diff options
| author | George Spelvin <lkml@sdf.org> | 2020-08-09 06:57:44 +0000 |
|---|---|---|
| committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2020-11-18 18:28:00 +0100 |
| commit | a16f026330fee92e434267dc35c21bceaa7fd573 (patch) | |
| tree | 2a8d87fc2eb12ed6dc7ccb57a8c98bae681dc531 /kernel | |
| parent | fc08b8e9d8928a21faf7fb5d31fc5d3f26a910a8 (diff) | |
random32: make prandom_u32() output unpredictable
commit c51f8f88d705e06bd696d7510aff22b33eb8e638 upstream.
Non-cryptographic PRNGs may have great statistical properties, but
are usually trivially predictable to someone who knows the algorithm,
given a small sample of their output. An LFSR like prandom_u32() is
particularly simple, even if the sample is widely scattered bits.
It turns out the network stack uses prandom_u32() for some things like
random port numbers which it would prefer are *not* trivially predictable.
Predictability led to a practical DNS spoofing attack. Oops.
This patch replaces the LFSR with a homebrew cryptographic PRNG based
on the SipHash round function, which is in turn seeded with 128 bits
of strong random key. (The authors of SipHash have *not* been consulted
about this abuse of their algorithm.) Speed is prioritized over security;
attacks are rare, while performance is always wanted.
Replacing all callers of prandom_u32() is the quick fix.
Whether to reinstate a weaker PRNG for uses which can tolerate it
is an open question.
Commit f227e3ec3b5c ("random32: update the net random state on interrupt
and activity") was an earlier attempt at a solution. This patch replaces
it.
Reported-by: Amit Klein <aksecurity@gmail.com>
Cc: Willy Tarreau <w@1wt.eu>
Cc: Eric Dumazet <edumazet@google.com>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: tytso@mit.edu
Cc: Florian Westphal <fw@strlen.de>
Cc: Marc Plumb <lkml.mplumb@gmail.com>
Fixes: f227e3ec3b5c ("random32: update the net random state on interrupt and activity")
Signed-off-by: George Spelvin <lkml@sdf.org>
Link: https://lore.kernel.org/netdev/20200808152628.GA27941@SDF.ORG/
[ willy: partial reversal of f227e3ec3b5c; moved SIPROUND definitions
to prandom.h for later use; merged George's prandom_seed() proposal;
inlined siprand_u32(); replaced the net_rand_state[] array with 4
members to fix a build issue; cosmetic cleanups to make checkpatch
happy; fixed RANDOM32_SELFTEST build ]
[wt: backported to 4.14 -- various context adjustments; timer API change]
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'kernel')
| -rw-r--r-- | kernel/time/timer.c | 7 |
1 files changed, 0 insertions, 7 deletions
diff --git a/kernel/time/timer.c b/kernel/time/timer.c index d4bc272e03ee..99f885d2904a 100644 --- a/kernel/time/timer.c +++ b/kernel/time/timer.c @@ -1596,13 +1596,6 @@ void update_process_times(int user_tick) scheduler_tick(); if (IS_ENABLED(CONFIG_POSIX_TIMERS)) run_posix_cpu_timers(p); - - /* The current CPU might make use of net randoms without receiving IRQs - * to renew them often enough. Let's update the net_rand_state from a - * non-constant value that's not affine to the number of calls to make - * sure it's updated when there's some activity (we don't care in idle). - */ - this_cpu_add(net_rand_state.s1, rol32(jiffies, 24) + user_tick); } /** |