Fedora has integrated the jitter entropy daemon to work around slow boot problems, especially on VM's that don't support virtio-rng:
https://bugzilla.redhat.com/show_bug.cgi?id=1572944
It's understandable why they did this, but the Jitter entropy daemon works fundamentally on the principle: "the CPU microarchitecture is **so** complicated and we can't figure it out, so it *must* be random". Yes, it uses statistical tests to "prove" it is secure, but AES_ENCRYPT(NSA_KEY, COUNTER++) will also pass statistical tests with flying colors.
So if RDRAND is available, mix it into entropy submitted from userspace. It can't hurt, and if you believe the NSA has backdoored RDRAND, then they probably have enough details about the Intel microarchitecture that they can reverse engineer how the Jitter entropy daemon affects the microarchitecture, and attack its output stream. And if RDRAND is in fact an honest DRNG, it will immeasurably improve on what the Jitter entropy daemon might produce.
This also provides some protection against someone who is able to read or set the entropy seed file.
Signed-off-by: Theodore Ts'o tytso@mit.edu Cc: stable@vger.kernel.org Cc: Arnd Bergmann arnd@arndb.de ---
Changes in v2: - Fix silly typo that Arnd pointed out in check the return value of arch_get_random_int() - Break out of the loop after the first failure reported by arch_get_random_int()
drivers/char/random.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-)
diff --git a/drivers/char/random.c b/drivers/char/random.c index cd888d4ee605..bd449ad52442 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -1895,14 +1895,22 @@ static int write_pool(struct entropy_store *r, const char __user *buffer, size_t count) { size_t bytes; - __u32 buf[16]; + __u32 t, buf[16]; const char __user *p = buffer;
while (count > 0) { + int b, i = 0; + bytes = min(count, sizeof(buf)); if (copy_from_user(&buf, p, bytes)) return -EFAULT;
+ for (b = bytes ; b > 0 ; b -= sizeof(__u32), i++) { + if (!arch_get_random_int(&t)) + break; + buf[i] ^= t; + } + count -= bytes; p += bytes;