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 <[email protected]>
Cc: Alexander Potapenko <[email protected]>
Cc: Marco Elver <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Reported-by: Geert Uytterhoeven <[email protected]>
Signed-off-by: Jason A. Donenfeld <[email protected]>
---
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
On Thu, 9 Jun 2022 at 14:17, Jason A. Donenfeld <[email protected]> wrote:
>
> 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 <[email protected]>
> Cc: Alexander Potapenko <[email protected]>
> Cc: Marco Elver <[email protected]>
> Cc: Dmitry Vyukov <[email protected]>
> Reported-by: Geert Uytterhoeven <[email protected]>
> Signed-off-by: Jason A. Donenfeld <[email protected]>
> ---
> 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)
The compiler should elide this branch entirely if
CONFIG_KFENCE_STRESS_TEST_FAULTS=0, but not sure it'll always do so
now. My suggestion is to make both new bools consts, to help out the
compiler a little.
Otherwise looks good, thanks for the quick fix!
Hi Jason,
On Thu, Jun 9, 2022 at 2:17 PM Jason A. Donenfeld <[email protected]> wrote:
> 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 <[email protected]>
> Cc: Alexander Potapenko <[email protected]>
> Cc: Marco Elver <[email protected]>
> Cc: Dmitry Vyukov <[email protected]>
> Reported-by: Geert Uytterhoeven <[email protected]>
> Signed-off-by: Jason A. Donenfeld <[email protected]>
Thank you, the splat is gone.
Tested-by: Geert Uytterhoeven <[email protected]>
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- [email protected]
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds