random: add backtracking protection to the CRNG

Signed-off-by: Theodore Ts'o <tytso@mit.edu>

1 files changed, 49 insertions, 5 deletions

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 2a30d9718a1b..783dee11cdc9 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -438,7 +438,8 @@ static int crng_init_cnt = 0;
 #define CRNG_INIT_CNT_THRESH (2*CHACHA20_KEY_SIZE)
 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);
 
 /**********************************************************************
@@ -826,8 +827,11 @@ 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;
@@ -889,9 +893,46 @@ 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)
+{
+        struct crng_state *crng = NULL;
+
+#ifdef CONFIG_NUMA
+        if (crng_node_pool)
+                crng = crng_node_pool[numa_node_id()];
+        if (crng == NULL)
+#endif
+                crng = &primary_crng;
+        _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);
 
@@ -916,6 +957,7 @@ 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 +1515,10 @@ 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);