Fix potential vulnerability in a cloned function

drivers/char/random.c

@@ -896,6 +896,7 @@ 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))

include/linux/random.h

@@ -7,6 +7,7 @@
 #define _LINUX_RANDOM_H
 
 #include <uapi/linux/random.h>
+#include <linux/percpu.h>
 
 extern void add_device_randomness(const void *, unsigned int);
 extern void add_input_randomness(unsigned int type, unsigned int code,
@@ -35,6 +36,8 @@ 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);
 

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>
@@ -1485,6 +1486,13 @@ 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);
 }
 
 /*

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.