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Subject[PATCH] random: add and use memzero_explicit() for clearing data
Date
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>
Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
---
drivers/char/random.c | 8 ++++----
include/linux/string.h | 5 +++--
lib/string.c | 16 ++++++++++++++++
3 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index c18d41d..8c86a95 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;
}
diff --git a/include/linux/string.h b/include/linux/string.h
index d36977e..3b42b37 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -132,7 +132,7 @@ int bprintf(u32 *bin_buf, size_t size, const char *fmt, ...) __printf(3, 4);
#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?
@@ -144,7 +144,8 @@ static inline bool strstarts(const char *str, const char *prefix)
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.
diff --git a/lib/string.c b/lib/string.c
index 992bf30..0f3a927 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -604,6 +604,22 @@ 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
--
1.7.11.7


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