random32: update the net random state on interrupt and activity

commit f227e3ec3b5cad859ad15666874405e8c1bbc1d4 upstream.

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
state.

Note that depending on some network devices' interrupt rate moderation
or binding, this re-seeding might happen on every packet or even almost
never.

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@gmail.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
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: <stable@vger.kernel.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

4 files changed, 13 insertions, 1 deletions

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 661ed5ec546e..7bb1e423eb19 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -923,6 +923,7 @@ void add_interrupt_randomness(int irq, int irq_flags)
 
 	fast_mix(fast_pool);
 	add_interrupt_bench(cycles);
+	this_cpu_add(net_rand_state.s1, fast_pool->pool[cycles & 3]);
 
 	if ((fast_pool->count < 64) &&
 	    !time_after(now, fast_pool->last + HZ))
diff --git a/include/linux/random.h b/include/linux/random.h
index 9c29122037f9..fb9e3798f771 100644
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -8,6 +8,7 @@
 
 #include <linux/list.h>
 #include <linux/once.h>
+#include <linux/percpu.h>
 
 #include <uapi/linux/random.h>
 
@@ -46,6 +47,8 @@ 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 3d7588a2e97c..43bee4993187 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -42,6 +42,7 @@
 #include <linux/sched/sysctl.h>
 #include <linux/slab.h>
 #include <linux/compat.h>
+#include <linux/random.h>
 
 #include <asm/uaccess.h>
 #include <asm/unistd.h>
@@ -1431,6 +1432,13 @@ void update_process_times(int user_tick)
 #endif
 	scheduler_tick();
 	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);
 }
 
 /*
diff --git a/lib/random32.c b/lib/random32.c
index 12111910ccd0..8072ccd9eed5 100644
--- a/lib/random32.c
+++ b/lib/random32.c
@@ -47,7 +47,7 @@ static inline void prandom_state_selftest(void)
 }
 #endif
 
-static DEFINE_PER_CPU(struct rnd_state, net_rand_state);
+DEFINE_PER_CPU(struct rnd_state, net_rand_state);
 
 /**
  *	prandom_u32_state - seeded pseudo-random number generator.