Re: [PATCH v2 1/2] mm, kasan: improve double-free detection

From: Yury Norov
Date: Sat May 07 2016 - 06:29:47 EST

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In reply to: Kuthonuzo Luruo: "[PATCH v2 1/2] mm, kasan: improve double-free detection"
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On Fri, May 06, 2016 at 05:17:27PM +0530, Kuthonuzo Luruo wrote:
> Currently, KASAN may fail to detect concurrent deallocations of the same
> object due to a race in kasan_slab_free(). This patch makes double-free
> detection more reliable by serializing access to KASAN object metadata.
> New functions kasan_meta_lock() and kasan_meta_unlock() are provided to
> lock/unlock per-object metadata. Double-free errors are now reported via
> kasan_report().
>
> Testing:
> - Tested with a modified version of the 'slab_test' microbenchmark where
> allocs occur on CPU 0; then all other CPUs concurrently attempt to free
> the same object.
> - Tested with new 'test_kasan' kasan_double_free() test in accompanying
> patch.
>
> Signed-off-by: Kuthonuzo Luruo <kuthonuzo.luruo@xxxxxxx>
> ---
>
> Changes in v2:
> - Incorporated suggestions from Dmitry Vyukov. New per-object metadata
> lock/unlock functions; kasan_alloc_meta modified to add new state while
> using fewer bits overall.
> - Double-free pr_err promoted to kasan_report().
> - kasan_init_object() introduced to initialize KASAN object metadata
> during slab creation. KASAN_STATE_INIT initialization removed from
> kasan_poison_object_data().
>
> ---
> include/linux/kasan.h | 8 +++
> mm/kasan/kasan.c | 118 ++++++++++++++++++++++++++++++++++++-------------
> mm/kasan/kasan.h | 15 +++++-
> mm/kasan/quarantine.c | 7 +++-
> mm/kasan/report.c | 31 +++++++++++--
> mm/slab.c | 1 +
> 6 files changed, 142 insertions(+), 38 deletions(-)
>
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index 645c280..c7bf625 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -78,6 +78,10 @@ struct kasan_cache {
> int kasan_module_alloc(void *addr, size_t size);
> void kasan_free_shadow(const struct vm_struct *vm);
>
> +#ifdef CONFIG_SLAB
> +void kasan_init_object(struct kmem_cache *cache, void *object);
> +#endif
> +
> #else /* CONFIG_KASAN */
>
> static inline void kasan_unpoison_shadow(const void *address, size_t size) {}
> @@ -124,6 +128,10 @@ static inline void kasan_poison_slab_free(struct kmem_cache *s, void *object) {}
> static inline int kasan_module_alloc(void *addr, size_t size) { return 0; }
> static inline void kasan_free_shadow(const struct vm_struct *vm) {}
>
> +#ifdef CONFIG_SLAB
> +static inline void kasan_init_object(struct kmem_cache *cache, void *object) {}
> +#endif
> +
> #endif /* CONFIG_KASAN */
>
> #endif /* LINUX_KASAN_H */
> diff --git a/mm/kasan/kasan.c b/mm/kasan/kasan.c
> index ef2e87b..a840b49 100644
> --- a/mm/kasan/kasan.c
> +++ b/mm/kasan/kasan.c
> @@ -34,6 +34,7 @@
> #include <linux/string.h>
> #include <linux/types.h>
> #include <linux/vmalloc.h>
> +#include <linux/atomic.h>
>
> #include "kasan.h"
> #include "../slab.h"
> @@ -419,13 +420,6 @@ void kasan_poison_object_data(struct kmem_cache *cache, void *object)
> kasan_poison_shadow(object,
> round_up(cache->object_size, KASAN_SHADOW_SCALE_SIZE),
> KASAN_KMALLOC_REDZONE);
> -#ifdef CONFIG_SLAB
> - if (cache->flags & SLAB_KASAN) {
> - struct kasan_alloc_meta *alloc_info =
> - get_alloc_info(cache, object);
> - alloc_info->state = KASAN_STATE_INIT;
> - }
> -#endif
> }
>
> #ifdef CONFIG_SLAB
> @@ -470,6 +464,18 @@ static inline depot_stack_handle_t save_stack(gfp_t flags)
> return depot_save_stack(&trace, flags);
> }
>
> +void kasan_init_object(struct kmem_cache *cache, void *object)
> +{
> + struct kasan_alloc_meta *alloc_info;
> +
> + if (cache->flags & SLAB_KASAN) {
> + kasan_unpoison_object_data(cache, object);
> + alloc_info = get_alloc_info(cache, object);
> + __memset(alloc_info, 0, sizeof(*alloc_info));
> + kasan_poison_object_data(cache, object);
> + }
> +}
> +
> static inline void set_track(struct kasan_track *track, gfp_t flags)
> {
> track->pid = current->pid;
> @@ -489,6 +495,39 @@ struct kasan_free_meta *get_free_info(struct kmem_cache *cache,
> BUILD_BUG_ON(sizeof(struct kasan_free_meta) > 32);
> return (void *)object + cache->kasan_info.free_meta_offset;
> }
> +
> +/* acquire per-object lock for access to KASAN metadata. */

I believe there's strong reason not to use standard spin_lock() or
similar. I think it's proper place to explain it.

> +void kasan_meta_lock(struct kasan_alloc_meta *alloc_info)
> +{
> + union kasan_alloc_data old, new;
> +
> + preempt_disable();

It's better to disable and enable preemption inside the loop
on each iteration, to decrease contention.

> + for (;;) {
> + old.packed = READ_ONCE(alloc_info->data);
> + if (unlikely(old.lock)) {
> + cpu_relax();
> + continue;
> + }
> + new.packed = old.packed;
> + new.lock = 1;
> + if (likely(cmpxchg(&alloc_info->data, old.packed, new.packed)
> + == old.packed))
> + break;
> + }
> +}
> +
> +/* release lock after a kasan_meta_lock(). */
> +void kasan_meta_unlock(struct kasan_alloc_meta *alloc_info)
> +{
> + union kasan_alloc_data alloc_data;
> +
> + alloc_data.packed = READ_ONCE(alloc_info->data);
> + alloc_data.lock = 0;
> + if (unlikely(xchg(&alloc_info->data, alloc_data.packed) !=
> + (alloc_data.packed | 0x1U)))
> + WARN_ONCE(1, "%s: lock not held!\n", __func__);

Nitpick. It never happens in normal case, correct?. Why don't you place it under
some developer config, or even leave at dev branch? The function will
be twice shorter without it.

> + preempt_enable();
> +}
> #endif
>
> void kasan_slab_alloc(struct kmem_cache *cache, void *object, gfp_t flags)
> @@ -511,32 +550,41 @@ void kasan_poison_slab_free(struct kmem_cache *cache, void *object)
> bool kasan_slab_free(struct kmem_cache *cache, void *object)
> {
> #ifdef CONFIG_SLAB
> + struct kasan_alloc_meta *alloc_info;
> + struct kasan_free_meta *free_info;
> + union kasan_alloc_data alloc_data;
> +
> /* RCU slabs could be legally used after free within the RCU period */
> if (unlikely(cache->flags & SLAB_DESTROY_BY_RCU))
> return false;
>
> - if (likely(cache->flags & SLAB_KASAN)) {
> - struct kasan_alloc_meta *alloc_info =
> - get_alloc_info(cache, object);
> - struct kasan_free_meta *free_info =
> - get_free_info(cache, object);
> -
> - switch (alloc_info->state) {
> - case KASAN_STATE_ALLOC:
> - alloc_info->state = KASAN_STATE_QUARANTINE;
> - quarantine_put(free_info, cache);
> - set_track(&free_info->track, GFP_NOWAIT);
> - kasan_poison_slab_free(cache, object);
> - return true;
> - case KASAN_STATE_QUARANTINE:
> - case KASAN_STATE_FREE:
> - pr_err("Double free");
> - dump_stack();
> - break;
> - default:
> - break;
> - }
> + if (unlikely(!(cache->flags & SLAB_KASAN)))
> + return false;
> +
> + alloc_info = get_alloc_info(cache, object);
> + kasan_meta_lock(alloc_info);
> + alloc_data.packed = alloc_info->data;
> + if (alloc_data.state == KASAN_STATE_ALLOC) {
> + free_info = get_free_info(cache, object);
> + quarantine_put(free_info, cache);

I just pulled master and didn't find this function. If your patchset
is based on other branch, please notice it.

> + set_track(&free_info->track, GFP_NOWAIT);

It may fail for many reasons. Is it OK to ignore it? If OK, I think it
should be explained.

> + kasan_poison_slab_free(cache, object);
> + alloc_data.state = KASAN_STATE_QUARANTINE;
> + alloc_info->data = alloc_data.packed;
> + kasan_meta_unlock(alloc_info);
> + return true;
> }
> + switch (alloc_data.state) {
> + case KASAN_STATE_QUARANTINE:
> + case KASAN_STATE_FREE:
> + kasan_report((unsigned long)object, 0, false,
> + (unsigned long)__builtin_return_address(1));

__builtin_return_address() is unsafe if argument is non-zero. Use
return_address() instead.

> + kasan_meta_unlock(alloc_info);
> + return true;
> + default:
> + break;
> + }
> + kasan_meta_unlock(alloc_info);
> return false;
> #else
> kasan_poison_slab_free(cache, object);
> @@ -568,12 +616,20 @@ void kasan_kmalloc(struct kmem_cache *cache, const void *object, size_t size,
> KASAN_KMALLOC_REDZONE);
> #ifdef CONFIG_SLAB
> if (cache->flags & SLAB_KASAN) {
> + union kasan_alloc_data alloc_data;
> struct kasan_alloc_meta *alloc_info =
> get_alloc_info(cache, object);
> -
> - alloc_info->state = KASAN_STATE_ALLOC;
> - alloc_info->alloc_size = size;
> + unsigned long flags;
> +
> + local_irq_save(flags);
> + kasan_meta_lock(alloc_info);
> + alloc_data.packed = alloc_info->data;
> + alloc_data.state = KASAN_STATE_ALLOC;
> + alloc_data.size_delta = cache->object_size - size;
> + alloc_info->data = alloc_data.packed;
> set_track(&alloc_info->track, flags);

Same as above

> + kasan_meta_unlock(alloc_info);
> + local_irq_restore(flags);
> }
> #endif
> }
> diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> index 7da78a6..df2724d 100644
> --- a/mm/kasan/kasan.h
> +++ b/mm/kasan/kasan.h
> @@ -73,10 +73,19 @@ struct kasan_track {
> depot_stack_handle_t stack;
> };
>
> +union kasan_alloc_data {
> + struct {
> + u32 lock : 1;
> + u32 state : 2; /* enum kasan_state */
> + u32 size_delta : 24; /* object_size - alloc size */
> + u32 unused : 5;
> + };
> + u32 packed;
> +};
> +
> struct kasan_alloc_meta {
> struct kasan_track track;
> - u32 state : 2; /* enum kasan_state */
> - u32 alloc_size : 30;
> + u32 data; /* encoded as union kasan_alloc_data */
> u32 reserved;
> };
>
> @@ -112,4 +121,6 @@ void quarantine_put(struct kasan_free_meta *info, struct kmem_cache *cache);
> void quarantine_reduce(void);
> void quarantine_remove_cache(struct kmem_cache *cache);
>
> +void kasan_meta_lock(struct kasan_alloc_meta *alloc_info);
> +void kasan_meta_unlock(struct kasan_alloc_meta *alloc_info);
> #endif
> diff --git a/mm/kasan/quarantine.c b/mm/kasan/quarantine.c
> index 40159a6..d59a569 100644
> --- a/mm/kasan/quarantine.c
> +++ b/mm/kasan/quarantine.c
> @@ -144,10 +144,15 @@ static void qlink_free(void **qlink, struct kmem_cache *cache)
> {
> void *object = qlink_to_object(qlink, cache);
> struct kasan_alloc_meta *alloc_info = get_alloc_info(cache, object);
> + union kasan_alloc_data alloc_data;
> unsigned long flags;
>
> local_irq_save(flags);
> - alloc_info->state = KASAN_STATE_FREE;
> + kasan_meta_lock(alloc_info);
> + alloc_data.packed = alloc_info->data;
> + alloc_data.state = KASAN_STATE_FREE;
> + alloc_info->data = alloc_data.packed;
> + kasan_meta_unlock(alloc_info);
> ___cache_free(cache, object, _THIS_IP_);
> local_irq_restore(flags);
> }
> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> index b3c122d..cecf2fa 100644
> --- a/mm/kasan/report.c
> +++ b/mm/kasan/report.c
> @@ -53,6 +53,17 @@ static void print_error_description(struct kasan_access_info *info)
> const char *bug_type = "unknown-crash";
> u8 *shadow_addr;
>
> +#ifdef CONFIG_SLAB
> + if (!info->access_size) {
> + pr_err("BUG: KASAN: double-free attempt in %pS on object at addr %p\n",
> + (void *)info->ip, info->access_addr);
> + pr_err("Double free by task %s/%d\n",
> + current->comm, task_pid_nr(current));
> + info->first_bad_addr = info->access_addr;
> + return;
> + }
> +#endif
> +
> info->first_bad_addr = find_first_bad_addr(info->access_addr,
> info->access_size);
>
> @@ -131,29 +142,34 @@ static void print_track(struct kasan_track *track)
> }
>
> static void object_err(struct kmem_cache *cache, struct page *page,
> - void *object, char *unused_reason)
> + void *object, char *unused_reason,
> + struct kasan_access_info *info)
> {
> struct kasan_alloc_meta *alloc_info = get_alloc_info(cache, object);
> struct kasan_free_meta *free_info;
> + union kasan_alloc_data alloc_data;
>
> dump_stack();
> pr_err("Object at %p, in cache %s\n", object, cache->name);
> if (!(cache->flags & SLAB_KASAN))
> return;
> - switch (alloc_info->state) {
> + if (info->access_size)
> + kasan_meta_lock(alloc_info);
> + alloc_data.packed = alloc_info->data;
> + switch (alloc_data.state) {
> case KASAN_STATE_INIT:
> pr_err("Object not allocated yet\n");
> break;
> case KASAN_STATE_ALLOC:
> pr_err("Object allocated with size %u bytes.\n",
> - alloc_info->alloc_size);
> + (cache->object_size - alloc_data.size_delta));
> pr_err("Allocation:\n");
> print_track(&alloc_info->track);
> break;
> case KASAN_STATE_FREE:
> case KASAN_STATE_QUARANTINE:
> pr_err("Object freed, allocated with size %u bytes\n",
> - alloc_info->alloc_size);
> + (cache->object_size - alloc_data.size_delta));
> free_info = get_free_info(cache, object);
> pr_err("Allocation:\n");
> print_track(&alloc_info->track);
> @@ -161,6 +177,8 @@ static void object_err(struct kmem_cache *cache, struct page *page,
> print_track(&free_info->track);
> break;
> }
> + if (info->access_size)
> + kasan_meta_unlock(alloc_info);
> }
> #endif
>
> @@ -177,8 +195,13 @@ static void print_address_description(struct kasan_access_info *info)
> struct kmem_cache *cache = page->slab_cache;
> object = nearest_obj(cache, page,
> (void *)info->access_addr);
> +#ifdef CONFIG_SLAB
> + object_err(cache, page, object,
> + "kasan: bad access detected", info);
> +#else
> object_err(cache, page, object,
> "kasan: bad access detected");
> +#endif
> return;
> }
> dump_page(page, "kasan: bad access detected");
> diff --git a/mm/slab.c b/mm/slab.c
> index 3f20800..110d586 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -2651,6 +2651,7 @@ static void cache_init_objs(struct kmem_cache *cachep,
> cachep->ctor(objp);
> kasan_poison_object_data(cachep, objp);
> }
> + kasan_init_object(cachep, index_to_obj(cachep, page, i));
>
> if (!shuffled)
> set_free_obj(page, i, i);
> --
> 1.7.1

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