81d7c56d6f
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 4.19 -- various context adjustments]
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
111 lines
3.2 KiB
C
111 lines
3.2 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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/*
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* include/linux/prandom.h
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*
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* Include file for the fast pseudo-random 32-bit
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* generation.
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*/
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#ifndef _LINUX_PRANDOM_H
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#define _LINUX_PRANDOM_H
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#include <linux/types.h>
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#include <linux/percpu.h>
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u32 prandom_u32(void);
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void prandom_bytes(void *buf, size_t nbytes);
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void prandom_seed(u32 seed);
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void prandom_reseed_late(void);
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#if BITS_PER_LONG == 64
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/*
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* The core SipHash round function. Each line can be executed in
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* parallel given enough CPU resources.
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*/
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#define PRND_SIPROUND(v0, v1, v2, v3) ( \
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v0 += v1, v1 = rol64(v1, 13), v2 += v3, v3 = rol64(v3, 16), \
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v1 ^= v0, v0 = rol64(v0, 32), v3 ^= v2, \
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v0 += v3, v3 = rol64(v3, 21), v2 += v1, v1 = rol64(v1, 17), \
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v3 ^= v0, v1 ^= v2, v2 = rol64(v2, 32) \
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)
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#define PRND_K0 (0x736f6d6570736575 ^ 0x6c7967656e657261)
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#define PRND_K1 (0x646f72616e646f6d ^ 0x7465646279746573)
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#elif BITS_PER_LONG == 32
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/*
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* On 32-bit machines, we use HSipHash, a reduced-width version of SipHash.
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* This is weaker, but 32-bit machines are not used for high-traffic
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* applications, so there is less output for an attacker to analyze.
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*/
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#define PRND_SIPROUND(v0, v1, v2, v3) ( \
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v0 += v1, v1 = rol32(v1, 5), v2 += v3, v3 = rol32(v3, 8), \
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v1 ^= v0, v0 = rol32(v0, 16), v3 ^= v2, \
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v0 += v3, v3 = rol32(v3, 7), v2 += v1, v1 = rol32(v1, 13), \
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v3 ^= v0, v1 ^= v2, v2 = rol32(v2, 16) \
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)
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#define PRND_K0 0x6c796765
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#define PRND_K1 0x74656462
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#else
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#error Unsupported BITS_PER_LONG
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#endif
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struct rnd_state {
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__u32 s1, s2, s3, s4;
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};
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u32 prandom_u32_state(struct rnd_state *state);
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void prandom_bytes_state(struct rnd_state *state, void *buf, size_t nbytes);
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void prandom_seed_full_state(struct rnd_state __percpu *pcpu_state);
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#define prandom_init_once(pcpu_state) \
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DO_ONCE(prandom_seed_full_state, (pcpu_state))
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/**
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* prandom_u32_max - returns a pseudo-random number in interval [0, ep_ro)
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* @ep_ro: right open interval endpoint
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*
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* Returns a pseudo-random number that is in interval [0, ep_ro). Note
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* that the result depends on PRNG being well distributed in [0, ~0U]
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* u32 space. Here we use maximally equidistributed combined Tausworthe
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* generator, that is, prandom_u32(). This is useful when requesting a
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* random index of an array containing ep_ro elements, for example.
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*
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* Returns: pseudo-random number in interval [0, ep_ro)
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*/
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static inline u32 prandom_u32_max(u32 ep_ro)
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{
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return (u32)(((u64) prandom_u32() * ep_ro) >> 32);
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}
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/*
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* Handle minimum values for seeds
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*/
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static inline u32 __seed(u32 x, u32 m)
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{
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return (x < m) ? x + m : x;
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}
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/**
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* prandom_seed_state - set seed for prandom_u32_state().
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* @state: pointer to state structure to receive the seed.
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* @seed: arbitrary 64-bit value to use as a seed.
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*/
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static inline void prandom_seed_state(struct rnd_state *state, u64 seed)
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{
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u32 i = (seed >> 32) ^ (seed << 10) ^ seed;
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state->s1 = __seed(i, 2U);
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state->s2 = __seed(i, 8U);
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state->s3 = __seed(i, 16U);
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state->s4 = __seed(i, 128U);
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}
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/* Pseudo random number generator from numerical recipes. */
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static inline u32 next_pseudo_random32(u32 seed)
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{
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return seed * 1664525 + 1013904223;
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}
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#endif
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