Re: [PATCH v2 1/3] rcu/kasan: record and print call_rcu() call stack

Paul E. McKenney
2020-05-12T16:14:27Z

On Tue, May 12, 2020 at 05:50:28PM +0200, Dmitry Vyukov wrote:
> On Tue, May 12, 2020 at 4:25 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> >
> > On Tue, May 12, 2020 at 03:56:17PM +0200, Dmitry Vyukov wrote:
> > > On Tue, May 12, 2020 at 4:36 AM Walter Wu <walter-zh.wu@mediatek.com> wrote:
> > > >
> > > > On Mon, 2020-05-11 at 11:05 -0700, Paul E. McKenney wrote:
> > > > > On Mon, May 11, 2020 at 10:31:11AM +0800, Walter Wu wrote:
> > > > > > This feature will record first and last call_rcu() call stack and
> > > > > > print two call_rcu() call stack in KASAN report.
> > > > >
> > > > > Suppose that a given rcu_head structure is passed to call_rcu(), then
> > > > > the grace period elapses, the callback is invoked, and the enclosing
> > > > > data structure is freed. But then that same region of memory is
> > > > > immediately reallocated as the same type of structure and again
> > > > > passed to call_rcu(), and that this cycle repeats several times.
> > > > >
> > > > > Would the first call stack forever be associated with the first
> > > > > call_rcu() in this series? If so, wouldn't the last two usually
> > > > > be the most useful? Or am I unclear on the use case?
> > >
> > > 2 points here:
> > >
> > > 1. With KASAN the object won't be immediately reallocated. KASAN has
> > > 'quarantine' to delay reuse of heap objects. It is assumed that the
> > > object is still in quarantine when we detect a use-after-free. In such
> > > a case we will have proper call_rcu stacks as well.
> > > It is possible that the object is not in quarantine already and was
> > > reused several times (quarantine is not infinite), but then KASAN will
> > > report non-sense stacks for allocation/free as well. So wrong call_rcu
> > > stacks are less of a problem in such cases.
> > >
> > > 2. We would like to memorize 2 last call_rcu stacks regardless, but we
> > > just don't have a good place for the index (bit which of the 2 is the
> > > one to overwrite). Probably could shove it into some existing field,
> > > but then will require atomic operations, etc.
> > >
> > > Nobody knows how well/bad it will work. I think we need to get the
> > > first version in, deploy on syzbot, accumulate some base of example
> > > reports and iterate from there.
> >
> > If I understood the stack-index point below, why not just move the
> > previous stackm index to clobber the previous-to-previous stack index,
> > then put the current stack index into the spot thus opened up?
>
> We don't have any index in this change (don't have memory for such index).
> The pseudo code is"
>
> u32 aux_stacks[2]; // = {0,0}
>
> if (aux_stacks[0] != 0)
> aux_stacks[0] = stack;
> else
> aux_stacks[1] = stack;

I was thinking in terms of something like this:

u32 aux_stacks[2]; // = {0,0}

if (aux_stacks[0] != 0) {
aux_stacks[0] = stack;
} else {
if (aux_stacks[1])
aux_stacks[0] = aux_stacks[1];
aux_stacks[1] = stack;
}

Whether this actually makes sense in real life, I have no idea.
The theory is that you want the last two stacks. However, if these
elements get cleared at kfree() time, then I could easily believe that
the approach you already have (first and last) is the way to go.

Just asking the question, not arguing for a change!

Thanx, Paul

> > > > The first call stack doesn't forever associate with first call_rcu(),
> > > > if someone object freed and reallocated, then the first call stack will
> > > > replace with new object.
> > > >
> > > > > > When call_rcu() is called, we store the call_rcu() call stack into
> > > > > > slub alloc meta-data, so that KASAN report can print rcu stack.
> > > > > >
> > > > > > It 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
> > > > > > [2]https://groups.google.com/forum/#!searchin/kasan-dev/better$20stack$20traces$20for$20rcu%7Csort:date/kasan-dev/KQsjT_88hDE/7rNUZprRBgAJ
> > > > > >
> > > > > > Signed-off-by: Walter Wu <walter-zh.wu@mediatek.com>
> > > > > > Suggested-by: Dmitry Vyukov <dvyukov@google.com>
> > > > > > Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
> > > > > > Cc: Dmitry Vyukov <dvyukov@google.com>
> > > > > > Cc: Alexander Potapenko <glider@google.com>
> > > > > > Cc: Andrew Morton <akpm@linux-foundation.org>
> > > > > > Cc: Paul E. McKenney <paulmck@kernel.org>
> > > > > > Cc: Josh Triplett <josh@joshtriplett.org>
> > > > > > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > > > > > Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> > > > > > Cc: Joel Fernandes <joel@joelfernandes.org>
> > > > > > ---
> > > > > > include/linux/kasan.h | 2 ++
> > > > > > kernel/rcu/tree.c | 3 +++
> > > > > > lib/Kconfig.kasan | 2 ++
> > > > > > mm/kasan/common.c | 4 ++--
> > > > > > mm/kasan/generic.c | 29 +++++++++++++++++++++++++++++
> > > > > > mm/kasan/kasan.h | 19 +++++++++++++++++++
> > > > > > mm/kasan/report.c | 21 +++++++++++++++++----
> > > > > > 7 files changed, 74 insertions(+), 6 deletions(-)
> > > > > >
> > > > > > diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> > > > > > index 31314ca7c635..23b7ee00572d 100644
> > > > > > --- a/include/linux/kasan.h
> > > > > > +++ b/include/linux/kasan.h
> > > > > > @@ -174,11 +174,13 @@ static inline size_t kasan_metadata_size(struct kmem_cache *cache) { return 0; }
> > > > > >
> > > > > > void kasan_cache_shrink(struct kmem_cache *cache);
> > > > > > void kasan_cache_shutdown(struct kmem_cache *cache);
> > > > > > +void kasan_record_aux_stack(void *ptr);
> > > > > >
> > > > > > #else /* CONFIG_KASAN_GENERIC */
> > > > > >
> > > > > > 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) {}
> > > > > >
> > > > > > #endif /* CONFIG_KASAN_GENERIC */
> > > > > >
> > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > > > index 06548e2ebb72..de872b6cc261 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 +2695,8 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func)
> > > > > > trace_rcu_callback(rcu_state.name, head,
> > > > > > rcu_segcblist_n_cbs(&rdp->cblist));
> > > > > >
> > > > > > + kasan_record_aux_stack(head);
> > > > >
> > > > > Just for the record, at this point we have not yet queued the callback.
> > > > > We have also not yet disabled interrupts. Which might be OK, but I
> > > > > figured I should call out the possibility of moving this down a few
> > > > > lines to follow the local_irq_save().
> > > > >
> > > >
> > > > We will intend to do it.
> > >
> > > I will sleep better if we move it up :)
> > > It qualifies a "debug check", which are generally done on entrance to
> > > the function. Or are these all debug checks up to this point?
> > > But if the callback did not leak anywhere up to this point and we will
> > > maintain it that way, then formally it is fine.
> >
> > There are debug checks, then initialization of presumed private
> > structures, disabling of interrupts, more check that are now safe given
> > that we are pinned to a specific CPU, and so on.
> >
> > I am OK with it being at the beginning of the function.
> >
> > > > > If someone incorrectly invokes concurrently invokes call_rcu() on this
> > > > > same region of memory, possibly from an interrupt handler, we are OK
> > > > > corrupting the stack traces, right?
> > > > >
> > > >
> > > > Yes, and the wrong invoking call_rcu should be recorded.
> > > >
> > > > > But what happens if a given structure has more than one rcu_head
> > > > > structure? In that case, RCU would be just fine with it being
> > > > > concurrently passed to different call_rcu() invocations as long as the
> > > > > two invocations didn't both use the same rcu_head structure. (In that
> > > > > case, they had best not be both freeing the object, and if even one of
> > > > > them is freeing the object, coordination is necessary.)
> > > > >
> > > > > If this is a problem, one approach would be to move the
> > > > > kasan_record_aux_stack(head) call to kfree_rcu(). After all, it is
> > > > > definitely illegal to pass the same memory to a pair of kfree_rcu()
> > > > > invocations! ;-)
> > > > >
> > > >
> > > > The function of kasan_record_aux_stack(head) is simple, it is only to
> > > > record call stack by the 'head' object.
> > >
> > > I would say "corrupting" stacks on some races is fine-ish. In the end
> > > we are just storing an u32 stack id.
> > > On syzbot we generally have multiple samples of the same crash, so
> > > even if one is "corrupted" there may be others that are not corrupted.
> > > Just protecting from this looks too complex and expensive. And in the
> > > end there is not much we can do anyway.
> > >
> > > Recording all call_rcu stacks (not just kfree_rcu) is intentional. I
> > > think it may be useful to even extend to recording workqueue and timer
> > > stacks as well.
> >
> > Given the u32 nature of the stack ID, I agree that there is no point
> > in excluding call_rcu(). At least until such time as we start getting
> > false positives due to multiple rcu_head structures in the same structure.
> >
> > Thanx, Paul
> >
> > > > > > +
> > > > > > /* 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..56a89291f1cc 100644
> > > > > > --- a/lib/Kconfig.kasan
> > > > > > +++ b/lib/Kconfig.kasan
> > > > > > @@ -58,6 +58,8 @@ config KASAN_GENERIC
> > > > > > For better error detection enable CONFIG_STACKTRACE.
> > > > > > Currently CONFIG_KASAN_GENERIC doesn't work with CONFIG_DEBUG_SLAB
> > > > > > (the resulting kernel does not boot).
> > > > > > + Currently CONFIG_KASAN_GENERIC will print first and last call_rcu()
> > > > > > + call stack. It doesn't increase the cost of memory consumption.
> > > > > >
> > > > > > config KASAN_SW_TAGS
> > > > > > bool "Software tag-based mode"
> > > > > > diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> > > > > > index 2906358e42f0..8bc618289bb1 100644
> > > > > > --- a/mm/kasan/common.c
> > > > > > +++ b/mm/kasan/common.c
> > > > > > @@ -41,7 +41,7 @@
> > > > > > #include "kasan.h"
> > > > > > #include "../slab.h"
> > > > > >
> > > > > > -static inline depot_stack_handle_t save_stack(gfp_t flags)
> > > > > > +depot_stack_handle_t kasan_save_stack(gfp_t flags)
> > > > > > {
> > > > > > unsigned long entries[KASAN_STACK_DEPTH];
> > > > > > unsigned int nr_entries;
> > > > > > @@ -54,7 +54,7 @@ static inline depot_stack_handle_t save_stack(gfp_t flags)
> > > > > > static inline void set_track(struct kasan_track *track, gfp_t flags)
> > > > > > {
> > > > > > track->pid = current->pid;
> > > > > > - track->stack = save_stack(flags);
> > > > > > + track->stack = kasan_save_stack(flags);
> > > > > > }
> > > > > >
> > > > > > void kasan_enable_current(void)
> > > > > > diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
> > > > > > index 56ff8885fe2e..b86880c338e2 100644
> > > > > > --- a/mm/kasan/generic.c
> > > > > > +++ b/mm/kasan/generic.c
> > > > > > @@ -325,3 +325,32 @@ DEFINE_ASAN_SET_SHADOW(f2);
> > > > > > DEFINE_ASAN_SET_SHADOW(f3);
> > > > > > DEFINE_ASAN_SET_SHADOW(f5);
> > > > > > DEFINE_ASAN_SET_SHADOW(f8);
> > > > > > +
> > > > > > +void kasan_record_aux_stack(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_stack[0])
> > > > > > + /* record first call_rcu() call stack */
> > > > > > + alloc_info->rcu_stack[0] = kasan_save_stack(GFP_NOWAIT);
> > > > > > + else
> > > > > > + /* record last call_rcu() call stack */
> > > > > > + alloc_info->rcu_stack[1] = kasan_save_stack(GFP_NOWAIT);
> > > > > > +}
> > > > > > +
> > > > > > +struct kasan_track *kasan_get_aux_stack(struct kasan_alloc_meta *alloc_info,
> > > > > > + u8 idx)
> > > > > > +{
> > > > > > + return container_of(&alloc_info->rcu_stack[idx],
> > > > > > + struct kasan_track, stack);
> > > > > > +}
> > > > > > diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> > > > > > index e8f37199d885..1cc1fb7b0de3 100644
> > > > > > --- a/mm/kasan/kasan.h
> > > > > > +++ b/mm/kasan/kasan.h
> > > > > > @@ -96,15 +96,28 @@ struct kasan_track {
> > > > > > depot_stack_handle_t stack;
> > > > > > };
> > > > > >
> > > > > > +#ifdef CONFIG_KASAN_GENERIC
> > > > > > +#define SIZEOF_PTR sizeof(void *)
> > > > > > +#define KASAN_NR_RCU_CALL_STACKS 2
> > > > > > +#else /* CONFIG_KASAN_GENERIC */
> > > > > > #ifdef CONFIG_KASAN_SW_TAGS_IDENTIFY
> > > > > > #define KASAN_NR_FREE_STACKS 5
> > > > > > #else
> > > > > > #define KASAN_NR_FREE_STACKS 1
> > > > > > #endif
> > > > > > +#endif /* CONFIG_KASAN_GENERIC */
> > > > > >
> > > > > > struct kasan_alloc_meta {
> > > > > > struct kasan_track alloc_track;
> > > > > > +#ifdef CONFIG_KASAN_GENERIC
> > > > > > + /*
> > > > > > + * call_rcu() call stack is stored into struct kasan_alloc_meta.
> > > > > > + * The free stack is stored into freed object.
> > > > > > + */
> > > > > > + depot_stack_handle_t rcu_stack[KASAN_NR_RCU_CALL_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;
> > > > > > @@ -159,16 +172,22 @@ void kasan_report_invalid_free(void *object, unsigned long ip);
> > > > > >
> > > > > > struct page *kasan_addr_to_page(const void *addr);
> > > > > >
> > > > > > +depot_stack_handle_t kasan_save_stack(gfp_t flags);
> > > > > > +
> > > > > > #if defined(CONFIG_KASAN_GENERIC) && \
> > > > > > (defined(CONFIG_SLAB) || defined(CONFIG_SLUB))
> > > > > > void quarantine_put(struct kasan_free_meta *info, struct kmem_cache *cache);
> > > > > > void quarantine_reduce(void);
> > > > > > void quarantine_remove_cache(struct kmem_cache *cache);
> > > > > > +struct kasan_track *kasan_get_aux_stack(struct kasan_alloc_meta *alloc_info,
> > > > > > + u8 idx);
> > > > > > #else
> > > > > > static inline void quarantine_put(struct kasan_free_meta *info,
> > > > > > struct kmem_cache *cache) { }
> > > > > > static inline void quarantine_reduce(void) { }
> > > > > > static inline void quarantine_remove_cache(struct kmem_cache *cache) { }
> > > > > > +static inline struct kasan_track *kasan_get_aux_stack(
> > > > > > + struct kasan_alloc_meta *alloc_info, u8 idx) { return NULL; }
> > > > > > #endif
> > > > > >
> > > > > > #ifdef CONFIG_KASAN_SW_TAGS
> > > > > > diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> > > > > > index 80f23c9da6b0..f16a1a210815 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;
> > > > > > @@ -187,11 +191,20 @@ 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");
> > > > > > + print_track(free_track, "Freed", false);
> > > > > > pr_err("\n");
> > > > > > +
> > > > > > + if (IS_ENABLED(CONFIG_KASAN_GENERIC)) {
> > > > > > + free_track = kasan_get_aux_stack(alloc_info, 0);
> > > > > > + print_track(free_track, "First call_rcu() call stack", true);
> > > > > > + pr_err("\n");
> > > > > > + free_track = kasan_get_aux_stack(alloc_info, 1);
> > > > > > + print_track(free_track, "Last call_rcu() call stack", true);
> > > > > > + pr_err("\n");
> > > > > > + }
> > > > > > }
> > > > > >
> > > > > > describe_object_addr(cache, object, addr);
> > > > > > --
> > > > > I> 2.18.0
> > > >
> > > > --
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