[PATCH v2 2/2] kasan: revert eviction of stack traces in generic mode

Marco Elver
2024-01-29T10:07:17Z

This partially reverts commits cc478e0b6bdf, 63b85ac56a64, 08d7c94d9635,
a414d4286f34, and 773688a6cb24 to make use of variable-sized stack depot
records, since eviction of stack entries from stack depot forces fixed-
sized stack records. Care was taken to retain the code cleanups by the
above commits.

Eviction was added to generic KASAN as a response to alleviating the
additional memory usage from fixed-sized stack records, but this still
uses more memory than previously.

With the re-introduction of variable-sized records for stack depot, we
can just switch back to non-evictable stack records again, and return
back to the previous performance and memory usage baseline.

Before (observed after a KASAN kernel boot):

pools: 597
refcounted_allocations: 17547
refcounted_frees: 6477
refcounted_in_use: 11070
freelist_size: 3497
persistent_count: 12163
persistent_bytes: 1717008

After:

pools: 319
refcounted_allocations: 0
refcounted_frees: 0
refcounted_in_use: 0
freelist_size: 0
persistent_count: 29397
persistent_bytes: 5183536

As can be seen from the counters, with a generic KASAN config,
refcounted allocations and evictions are no longer used. Due to using
variable-sized records, I observe a reduction of 278 stack depot pools
(saving 4448 KiB) with my test setup.

Fixes: cc478e0b6bdf ("kasan: avoid resetting aux_lock")
Fixes: 63b85ac56a64 ("kasan: stop leaking stack trace handles")
Fixes: 08d7c94d9635 ("kasan: memset free track in qlink_free")
Fixes: a414d4286f34 ("kasan: handle concurrent kasan_record_aux_stack calls")
Fixes: 773688a6cb24 ("kasan: use stack_depot_put for Generic mode")
Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
---
v2:
* Revert kasan_release_object_meta() as well (no longer needed) to catch
use-after-free-before-realloc bugs.
* Add more comments.
---
mm/kasan/common.c | 8 ++---
mm/kasan/generic.c | 68 +++++--------------------------------------
mm/kasan/kasan.h | 10 -------
mm/kasan/quarantine.c | 5 +++-
4 files changed, 14 insertions(+), 77 deletions(-)

diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index 610efae91220..6ca63e8dda74 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -65,8 +65,7 @@ void kasan_save_track(struct kasan_track *track, gfp_t flags)
{
depot_stack_handle_t stack;

- stack = kasan_save_stack(flags,
- STACK_DEPOT_FLAG_CAN_ALLOC | STACK_DEPOT_FLAG_GET);
+ stack = kasan_save_stack(flags, STACK_DEPOT_FLAG_CAN_ALLOC);
kasan_set_track(track, stack);
}

@@ -266,10 +265,9 @@ bool __kasan_slab_free(struct kmem_cache *cache, void *object,
return true;

/*
- * If the object is not put into quarantine, it will likely be quickly
- * reallocated. Thus, release its metadata now.
+ * Note: Keep per-object metadata to allow KASAN print stack traces for
+ * use-after-free-before-realloc bugs.
*/
- kasan_release_object_meta(cache, object);

/* Let slab put the object onto the freelist. */
return false;
diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
index df6627f62402..fc9cf1860efb 100644
--- a/mm/kasan/generic.c
+++ b/mm/kasan/generic.c
@@ -485,16 +485,6 @@ void kasan_init_object_meta(struct kmem_cache *cache, const void *object)
if (alloc_meta) {
/* Zero out alloc meta to mark it as invalid. */
__memset(alloc_meta, 0, sizeof(*alloc_meta));
-
- /*
- * Prepare the lock for saving auxiliary stack traces.
- * Temporarily disable KASAN bug reporting to allow instrumented
- * raw_spin_lock_init to access aux_lock, which resides inside
- * of a redzone.
- */
- kasan_disable_current();
- raw_spin_lock_init(&alloc_meta->aux_lock);
- kasan_enable_current();
}

/*
@@ -506,18 +496,8 @@ void kasan_init_object_meta(struct kmem_cache *cache, const void *object)

static void release_alloc_meta(struct kasan_alloc_meta *meta)
{
- /* Evict the stack traces from stack depot. */
- stack_depot_put(meta->alloc_track.stack);
- stack_depot_put(meta->aux_stack[0]);
- stack_depot_put(meta->aux_stack[1]);
-
- /*
- * Zero out alloc meta to mark it as invalid but keep aux_lock
- * initialized to avoid having to reinitialize it when another object
- * is allocated in the same slot.
- */
- __memset(&meta->alloc_track, 0, sizeof(meta->alloc_track));
- __memset(meta->aux_stack, 0, sizeof(meta->aux_stack));
+ /* Zero out alloc meta to mark it as invalid. */
+ __memset(meta, 0, sizeof(*meta));
}

static void release_free_meta(const void *object, struct kasan_free_meta *meta)
@@ -526,27 +506,10 @@ static void release_free_meta(const void *object, struct kasan_free_meta *meta)
if (*(u8 *)kasan_mem_to_shadow(object) != KASAN_SLAB_FREE_META)
return;

- /* Evict the stack trace from the stack depot. */
- stack_depot_put(meta->free_track.stack);
-
/* Mark free meta as invalid. */
*(u8 *)kasan_mem_to_shadow(object) = KASAN_SLAB_FREE;
}

-void kasan_release_object_meta(struct kmem_cache *cache, const void *object)
-{
- struct kasan_alloc_meta *alloc_meta;
- struct kasan_free_meta *free_meta;
-
- alloc_meta = kasan_get_alloc_meta(cache, object);
- if (alloc_meta)
- release_alloc_meta(alloc_meta);
-
- free_meta = kasan_get_free_meta(cache, object);
- if (free_meta)
- release_free_meta(object, free_meta);
-}
-
size_t kasan_metadata_size(struct kmem_cache *cache, bool in_object)
{
struct kasan_cache *info = &cache->kasan_info;
@@ -571,8 +534,6 @@ static void __kasan_record_aux_stack(void *addr, depot_flags_t depot_flags)
struct kmem_cache *cache;
struct kasan_alloc_meta *alloc_meta;
void *object;
- depot_stack_handle_t new_handle, old_handle;
- unsigned long flags;

if (is_kfence_address(addr) || !slab)
return;
@@ -583,33 +544,18 @@ static void __kasan_record_aux_stack(void *addr, depot_flags_t depot_flags)
if (!alloc_meta)
return;

- new_handle = kasan_save_stack(0, depot_flags);
-
- /*
- * Temporarily disable KASAN bug reporting to allow instrumented
- * spinlock functions to access aux_lock, which resides inside of a
- * redzone.
- */
- kasan_disable_current();
- raw_spin_lock_irqsave(&alloc_meta->aux_lock, flags);
- old_handle = alloc_meta->aux_stack[1];
alloc_meta->aux_stack[1] = alloc_meta->aux_stack[0];
- alloc_meta->aux_stack[0] = new_handle;
- raw_spin_unlock_irqrestore(&alloc_meta->aux_lock, flags);
- kasan_enable_current();
-
- stack_depot_put(old_handle);
+ 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 | STACK_DEPOT_FLAG_GET);
+ 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, STACK_DEPOT_FLAG_GET);
+ return __kasan_record_aux_stack(addr, 0);
}

void kasan_save_alloc_info(struct kmem_cache *cache, void *object, gfp_t flags)
@@ -620,7 +566,7 @@ void kasan_save_alloc_info(struct kmem_cache *cache, void *object, gfp_t flags)
if (!alloc_meta)
return;

- /* Evict previous stack traces (might exist for krealloc or mempool). */
+ /* Invalidate previous stack traces (might exist for krealloc or mempool). */
release_alloc_meta(alloc_meta);

kasan_save_track(&alloc_meta->alloc_track, flags);
@@ -634,7 +580,7 @@ void kasan_save_free_info(struct kmem_cache *cache, void *object)
if (!free_meta)
return;

- /* Evict previous stack trace (might exist for mempool). */
+ /* Invalidate previous stack trace (might exist for mempool). */
release_free_meta(object, free_meta);

kasan_save_track(&free_meta->free_track, 0);
diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
index d0f172f2b978..fb2b9ac0659a 100644
--- a/mm/kasan/kasan.h
+++ b/mm/kasan/kasan.h
@@ -6,7 +6,6 @@
#include <linux/kasan.h>
#include <linux/kasan-tags.h>
#include <linux/kfence.h>
-#include <linux/spinlock.h>
#include <linux/stackdepot.h>

#if defined(CONFIG_KASAN_SW_TAGS) || defined(CONFIG_KASAN_HW_TAGS)
@@ -265,13 +264,6 @@ struct kasan_global {
struct kasan_alloc_meta {
struct kasan_track alloc_track;
/* Free track is stored in kasan_free_meta. */
- /*
- * aux_lock protects aux_stack from accesses from concurrent
- * kasan_record_aux_stack calls. It is a raw spinlock to avoid sleeping
- * on RT kernels, as kasan_record_aux_stack_noalloc can be called from
- * non-sleepable contexts.
- */
- raw_spinlock_t aux_lock;
depot_stack_handle_t aux_stack[2];
};

@@ -398,10 +390,8 @@ struct kasan_alloc_meta *kasan_get_alloc_meta(struct kmem_cache *cache,
struct kasan_free_meta *kasan_get_free_meta(struct kmem_cache *cache,
const void *object);
void kasan_init_object_meta(struct kmem_cache *cache, const void *object);
-void kasan_release_object_meta(struct kmem_cache *cache, const void *object);
#else
static inline void kasan_init_object_meta(struct kmem_cache *cache, const void *object) { }
-static inline void kasan_release_object_meta(struct kmem_cache *cache, const void *object) { }
#endif

depot_stack_handle_t kasan_save_stack(gfp_t flags, depot_flags_t depot_flags);
diff --git a/mm/kasan/quarantine.c b/mm/kasan/quarantine.c
index 3ba02efb952a..6958aa713c67 100644
--- a/mm/kasan/quarantine.c
+++ b/mm/kasan/quarantine.c
@@ -145,7 +145,10 @@ static void qlink_free(struct qlist_node *qlink, struct kmem_cache *cache)
void *object = qlink_to_object(qlink, cache);
struct kasan_free_meta *free_meta = kasan_get_free_meta(cache, object);

- kasan_release_object_meta(cache, object);
+ /*
+ * Note: Keep per-object metadata to allow KASAN print stack traces for
+ * use-after-free-before-realloc bugs.
+ */

/*
* If init_on_free is enabled and KASAN's free metadata is stored in
--
2.43.0.429.g432eaa2c6b-goog