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Subject[PATCH 4/5] random: add backtracking protection to the CRNG
Date
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
---
drivers/char/random.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++-----
1 file changed, 47 insertions(+), 5 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index bf370a3..860862f 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -436,7 +436,8 @@ static int crng_init = 0;
#define crng_ready() (likely(crng_init >= 2))
static void _extract_crng(struct crng_state *crng,
__u8 out[CHACHA20_BLOCK_SIZE]);
-static void extract_crng(__u8 out[CHACHA20_BLOCK_SIZE]);
+static void _crng_backtrack_protect(struct crng_state *crng,
+ __u8 tmp[CHACHA20_BLOCK_SIZE], int used);
static void process_random_ready_list(void);

/**********************************************************************
@@ -832,8 +833,11 @@ static void crng_reseed(struct crng_state *crng, struct entropy_store *r)
num = extract_entropy(r, &buf, 32, 16, 0);
if (num == 0)
return;
- } else
+ } else {
_extract_crng(&primary_crng, buf.block);
+ _crng_backtrack_protect(&primary_crng, buf.block,
+ CHACHA20_KEY_SIZE);
+ }
spin_lock_irqsave(&primary_crng.lock, flags);
for (i = 0; i < 8; i++) {
unsigned long rv;
@@ -893,9 +897,44 @@ static void extract_crng(__u8 out[CHACHA20_BLOCK_SIZE])
_extract_crng(crng, out);
}

+/*
+ * 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(struct crng_state *crng,
+ __u8 tmp[CHACHA20_BLOCK_SIZE], int used)
+{
+ unsigned long flags;
+ __u32 *s, *d;
+ int i;
+
+ used = round_up(used, sizeof(__u32));
+ if (used + CHACHA20_KEY_SIZE > CHACHA20_BLOCK_SIZE) {
+ extract_crng(tmp);
+ used = 0;
+ }
+ spin_lock_irqsave(&crng->lock, flags);
+ s = (__u32 *) &tmp[used];
+ d = &crng->state[4];
+ for (i=0; i < 8; i++)
+ *d++ ^= *s++;
+ spin_unlock_irqrestore(&crng->lock, flags);
+}
+
+static void crng_backtrack_protect(__u8 tmp[CHACHA20_BLOCK_SIZE], int used)
+{
+#ifdef CONFIG_NUMA
+ struct crng_state *crng = crng_node_pool[numa_node_id()];
+#else
+ struct crng_state *crng = &primary_crng;
+#endif
+
+ _crng_backtrack_protect(crng, tmp, used);
+}
+
static ssize_t extract_crng_user(void __user *buf, size_t nbytes)
{
- ssize_t ret = 0, i;
+ ssize_t ret = 0, i = CHACHA20_BLOCK_SIZE;
__u8 tmp[CHACHA20_BLOCK_SIZE];
int large_request = (nbytes > 256);

@@ -920,6 +959,7 @@ 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));
@@ -1473,8 +1513,10 @@ 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, CHACHA20_BLOCK_SIZE);
+ memzero_explicit(tmp, sizeof(tmp));
}
EXPORT_SYMBOL(get_random_bytes);

--
2.5.0
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