Re: [PATCH 1/3] rcu/kasan: record and print call_rcu() call stack

[Walter Wu]

On Wed, 2020-05-06 at 11:46 +0200, Dmitry Vyukov wrote:
> On Wed, May 6, 2020 at 7:21 AM Walter Wu <walter-zh.wu@xxxxxxxxxxxx> wrote:
> >
> > When call_rcu() is called, we store the call_rcu() call stack into
> > slub alloc meta-data, so that KASAN report prints call_rcu() information.
> >
> > We add new KASAN_RCU_STACK_RECORD configuration option. It will record
> > first and last call_rcu() call stack and KASAN report will print two
> > call_rcu() call stack.
> >
> > This option doesn't increase the cost of memory consumption. Because
> > we don't enlarge struct kasan_alloc_meta size.
> > - add two call_rcu() call stack into kasan_alloc_meta, size is 8 bytes.
> > - remove free track from kasan_alloc_meta, size is 8 bytes.
> >
> > [1]https://bugzilla.kernel.org/show_bug.cgi?id=198437
> >
> > Signed-off-by: Walter Wu <walter-zh.wu@xxxxxxxxxxxx>
> > Suggested-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> > Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> > Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> > Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
> > Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> > ---
> >  include/linux/kasan.h |  7 +++++++
> >  kernel/rcu/tree.c     |  4 ++++
> >  lib/Kconfig.kasan     | 11 +++++++++++
> >  mm/kasan/common.c     | 23 +++++++++++++++++++++++
> >  mm/kasan/kasan.h      | 12 ++++++++++++
> >  mm/kasan/report.c     | 33 +++++++++++++++++++++++++++------
> >  6 files changed, 84 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> > index 31314ca7c635..5eeece6893cd 100644
> > --- a/include/linux/kasan.h
> > +++ b/include/linux/kasan.h
> > @@ -96,6 +96,12 @@ size_t kasan_metadata_size(struct kmem_cache *cache);
> >  bool kasan_save_enable_multi_shot(void);
> >  void kasan_restore_multi_shot(bool enabled);
> >
> > +#ifdef CONFIG_KASAN_RCU_STACK_RECORD
> > +void kasan_record_callrcu(void *ptr);
> 
> The issue also mentions workqueue and timer stacks.
> Have you considered supporting them as well? What was your motivation
> for doing only rcu?
> 
I will try to implement them when I have free time, maybe we can do it
step by step, finish the printing call_rcu() first.
I remember that I saw the following link, recording call_rcu seems like
be needed.

https://groups.google.com/forum/#!searchin/kasan-dev/better$20stack
$20traces$20for$20rcu%7Csort:date/kasan-dev/KQsjT_88hDE/7rNUZprRBgAJ


> Looking at the first report for "workqueue use-after-free":
> https://syzkaller.appspot.com/bug?extid=9cba1e478f91aad39876
> This is exactly the same situation as for call_rcu, just a workqueue
> is used to invoke a callback that frees the object.
> 
> If you don't want to do all at the same time, I would at least
> name/branch everything inside of KASAN more generally (I think in the
> issue I called it "aux" (auxiliary), or maybe something like
> "additional"). But then call this kasan_record_aux_stack() only from
> rcu for now. But then later we can separately decide and extend to
> other callers.
> It just feels wrong to have KASAN over-specialized for rcu only in this way.
> And I think if the UAF is really caused by call_rcu callback, then it
> sill will be recorded as last stack most of the time because rcu
> callbacks are invoked relatively fast and there should not be much
> else happening with the object since it's near end of life already.
> 
Yes, I agree with you. I will send v2 patchset as you saying.

> 
> 
> 
> > +#else
> > +static inline void kasan_record_callrcu(void *ptr) {}
> > +#endif
> > +
> >  #else /* CONFIG_KASAN */
> >
> >  static inline void kasan_unpoison_shadow(const void *address, size_t size) {}
> > @@ -165,6 +171,7 @@ static inline void kasan_remove_zero_shadow(void *start,
> >
> >  static inline void kasan_unpoison_slab(const void *ptr) { }
> >  static inline size_t kasan_metadata_size(struct kmem_cache *cache) { return 0; }
> > +static inline void kasan_record_callrcu(void *ptr) {}
> >
> >  #endif /* CONFIG_KASAN */
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 06548e2ebb72..145c79becf7b 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -57,6 +57,7 @@
> >  #include <linux/slab.h>
> >  #include <linux/sched/isolation.h>
> >  #include <linux/sched/clock.h>
> > +#include <linux/kasan.h>
> >  #include "../time/tick-internal.h"
> >
> >  #include "tree.h"
> > @@ -2694,6 +2695,9 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func)
> >                 trace_rcu_callback(rcu_state.name, head,
> >                                    rcu_segcblist_n_cbs(&rdp->cblist));
> >
> > +       if (IS_ENABLED(CONFIG_KASAN_RCU_STACK_RECORD))
> 
> The if is not necessary, this function is no-op when not enabled.
> 
> > +               kasan_record_callrcu(head);
> > +
> >         /* Go handle any RCU core processing required. */
> >         if (IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
> >             unlikely(rcu_segcblist_is_offloaded(&rdp->cblist))) {
> > diff --git a/lib/Kconfig.kasan b/lib/Kconfig.kasan
> > index 81f5464ea9e1..022934049cc2 100644
> > --- a/lib/Kconfig.kasan
> > +++ b/lib/Kconfig.kasan
> > @@ -158,6 +158,17 @@ config KASAN_VMALLOC
> >           for KASAN to detect more sorts of errors (and to support vmapped
> >           stacks), but at the cost of higher memory usage.
> >
> > +config KASAN_RCU_STACK_RECORD
> > +       bool "Record and print call_rcu() call stack"
> > +       depends on KASAN_GENERIC
> > +       help
> > +         By default, the KASAN report doesn't print call_rcu() call stack.
> > +         It is very difficult to analyze memory issues(e.g., use-after-free).
> > +
> > +         Enabling this option will print first and last call_rcu() call stack.
> > +         It doesn't enlarge slub alloc meta-data size, so it doesn't increase
> > +         the cost of memory consumption.
> > +
> >  config TEST_KASAN
> >         tristate "Module for testing KASAN for bug detection"
> >         depends on m && KASAN
> > diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> > index 2906358e42f0..32d422bdf127 100644
> > --- a/mm/kasan/common.c
> > +++ b/mm/kasan/common.c
> > @@ -299,6 +299,29 @@ struct kasan_free_meta *get_free_info(struct kmem_cache *cache,
> >         return (void *)object + cache->kasan_info.free_meta_offset;
> >  }
> >
> > +#ifdef CONFIG_KASAN_RCU_STACK_RECORD
> > +void kasan_record_callrcu(void *addr)
> > +{
> > +       struct page *page = kasan_addr_to_page(addr);
> > +       struct kmem_cache *cache;
> > +       struct kasan_alloc_meta *alloc_info;
> > +       void *object;
> > +
> > +       if (!(page && PageSlab(page)))
> > +               return;
> > +
> > +       cache = page->slab_cache;
> > +       object = nearest_obj(cache, page, addr);
> > +       alloc_info = get_alloc_info(cache, object);
> > +
> > +       if (!alloc_info->rcu_free_stack[0])
> > +               /* record first call_rcu() call stack */
> > +               alloc_info->rcu_free_stack[0] = save_stack(GFP_NOWAIT);
> > +       else
> > +               /* record last call_rcu() call stack */
> > +               alloc_info->rcu_free_stack[1] = save_stack(GFP_NOWAIT);
> > +}
> > +#endif
> >
> >  static void kasan_set_free_info(struct kmem_cache *cache,
> >                 void *object, u8 tag)
> > diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> > index e8f37199d885..adc105b9cd07 100644
> > --- a/mm/kasan/kasan.h
> > +++ b/mm/kasan/kasan.h
> > @@ -96,15 +96,27 @@ struct kasan_track {
> >         depot_stack_handle_t stack;
> >  };
> >
> > +#ifdef CONFIG_KASAN_RCU_STACK_RECORD
> > +#define BYTES_PER_WORD 4
> > +#define KASAN_NR_RCU_FREE_STACKS 2
> > +#else /* CONFIG_KASAN_RCU_STACK_RECORD */
> >  #ifdef CONFIG_KASAN_SW_TAGS_IDENTIFY
> >  #define KASAN_NR_FREE_STACKS 5
> >  #else
> >  #define KASAN_NR_FREE_STACKS 1
> >  #endif
> > +#endif /* CONFIG_KASAN_RCU_STACK_RECORD */
> >
> >  struct kasan_alloc_meta {
> >         struct kasan_track alloc_track;
> > +#ifdef CONFIG_KASAN_RCU_STACK_RECORD
> > +       /* call_rcu() call stack is stored into kasan_alloc_meta.
> > +        * free stack is stored into freed object.
> > +        */
> > +       depot_stack_handle_t rcu_free_stack[KASAN_NR_RCU_FREE_STACKS];
> > +#else
> >         struct kasan_track free_track[KASAN_NR_FREE_STACKS];
> > +#endif
> >  #ifdef CONFIG_KASAN_SW_TAGS_IDENTIFY
> >         u8 free_pointer_tag[KASAN_NR_FREE_STACKS];
> >         u8 free_track_idx;
> > diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> > index 80f23c9da6b0..7aaccc70b65b 100644
> > --- a/mm/kasan/report.c
> > +++ b/mm/kasan/report.c
> > @@ -105,9 +105,13 @@ static void end_report(unsigned long *flags)
> >         kasan_enable_current();
> >  }
> >
> > -static void print_track(struct kasan_track *track, const char *prefix)
> > +static void print_track(struct kasan_track *track, const char *prefix,
> > +                                               bool is_callrcu)
> >  {
> > -       pr_err("%s by task %u:\n", prefix, track->pid);
> > +       if (is_callrcu)
> > +               pr_err("%s:\n", prefix);
> > +       else
> > +               pr_err("%s by task %u:\n", prefix, track->pid);
> >         if (track->stack) {
> >                 unsigned long *entries;
> >                 unsigned int nr_entries;
> > @@ -159,8 +163,22 @@ static void describe_object_addr(struct kmem_cache *cache, void *object,
> >                 (void *)(object_addr + cache->object_size));
> >  }
> >
> > +#ifdef CONFIG_KASAN_RCU_STACK_RECORD
> > +static void kasan_print_rcu_free_stack(struct kasan_alloc_meta *alloc_info)
> > +{
> > +       struct kasan_track free_track;
> > +
> > +       free_track.stack  = alloc_info->rcu_free_stack[0];
> > +       print_track(&free_track, "First call_rcu() call stack", true);
> > +       pr_err("\n");
> > +       free_track.stack  = alloc_info->rcu_free_stack[1];
> > +       print_track(&free_track, "Last call_rcu() call stack", true);
> > +       pr_err("\n");
> > +}
> > +#endif
> > +
> >  static struct kasan_track *kasan_get_free_track(struct kmem_cache *cache,
> > -               void *object, u8 tag)
> > +               void *object, u8 tag, const void *addr)
> >  {
> >         struct kasan_alloc_meta *alloc_meta;
> >         int i = 0;
> > @@ -187,11 +205,14 @@ static void describe_object(struct kmem_cache *cache, void *object,
> >         if (cache->flags & SLAB_KASAN) {
> >                 struct kasan_track *free_track;
> >
> > -               print_track(&alloc_info->alloc_track, "Allocated");
> > +               print_track(&alloc_info->alloc_track, "Allocated", false);
> >                 pr_err("\n");
> > -               free_track = kasan_get_free_track(cache, object, tag);
> > -               print_track(free_track, "Freed");
> > +               free_track = kasan_get_free_track(cache, object, tag, addr);
> > +               print_track(free_track, "Freed", false);
> >                 pr_err("\n");
> > +#ifdef CONFIG_KASAN_RCU_STACK_RECORD
> > +               kasan_print_rcu_free_stack(alloc_info);
> > +#endif
> >         }
> >
> >         describe_object_addr(cache, object, addr);
> > --
> > 2.18.0
> >
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