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authorMarco Elver <elver@google.com>2020-11-24 12:02:09 +0100
committerPaul E. McKenney <paulmck@kernel.org>2021-01-04 14:39:07 -0800
commit71a076f4a61a6c779794ad286f356b39725edc3b (patch)
treeaac4f289d877fee6e9ad7dbed32c3c0fc80efce0 /kernel/kcsan
parent5c8fe583cce542aa0b84adc939ce85293de36e5e (diff)
downloadlinux-71a076f4a61a6c779794ad286f356b39725edc3b.tar.bz2
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: cd290ec24633 ("kcsan: Use tracing-safe version of prandom") Signed-off-by: Marco Elver <elver@google.com> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Diffstat (limited to 'kernel/kcsan')
-rw-r--r--kernel/kcsan/core.c26
1 files changed, 13 insertions, 13 deletions
diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index 3994a217bde7..3bf98db9c702 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -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;