| Message ID | 20220609121709.12939-1-Jason@zx2c4.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm/kfence: select random number before taking raw lock | expand |
On Thu, 9 Jun 2022 at 14:17, Jason A. Donenfeld <Jason@zx2c4.com> 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 <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) 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!
On Thu, Jun 9, 2022 at 2:27 PM Marco Elver <elver@google.com> wrote: > 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! Disassembling reveals it still does elide. (gcc 11.3) But I'll make them const just in case and send a v2. Jason
Hi Jason, On Thu, Jun 9, 2022 at 2:17 PM Jason A. Donenfeld <Jason@zx2c4.com> 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 <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> Thank you, the splat is gone. Tested-by: Geert Uytterhoeven <geert+renesas@glider.be> Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org 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
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]);
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(-)