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@xxxxxxxxxxxxx>
Cc: Alexander Potapenko <glider@xxxxxxxxxx>
Cc: Marco Elver <elver@xxxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Reported-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
Tested-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
---
Changes v1->v2:
- Make the bools const to help compiler elide branch when possible,
suggested by Marco.
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..4b5e5a3d3a63 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;
+ const bool random_right_allocate = prandom_u32_max(2);
+ const 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
