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Subject[PATCH 4/4] random: add backtracking protection to the CRNG
Date
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
---
drivers/char/random.c | 35 +++++++++++++++++++++++++++++++++--
1 file changed, 33 insertions(+), 2 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 897c75e..028d085 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -886,6 +886,34 @@ static void extract_crng(__u8 out[CHACHA20_BLOCK_SIZE])
#endif
}

+/*
+ * Use the leftover bytes from the CRNG block output (if there is
+ * enough) to mutate the CRNG key to provide backtracking protection.
+ */
+static void crng_backtrack_protect(__u8 tmp[CHACHA20_BLOCK_SIZE], int unused)
+{
+#ifdef CONFIG_NUMA
+ struct crng_state *crng = crng_node_pool[numa_node_id()];
+#else
+ struct crng_state *crng = &primary_crng;
+#endif
+ unsigned long flags;
+ __u32 *s, *d;
+ int i;
+
+ unused = round_up(unused, sizeof(__u32));
+ if (unused + CHACHA20_KEY_SIZE > CHACHA20_BLOCK_SIZE) {
+ extract_crng(tmp);
+ unused = 0;
+ }
+ spin_lock_irqsave(&crng->lock, flags);
+ s = (__u32 *) &tmp[unused];
+ d = &crng->state[4];
+ for (i=0; i < 8; i++)
+ *d++ ^= *s++;
+ spin_unlock_irqrestore(&crng->lock, flags);
+}
+
static ssize_t extract_crng_user(void __user *buf, size_t nbytes)
{
ssize_t ret = 0, i;
@@ -913,6 +941,7 @@ static ssize_t extract_crng_user(void __user *buf, size_t nbytes)
buf += i;
ret += i;
}
+ crng_backtrack_protect(tmp, i);

/* Wipe data just written to memory */
memzero_explicit(tmp, sizeof(tmp));
@@ -1457,8 +1486,10 @@ void get_random_bytes(void *buf, int nbytes)
if (nbytes > 0) {
extract_crng(tmp);
memcpy(buf, tmp, nbytes);
- memzero_explicit(tmp, nbytes);
- }
+ crng_backtrack_protect(tmp, nbytes);
+ } else
+ crng_backtrack_protect(tmp, 0);
+ memzero_explicit(tmp, sizeof(tmp));
}
EXPORT_SYMBOL(get_random_bytes);

--
2.5.0
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