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random: mix rdrand with entropy sent in from userspace 

commit 81e69df38e2911b642ec121dec319fad2a4782f3 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Change-Id: I4d4ac36af58c5f3d1a5066b54afff4b8f478affd
This commit is contained in:
Theodore Ts'o 2018-07-14 23:55:57 -04:00 committed by followmsi
parent f44d2c1f42
commit 095f11b227
1 changed files with 9 additions and 1 deletions
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10
drivers/char/random.c
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@ -1270,14 +1270,22 @@ 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;