[PATCH 4.9 017/264] random: use a different mixing algorithm for add_device_randomness()

Greg Kroah-Hartman
2022-06-23T16:47:14Z

From: Theodore Ts'o <tytso@mit.edu>

commit dc12baacb95f205948f64dc936a47d89ee110117 upstream.

add_device_randomness() use of crng_fast_load() was highly
problematic. Some callers of add_device_randomness() can pass in a
large amount of static information. This would immediately promote
the crng_init state from 0 to 1, without really doing much to
initialize the primary_crng's internal state with something even
vaguely unpredictable.

Since we don't have the speed constraints of add_interrupt_randomness(),
we can do a better job mixing in the what unpredictability a device
driver or architecture maintainer might see fit to give us, and do it
in a way which does not bump the crng_init_cnt variable.

Also, since add_device_randomness() doesn't bump any entropy
accounting in crng_init state 0, mix the device randomness into the
input_pool entropy pool as well. This is related to CVE-2018-1108.

Reported-by: Jann Horn <jannh@google.com>
Fixes: ee7998c50c26 ("random: do not ignore early device randomness")
Cc: stable@kernel.org # 4.13+
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/char/random.c | 55 ++++++++++++++++++++++++++++++++++++++++++++++----
1 file changed, 51 insertions(+), 4 deletions(-)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -799,6 +799,10 @@ static void crng_initialize(struct crng_
crng->init_time = jiffies - CRNG_RESEED_INTERVAL - 1;
}

+/*
+ * crng_fast_load() can be called by code in the interrupt service
+ * path. So we can't afford to dilly-dally.
+ */
static int crng_fast_load(const char *cp, size_t len)
{
unsigned long flags;
@@ -876,6 +880,51 @@ static struct crng_state *select_crng(vo
}
#endif

+/*
+ * crng_slow_load() is called by add_device_randomness, which has two
+ * attributes. (1) We can't trust the buffer passed to it is
+ * guaranteed to be unpredictable (so it might not have any entropy at
+ * all), and (2) it doesn't have the performance constraints of
+ * crng_fast_load().
+ *
+ * So we do something more comprehensive which is guaranteed to touch
+ * all of the primary_crng's state, and which uses a LFSR with a
+ * period of 255 as part of the mixing algorithm. Finally, we do
+ * *not* advance crng_init_cnt since buffer we may get may be something
+ * like a fixed DMI table (for example), which might very well be
+ * unique to the machine, but is otherwise unvarying.
+ */
+static int crng_slow_load(const char *cp, size_t len)
+{
+ unsigned long flags;
+ static unsigned char lfsr = 1;
+ unsigned char tmp;
+ unsigned i, max = CHACHA20_KEY_SIZE;
+ const char * src_buf = cp;
+ char * dest_buf = (char *) &primary_crng.state[4];
+
+ if (!spin_trylock_irqsave(&primary_crng.lock, flags))
+ return 0;
+ if (crng_init != 0) {
+ spin_unlock_irqrestore(&primary_crng.lock, flags);
+ return 0;
+ }
+ if (len > max)
+ max = len;
+
+ for (i = 0; i < max ; i++) {
+ tmp = lfsr;
+ lfsr >>= 1;
+ if (tmp & 1)
+ lfsr ^= 0xE1;
+ tmp = dest_buf[i % CHACHA20_KEY_SIZE];
+ dest_buf[i % CHACHA20_KEY_SIZE] ^= src_buf[i % len] ^ lfsr;
+ lfsr += (tmp << 3) | (tmp >> 5);
+ }
+ spin_unlock_irqrestore(&primary_crng.lock, flags);
+ return 1;
+}
+
static void crng_reseed(struct crng_state *crng, struct entropy_store *r)
{
unsigned long flags;
@@ -1046,10 +1095,8 @@ void add_device_randomness(const void *b
unsigned long time = random_get_entropy() ^ jiffies;
unsigned long flags;

- if (!crng_ready()) {
- crng_fast_load(buf, size);
- return;
- }
+ if (!crng_ready() && size)
+ crng_slow_load(buf, size);

trace_add_device_randomness(size, _RET_IP_);
spin_lock_irqsave(&input_pool.lock, flags);