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authorGeorge Spelvin <lkml@sdf.org>2020-08-09 06:57:44 +0000
committerWilly Tarreau <w@1wt.eu>2020-10-24 20:21:57 +0200
commitc51f8f88d705e06bd696d7510aff22b33eb8e638 (patch)
tree3fa567b3e70dc16bd397c3b01b987ea932393ccb /kernel
parentf11901ed723d1351843771c3a84b03a253bbf8b2 (diff)
random32: make prandom_u32() output unpredictable
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 ] Signed-off-by: Willy Tarreau <w@1wt.eu>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/time/timer.c7
1 files changed, 0 insertions, 7 deletions
diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index dda05f4b7a1f..3e341af741b9 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1717,13 +1717,6 @@ void update_process_times(int user_tick)
scheduler_tick();
if (IS_ENABLED(CONFIG_POSIX_TIMERS))
run_posix_cpu_timers();
-
- /* 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);
}
/**