[SRU Focal:oem-5.6] random32: update the net random state on interrupt and activity

Thadeu Lima de Souza Cascardo cascardo at canonical.com
Wed Sep 9 20:01:34 UTC 2020

From: Willy Tarreau <w at 1wt.eu>

This modifies the first 32 bits out of the 128 bits of a random CPU's
net_rand_state on interrupt or CPU activity to complicate remote
observations that could lead to guessing the network RNG's internal

Note that depending on some network devices' interrupt rate moderation
or binding, this re-seeding might happen on every packet or even almost

In addition, with NOHZ some CPUs might not even get timer interrupts,
leaving their local state rarely updated, while they are running
networked processes making use of the random state.  For this reason, we
also perform this update in update_process_times() in order to at least
update the state when there is user or system activity, since it's the
only case we care about.

Reported-by: Amit Klein <aksecurity at gmail.com>
Suggested-by: Linus Torvalds <torvalds at linux-foundation.org>
Cc: Eric Dumazet <edumazet at google.com>
Cc: "Jason A. Donenfeld" <Jason at zx2c4.com>
Cc: Andy Lutomirski <luto at kernel.org>
Cc: Kees Cook <keescook at chromium.org>
Cc: Thomas Gleixner <tglx at linutronix.de>
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: <stable at vger.kernel.org>
Signed-off-by: Willy Tarreau <w at 1wt.eu>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
(cherry picked from commit f227e3ec3b5cad859ad15666874405e8c1bbc1d4)
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
 drivers/char/random.c  | 1 +
 include/linux/random.h | 3 +++
 kernel/time/timer.c    | 8 ++++++++
 lib/random32.c         | 2 +-
 4 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 6cf5a5f416ff..eb5ed0455e34 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1249,6 +1249,7 @@ void add_interrupt_randomness(int irq, int irq_flags)
+	this_cpu_add(net_rand_state.s1, fast_pool->pool[cycles & 3]);
 	if (unlikely(crng_init == 0)) {
 		if ((fast_pool->count >= 64) &&
diff --git a/include/linux/random.h b/include/linux/random.h
index d319f9a1e429..114db225ac3e 100644
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -9,6 +9,7 @@
 #include <linux/list.h>
 #include <linux/once.h>
+#include <linux/percpu.h>
 #include <uapi/linux/random.h>
@@ -117,6 +118,8 @@ struct rnd_state {
 	__u32 s1, s2, s3, s4;
+DECLARE_PER_CPU(struct rnd_state, net_rand_state) __latent_entropy;
 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);
diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 4820823515e9..226da9217f26 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -43,6 +43,7 @@
 #include <linux/sched/debug.h>
 #include <linux/slab.h>
 #include <linux/compat.h>
+#include <linux/random.h>
 #include <linux/uaccess.h>
 #include <asm/unistd.h>
@@ -1731,6 +1732,13 @@ void update_process_times(int user_tick)
+	/* 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);
diff --git a/lib/random32.c b/lib/random32.c
index 763b920a6206..c4d317be2997 100644
--- a/lib/random32.c
+++ b/lib/random32.c
@@ -48,7 +48,7 @@ static inline void prandom_state_selftest(void)
-static DEFINE_PER_CPU(struct rnd_state, net_rand_state) __latent_entropy;
+DEFINE_PER_CPU(struct rnd_state, net_rand_state) __latent_entropy;
  *	prandom_u32_state - seeded pseudo-random number generator.

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