}
@@ -1043,7 +1043,7 @@ 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
@@ -129,7 +129,7 @@ int bprintf(u32 *bin_buf, size_t size, c
#endif

extern ssize_t memory_read_from_buffer(void *to, size_t count, loff_t *ppos,
- const void *from, size_t available);
+ const void *from, size_t available);

/**
* strstarts - does @str start with @prefix?
@@ -141,7 +141,8 @@ static inline bool strstarts(const char
return strncmp(str, prefix, strlen(prefix)) == 0;
}

-extern size_t memweight(const void *ptr, size_t bytes);
+size_t memweight(const void *ptr, size_t bytes);
+void memzero_explicit(void *s, size_t count);

/**
* kbasename - return the last part of a pathname.
--- a/lib/string.c
+++ b/lib/string.c
@@ -586,6 +586,22 @@ 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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