Re: [PATCH] kasan: Remove kasan_record_aux_stack_noalloc().

[Marco Elver <elver@google.com>]

On Tue, 19 Nov 2024 at 16:57, Sebastian Andrzej Siewior
<bigeasy@linutronix.de> wrote:
>
> From: Peter Zijlstra <peterz@infradead.org>

The patch title is misleading - it might suggest the opposite of what
it's doing. I think this might be clearer:

"kasan: Make kasan_record_aux_stack_noalloc() the default behaviour"

Which is also more or less what you say below.

> kasan_record_aux_stack_noalloc() was introduced to record a stack trace
> without allocating memory in the process. It has been added to callers
> which were invoked while a raw_spinlock_t was held.
> More and more callers were identified and changed over time. Is it a
> good thing to have this while functions try their best to do a
> locklessly setup? The only downside of having kasan_record_aux_stack()
> not allocate any memory is that we end up without a stacktrace if
> stackdepot runs out of memory and at the same stacktrace was not
> recorded before. Marco Elver said in
>         https://lore.kernel.org/all/20210913112609.2651084-1-elver@google.com/
> that this is rare.
>
> Make the kasan_record_aux_stack_noalloc() behaviour default as
> kasan_record_aux_stack().
>
> [bigeasy: Dressed the diff as patch. ]
>
> Reported-by: syzbot+39f85d612b7c20d8db48@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/67275485.050a0220.3c8d68.0a37.GAE@google.com
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Reviewed-by: Marco Elver <elver@google.com>

As I wrote in https://lore.kernel.org/all/CANpmjNPmQYJ7pv1N3cuU8cP18u7PP_uoZD8YxwZd4jtbof9nVQ@mail.gmail.com/:

> I'd be in favor, it simplifies things. And stack depot should be
> able to replenish its pool sufficiently in the "non-aux" cases
> i.e. regular allocations. Worst case we fail to record some
> aux stacks, but I think that's only really bad if there's a bug
> around one of these allocations. In general the probabilities
> of this being a regression are extremely small [...]

Good riddance.

Thanks,
-- Marco

> ---
>
> Didn't add a Fixes tag, didn't want to put
>    7cb3007ce2da2 ("kasan: generic: introduce kasan_record_aux_stack_noalloc()")
>
> there.
>
>  include/linux/kasan.h     |  2 --
>  include/linux/task_work.h |  3 ---
>  kernel/irq_work.c         |  2 +-
>  kernel/rcu/tiny.c         |  2 +-
>  kernel/rcu/tree.c         |  4 ++--
>  kernel/sched/core.c       |  2 +-
>  kernel/task_work.c        | 14 +-------------
>  kernel/workqueue.c        |  2 +-
>  mm/kasan/generic.c        | 14 ++------------
>  mm/slub.c                 |  2 +-
>  10 files changed, 10 insertions(+), 37 deletions(-)
>
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index 00a3bf7c0d8f0..1a623818e8b39 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -488,7 +488,6 @@ void kasan_cache_create(struct kmem_cache *cache, unsigned int *size,
>  void kasan_cache_shrink(struct kmem_cache *cache);
>  void kasan_cache_shutdown(struct kmem_cache *cache);
>  void kasan_record_aux_stack(void *ptr);
> -void kasan_record_aux_stack_noalloc(void *ptr);
>
>  #else /* CONFIG_KASAN_GENERIC */
>
> @@ -506,7 +505,6 @@ static inline void kasan_cache_create(struct kmem_cache *cache,
>  static inline void kasan_cache_shrink(struct kmem_cache *cache) {}
>  static inline void kasan_cache_shutdown(struct kmem_cache *cache) {}
>  static inline void kasan_record_aux_stack(void *ptr) {}
> -static inline void kasan_record_aux_stack_noalloc(void *ptr) {}
>
>  #endif /* CONFIG_KASAN_GENERIC */
>
> diff --git a/include/linux/task_work.h b/include/linux/task_work.h
> index 2964171856e00..0646804860ff1 100644
> --- a/include/linux/task_work.h
> +++ b/include/linux/task_work.h
> @@ -19,9 +19,6 @@ enum task_work_notify_mode {
>         TWA_SIGNAL,
>         TWA_SIGNAL_NO_IPI,
>         TWA_NMI_CURRENT,
> -
> -       TWA_FLAGS = 0xff00,
> -       TWAF_NO_ALLOC = 0x0100,
>  };
>
>  static inline bool task_work_pending(struct task_struct *task)
> diff --git a/kernel/irq_work.c b/kernel/irq_work.c
> index 2f4fb336dda17..73f7e1fd4ab4d 100644
> --- a/kernel/irq_work.c
> +++ b/kernel/irq_work.c
> @@ -147,7 +147,7 @@ bool irq_work_queue_on(struct irq_work *work, int cpu)
>         if (!irq_work_claim(work))
>                 return false;
>
> -       kasan_record_aux_stack_noalloc(work);
> +       kasan_record_aux_stack(work);
>
>         preempt_disable();
>         if (cpu != smp_processor_id()) {
> diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
> index b3b3ce34df631..4b3f319114650 100644
> --- a/kernel/rcu/tiny.c
> +++ b/kernel/rcu/tiny.c
> @@ -250,7 +250,7 @@ EXPORT_SYMBOL_GPL(poll_state_synchronize_rcu);
>  void kvfree_call_rcu(struct rcu_head *head, void *ptr)
>  {
>         if (head)
> -               kasan_record_aux_stack_noalloc(ptr);
> +               kasan_record_aux_stack(ptr);
>
>         __kvfree_call_rcu(head, ptr);
>  }
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index b1f883fcd9185..7eae9bd818a90 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3083,7 +3083,7 @@ __call_rcu_common(struct rcu_head *head, rcu_callback_t func, bool lazy_in)
>         }
>         head->func = func;
>         head->next = NULL;
> -       kasan_record_aux_stack_noalloc(head);
> +       kasan_record_aux_stack(head);
>         local_irq_save(flags);
>         rdp = this_cpu_ptr(&rcu_data);
>         lazy = lazy_in && !rcu_async_should_hurry();
> @@ -3807,7 +3807,7 @@ void kvfree_call_rcu(struct rcu_head *head, void *ptr)
>                 return;
>         }
>
> -       kasan_record_aux_stack_noalloc(ptr);
> +       kasan_record_aux_stack(ptr);
>         success = add_ptr_to_bulk_krc_lock(&krcp, &flags, ptr, !head);
>         if (!success) {
>                 run_page_cache_worker(krcp);
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index a1c353a62c568..3717360a940d2 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -10485,7 +10485,7 @@ void task_tick_mm_cid(struct rq *rq, struct task_struct *curr)
>                 return;
>
>         /* No page allocation under rq lock */
> -       task_work_add(curr, work, TWA_RESUME | TWAF_NO_ALLOC);
> +       task_work_add(curr, work, TWA_RESUME);
>  }
>
>  void sched_mm_cid_exit_signals(struct task_struct *t)
> diff --git a/kernel/task_work.c b/kernel/task_work.c
> index c969f1f26be58..d1efec571a4a4 100644
> --- a/kernel/task_work.c
> +++ b/kernel/task_work.c
> @@ -55,26 +55,14 @@ int task_work_add(struct task_struct *task, struct callback_head *work,
>                   enum task_work_notify_mode notify)
>  {
>         struct callback_head *head;
> -       int flags = notify & TWA_FLAGS;
>
> -       notify &= ~TWA_FLAGS;
>         if (notify == TWA_NMI_CURRENT) {
>                 if (WARN_ON_ONCE(task != current))
>                         return -EINVAL;
>                 if (!IS_ENABLED(CONFIG_IRQ_WORK))
>                         return -EINVAL;
>         } else {
> -               /*
> -                * Record the work call stack in order to print it in KASAN
> -                * reports.
> -                *
> -                * Note that stack allocation can fail if TWAF_NO_ALLOC flag
> -                * is set and new page is needed to expand the stack buffer.
> -                */
> -               if (flags & TWAF_NO_ALLOC)
> -                       kasan_record_aux_stack_noalloc(work);
> -               else
> -                       kasan_record_aux_stack(work);
> +               kasan_record_aux_stack(work);
>         }
>
>         head = READ_ONCE(task->task_works);
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 9949ffad8df09..65b8314b2d538 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -2180,7 +2180,7 @@ static void insert_work(struct pool_workqueue *pwq, struct work_struct *work,
>         debug_work_activate(work);
>
>         /* record the work call stack in order to print it in KASAN reports */
> -       kasan_record_aux_stack_noalloc(work);
> +       kasan_record_aux_stack(work);
>
>         /* we own @work, set data and link */
>         set_work_pwq(work, pwq, extra_flags);
> diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
> index 6310a180278b6..b18b5944997f8 100644
> --- a/mm/kasan/generic.c
> +++ b/mm/kasan/generic.c
> @@ -521,7 +521,7 @@ size_t kasan_metadata_size(struct kmem_cache *cache, bool in_object)
>                         sizeof(struct kasan_free_meta) : 0);
>  }
>
> -static void __kasan_record_aux_stack(void *addr, depot_flags_t depot_flags)
> +void kasan_record_aux_stack(void *addr)
>  {
>         struct slab *slab = kasan_addr_to_slab(addr);
>         struct kmem_cache *cache;
> @@ -538,17 +538,7 @@ static void __kasan_record_aux_stack(void *addr, depot_flags_t depot_flags)
>                 return;
>
>         alloc_meta->aux_stack[1] = alloc_meta->aux_stack[0];
> -       alloc_meta->aux_stack[0] = kasan_save_stack(0, depot_flags);
> -}
> -
> -void kasan_record_aux_stack(void *addr)
> -{
> -       return __kasan_record_aux_stack(addr, STACK_DEPOT_FLAG_CAN_ALLOC);
> -}
> -
> -void kasan_record_aux_stack_noalloc(void *addr)
> -{
> -       return __kasan_record_aux_stack(addr, 0);
> +       alloc_meta->aux_stack[0] = kasan_save_stack(0, 0);
>  }
>
>  void kasan_save_alloc_info(struct kmem_cache *cache, void *object, gfp_t flags)
> diff --git a/mm/slub.c b/mm/slub.c
> index 5b832512044e3..b8c4bf3fe0d07 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2300,7 +2300,7 @@ bool slab_free_hook(struct kmem_cache *s, void *x, bool init,
>                          * We have to do this manually because the rcu_head is
>                          * not located inside the object.
>                          */
> -                       kasan_record_aux_stack_noalloc(x);
> +                       kasan_record_aux_stack(x);
>
>                         delayed_free->object = x;
>                         call_rcu(&delayed_free->head, slab_free_after_rcu_debug);
> --
> 2.45.2
>