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random32: make prandom_u32() output unpredictable

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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 f227e3ec3b ("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: f227e3ec3b ("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 f227e3ec3b; 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>
This commit is contained in:
George Spelvin 2020-08-09 06:57:44 +00:00 • committed by Willy Tarreau
parent f11901ed72
commit c51f8f88d7
4 changed files with 318 additions and 190 deletions
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36
include/linux/prandom.h
View file

@ -16,12 +16,44 @@ void prandom_bytes(void *buf, size_t nbytes);
void prandom_seed(u32 seed);
void prandom_reseed_late(void);
#if BITS_PER_LONG == 64
/*
* The core SipHash round function. Each line can be executed in
* parallel given enough CPU resources.
*/
#define PRND_SIPROUND(v0, v1, v2, v3) ( \
v0 += v1, v1 = rol64(v1, 13), v2 += v3, v3 = rol64(v3, 16), \
v1 ^= v0, v0 = rol64(v0, 32), v3 ^= v2, \
v0 += v3, v3 = rol64(v3, 21), v2 += v1, v1 = rol64(v1, 17), \
v3 ^= v0, v1 ^= v2, v2 = rol64(v2, 32) \
)
#define PRND_K0 (0x736f6d6570736575 ^ 0x6c7967656e657261)
#define PRND_K1 (0x646f72616e646f6d ^ 0x7465646279746573)
#elif BITS_PER_LONG == 32
/*
* On 32-bit machines, we use HSipHash, a reduced-width version of SipHash.
* This is weaker, but 32-bit machines are not used for high-traffic
* applications, so there is less output for an attacker to analyze.
*/
#define PRND_SIPROUND(v0, v1, v2, v3) ( \
v0 += v1, v1 = rol32(v1, 5), v2 += v3, v3 = rol32(v3, 8), \
v1 ^= v0, v0 = rol32(v0, 16), v3 ^= v2, \
v0 += v3, v3 = rol32(v3, 7), v2 += v1, v1 = rol32(v1, 13), \
v3 ^= v0, v1 ^= v2, v2 = rol32(v2, 16) \
)
#define PRND_K0 0x6c796765
#define PRND_K1 0x74656462
#else
#error Unsupported BITS_PER_LONG
#endif
struct rnd_state {
__u32 s1, s2, s3, s4;
};
DECLARE_PER_CPU(struct rnd_state, net_rand_state);
u32 prandom_u32_state(struct rnd_state *state);
void prandom_bytes_state(struct rnd_state *state, void *buf, size_t nbytes);
void prandom_seed_full_state(struct rnd_state __percpu *pcpu_state);