Commit 71a076f4 authored by Marco Elver's avatar Marco Elver Committed by Paul E. McKenney
Browse files

kcsan: Rewrite kcsan_prandom_u32_max() without prandom_u32_state() 



Rewrite kcsan_prandom_u32_max() to not depend on code that might be
instrumented, removing any dependency on lib/random32.c. The rewrite
implements a simple linear congruential generator, that is sufficient
for our purposes (for udelay() and skip_watch counter randomness).

The initial motivation for this was to allow enabling KCSAN for
kernel/sched (remove KCSAN_SANITIZE := n from kernel/sched/Makefile),
with CONFIG_DEBUG_PREEMPT=y. Without this change, we could observe
recursion:

	check_access() [via instrumentation]
	  kcsan_setup_watchpoint()
	    reset_kcsan_skip()
	      kcsan_prandom_u32_max()
	        get_cpu_var()
		  preempt_disable()
		    preempt_count_add() [in kernel/sched/core.c]
		      check_access() [via instrumentation]

Note, while this currently does not affect an unmodified kernel, it'd be
good to keep a KCSAN kernel working when KCSAN_SANITIZE := n is removed
from kernel/sched/Makefile to permit testing scheduler code with KCSAN
if desired.

Fixes: cd290ec2 ("kcsan: Use tracing-safe version of prandom")
Signed-off-by: default avatarMarco Elver <elver@google.com>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
parent 5c8fe583

kernel/kcsan/core.c

+13 −13
Original line number Diff line number Diff line
@@ -12,7 +12,6 @@ 
#include <linux/moduleparam.h>

#include <linux/percpu.h>

#include <linux/preempt.h>

#include <linux/random.h>

#include <linux/sched.h>

#include <linux/uaccess.h>
@@ -101,7 +100,7 @@ static atomic_long_t watchpoints[CONFIG_KCSAN_NUM_WATCHPOINTS + NUM_SLOTS-1]; 
static DEFINE_PER_CPU(long, kcsan_skip);



/* For kcsan_prandom_u32_max(). */

static DEFINE_PER_CPU(struct rnd_state, kcsan_rand_state);

static DEFINE_PER_CPU(u32, kcsan_rand_state);



static __always_inline atomic_long_t *find_watchpoint(unsigned long addr,

						      size_t size,
@@ -275,20 +274,17 @@ should_watch(const volatile void *ptr, size_t size, int type, struct kcsan_ctx * 
}



/*

 * Returns a pseudo-random number in interval [0, ep_ro). See prandom_u32_max()

 * for more details.

 *

 * The open-coded version here is using only safe primitives for all contexts

 * where we can have KCSAN instrumentation. In particular, we cannot use

 * prandom_u32() directly, as its tracepoint could cause recursion.

 * Returns a pseudo-random number in interval [0, ep_ro). Simple linear

 * congruential generator, using constants from "Numerical Recipes".

 */

static u32 kcsan_prandom_u32_max(u32 ep_ro)

{

	struct rnd_state *state = &get_cpu_var(kcsan_rand_state);

	const u32 res = prandom_u32_state(state);

	u32 state = this_cpu_read(kcsan_rand_state);



	state = 1664525 * state + 1013904223;

	this_cpu_write(kcsan_rand_state, state);



	put_cpu_var(kcsan_rand_state);

	return (u32)(((u64) res * ep_ro) >> 32);

	return state % ep_ro;

}



static inline void reset_kcsan_skip(void)
@@ -639,10 +635,14 @@ static __always_inline void check_access(const volatile void *ptr, size_t size, 


void __init kcsan_init(void)

{

	int cpu;



	BUG_ON(!in_task());



	kcsan_debugfs_init();

	prandom_seed_full_state(&kcsan_rand_state);



	for_each_possible_cpu(cpu)

		per_cpu(kcsan_rand_state, cpu) = (u32)get_cycles();



	/*

	 * We are in the init task, and no other tasks should be running;