[PATCH] mm/kfence: select random number before taking raw lock

["Jason A. Donenfeld" <Jason@zx2c4.com>]

The RNG uses vanilla spinlocks, not raw spinlocks, so kfence should pick
its random numbers before taking its raw spinlocks. This also has the
nice effect of doing less work inside the lock. It should fix a splat
that Geert saw with CONFIG_PROVE_RAW_LOCK_NESTING:

     dump_backtrace.part.0+0x98/0xc0
     show_stack+0x14/0x28
     dump_stack_lvl+0xac/0xec
     dump_stack+0x14/0x2c
     __lock_acquire+0x388/0x10a0
     lock_acquire+0x190/0x2c0
     _raw_spin_lock_irqsave+0x6c/0x94
     crng_make_state+0x148/0x1e4
     _get_random_bytes.part.0+0x4c/0xe8
     get_random_u32+0x4c/0x140
     __kfence_alloc+0x460/0x5c4
     kmem_cache_alloc_trace+0x194/0x1dc
     __kthread_create_on_node+0x5c/0x1a8
     kthread_create_on_node+0x58/0x7c
     printk_start_kthread.part.0+0x34/0xa8
     printk_activate_kthreads+0x4c/0x54
     do_one_initcall+0xec/0x278
     kernel_init_freeable+0x11c/0x214
     kernel_init+0x24/0x124
     ret_from_fork+0x10/0x20

Cc: John Ogness <john.ogness@linutronix.de>
Cc: Alexander Potapenko <glider@google.com>
Cc: Marco Elver <elver@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 mm/kfence/core.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/kfence/core.c b/mm/kfence/core.c
index 4e7cd4c8e687..6322b7729b50 100644
--- a/mm/kfence/core.c
+++ b/mm/kfence/core.c
@@ -360,6 +360,9 @@ static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t g
 	unsigned long flags;
 	struct slab *slab;
 	void *addr;
+	bool random_right_allocate = prandom_u32_max(2);
+	bool random_fault = CONFIG_KFENCE_STRESS_TEST_FAULTS &&
+			    !prandom_u32_max(CONFIG_KFENCE_STRESS_TEST_FAULTS);
 
 	/* Try to obtain a free object. */
 	raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
@@ -404,7 +407,7 @@ static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t g
 	 * is that the out-of-bounds accesses detected are deterministic for
 	 * such allocations.
 	 */
-	if (prandom_u32_max(2)) {
+	if (random_right_allocate) {
 		/* Allocate on the "right" side, re-calculate address. */
 		meta->addr += PAGE_SIZE - size;
 		meta->addr = ALIGN_DOWN(meta->addr, cache->align);
@@ -444,7 +447,7 @@ static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t g
 	if (cache->ctor)
 		cache->ctor(addr);
 
-	if (CONFIG_KFENCE_STRESS_TEST_FAULTS && !prandom_u32_max(CONFIG_KFENCE_STRESS_TEST_FAULTS))
+	if (random_fault)
 		kfence_protect(meta->addr); /* Random "faults" by protecting the object. */
 
 	atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCATED]);
-- 
2.35.1