[PATCH v5 0/2] kasan: solve redzone overwritten issue at debug

[PATCH v5 0/2] kasan: solve redzone overwritten issue at debug

[yee.lee]

From: Yee Lee <yee.lee@mediatek.com>

Issue: In SLUB debug, hwtag kasan_unpoison() would overwrite the redzone
in those objects with unaligned size.

The first patch Introduces slub_debug_enable_unlikely() to check
the state of debug mode.

The second patch Adds memzero_explict() to separate the initialization for
such condition. The new code path is executed about 1.1% during nromal
booting process. The penalty is acceptable since it only works in debug
mode.


=============
Exp: QEMUv5.2(+mte)/SLUB_debug mode
code path exec : 941/80854 (1.1%)

Changed since v5:
 - Fix format

Changed since v4:
 - Introduce slub_debug_enable_unlikly() to check the debug state.
 - Include "slab.h" and Add slub_debug_enable_unlikly() to lead
   the condition statement.
 - Add comment block about this new code path in source code.

Changed since v3:
 - Apply IS_ENABLED to wrap codes under SLUB debug mode.
 - Replace memset() by memzero_explict().

---

Marco Elver (1):
  mm: introduce helper to check slub_debug_enabled

Yee Lee (1):
  kasan: Add memzero int for unaligned size at DEBUG

 mm/kasan/kasan.h | 12 ++++++++++++
 mm/slab.h        | 15 +++++++++++----
 2 files changed, 23 insertions(+), 4 deletions(-)

-- 
2.18.0
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[PATCH v5 1/2] mm: introduce helper to check slub_debug_enabled

[yee.lee]

From: Marco Elver <elver@google.com>

Introduce a helper to check slub_debug_enabled, so that we can confine
the use of #ifdef to the definition of the slub_debug_enabled_unlikely()
helper.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Yee Lee <yee.lee@mediatek.com>
---
 mm/slab.h | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/mm/slab.h b/mm/slab.h
index 7b60ef2f32c3..a11126afe24c 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -216,10 +216,18 @@ DECLARE_STATIC_KEY_FALSE(slub_debug_enabled);
 #endif
 extern void print_tracking(struct kmem_cache *s, void *object);
 long validate_slab_cache(struct kmem_cache *s);
+static inline bool slub_debug_enabled_unlikely(void)
+{
+	return static_branch_unlikely(&slub_debug_enabled);
+}
 #else
 static inline void print_tracking(struct kmem_cache *s, void *object)
 {
 }
+static inline bool slub_debug_enabled_unlikely(void)
+{
+	return false;
+}
 #endif
 
 /*
@@ -229,11 +237,10 @@ static inline void print_tracking(struct kmem_cache *s, void *object)
  */
 static inline bool kmem_cache_debug_flags(struct kmem_cache *s, slab_flags_t flags)
 {
-#ifdef CONFIG_SLUB_DEBUG
-	VM_WARN_ON_ONCE(!(flags & SLAB_DEBUG_FLAGS));
-	if (static_branch_unlikely(&slub_debug_enabled))
+	if (IS_ENABLED(CONFIG_SLUB_DEBUG))
+		VM_WARN_ON_ONCE(!(flags & SLAB_DEBUG_FLAGS));
+	if (slub_debug_enabled_unlikely())
 		return s->flags & flags;
-#endif
 	return false;
 }
 
-- 
2.18.0
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Re: [PATCH v5 1/2] mm: introduce helper to check slub_debug_enabled

[Matthew Wilcox]

On Mon, Jul 05, 2021 at 10:40:57AM +0800, yee.lee@mediatek.com wrote:
> From: Marco Elver <elver@google.com>
> 
> Introduce a helper to check slub_debug_enabled, so that we can confine
> the use of #ifdef to the definition of the slub_debug_enabled_unlikely()
> helper.

We don't usually embed '_unlikely' in function names; we
just do:

static inline bool slub_debug_enabled(void)
{
	return static_branch_unlikely(&slub_debug_enabled);
}

eg:

static inline bool cpusets_enabled(void)
{
        return static_branch_unlikely(&cpusets_enabled_key);
}

#define cgroup_bpf_enabled(type) static_branch_unlikely(&cgroup_bpf_enabled_key[type])

static inline int ip_tunnel_collect_metadata(void)
{
        return static_branch_unlikely(&ip_tunnel_metadata_cnt);
}

static inline bool frontswap_enabled(void)
{
        return static_branch_unlikely(&frontswap_enabled_key);
}


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Re: [PATCH v5 1/2] mm: introduce helper to check slub_debug_enabled

[Marco Elver]

On Mon, Jul 05, 2021 at 04:20AM +0100, Matthew Wilcox wrote:
> On Mon, Jul 05, 2021 at 10:40:57AM +0800, yee.lee@mediatek.com wrote:
> > From: Marco Elver <elver@google.com>
> > 
> > Introduce a helper to check slub_debug_enabled, so that we can confine
> > the use of #ifdef to the definition of the slub_debug_enabled_unlikely()
> > helper.
> 
> We don't usually embed '_unlikely' in function names; we
> just do:
> 
> static inline bool slub_debug_enabled(void)
> {
> 	return static_branch_unlikely(&slub_debug_enabled);
> }

The names are identical, this obviously won't work:

In file included from mm/slub.c:21:
mm/slab.h:219:20: error: ‘slub_debug_enabled’ redeclared as different kind of symbol
  219 | static inline bool slub_debug_enabled(void)

But it seems that someone had the same idea, and this merge window got a
__slub_debug_enabled() with 792702911f58.

Yee, can you replace this patch with the below:

From: Marco Elver <elver@google.com>
Date: Wed, 30 Jun 2021 20:56:57 +0200
Subject: [PATCH] mm: move helper to check slub_debug_enabled

Move the helper to check slub_debug_enabled, so that we can confine
the use of #ifdef outside slub.c as well.

Signed-off-by: Marco Elver <elver@google.com>
---
v6:
* Move helper instead of introducing a new one.
---
 mm/slab.h | 15 +++++++++++----
 mm/slub.c | 14 --------------
 2 files changed, 11 insertions(+), 18 deletions(-)

diff --git a/mm/slab.h b/mm/slab.h
index 67e06637ff2e..f997fd5e42c8 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -216,10 +216,18 @@ DECLARE_STATIC_KEY_FALSE(slub_debug_enabled);
 #endif
 extern void print_tracking(struct kmem_cache *s, void *object);
 long validate_slab_cache(struct kmem_cache *s);
+static inline bool __slub_debug_enabled(void)
+{
+	return static_branch_unlikely(&slub_debug_enabled);
+}
 #else
 static inline void print_tracking(struct kmem_cache *s, void *object)
 {
 }
+static inline bool __slub_debug_enabled(void)
+{
+	return false;
+}
 #endif
 
 /*
@@ -229,11 +237,10 @@ static inline void print_tracking(struct kmem_cache *s, void *object)
  */
 static inline bool kmem_cache_debug_flags(struct kmem_cache *s, slab_flags_t flags)
 {
-#ifdef CONFIG_SLUB_DEBUG
-	VM_WARN_ON_ONCE(!(flags & SLAB_DEBUG_FLAGS));
-	if (static_branch_unlikely(&slub_debug_enabled))
+	if (IS_ENABLED(CONFIG_SLUB_DEBUG))
+		VM_WARN_ON_ONCE(!(flags & SLAB_DEBUG_FLAGS));
+	if (__slub_debug_enabled())
 		return s->flags & flags;
-#endif
 	return false;
 }
 
diff --git a/mm/slub.c b/mm/slub.c
index 2ee43ff667a5..090fa14628f9 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -119,25 +119,11 @@
  */
 
 #ifdef CONFIG_SLUB_DEBUG
-
 #ifdef CONFIG_SLUB_DEBUG_ON
 DEFINE_STATIC_KEY_TRUE(slub_debug_enabled);
 #else
 DEFINE_STATIC_KEY_FALSE(slub_debug_enabled);
 #endif
-
-static inline bool __slub_debug_enabled(void)
-{
-	return static_branch_unlikely(&slub_debug_enabled);
-}
-
-#else		/* CONFIG_SLUB_DEBUG */
-
-static inline bool __slub_debug_enabled(void)
-{
-	return false;
-}
-
 #endif		/* CONFIG_SLUB_DEBUG */
 
 static inline bool kmem_cache_debug(struct kmem_cache *s)
-- 
2.32.0.93.g670b81a890-goog


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[PATCH v5 2/2] kasan: Add memzero int for unaligned size at DEBUG

[yee.lee]

From: Yee Lee <yee.lee@mediatek.com>

Issue: when SLUB debug is on, hwtag kasan_unpoison() would overwrite
the redzone of object with unaligned size.

An additional memzero_explicit() path is added to replacing init by
hwtag instruction for those unaligned size at SLUB debug mode.

The penalty is acceptable since they are only enabled in debug mode,
not production builds. A block of comment is added for explanation.

Signed-off-by: Yee Lee <yee.lee@mediatek.com>
Suggested-by: Marco Elver <elver@google.com>
Suggested-by: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 mm/kasan/kasan.h | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
index 98e3059bfea4..a9d837197302 100644
--- a/mm/kasan/kasan.h
+++ b/mm/kasan/kasan.h
@@ -9,6 +9,7 @@
 #ifdef CONFIG_KASAN_HW_TAGS
 
 #include <linux/static_key.h>
+#include "../slab.h"
 
 DECLARE_STATIC_KEY_FALSE(kasan_flag_stacktrace);
 extern bool kasan_flag_async __ro_after_init;
@@ -387,6 +388,17 @@ static inline void kasan_unpoison(const void *addr, size_t size, bool init)
 
 	if (WARN_ON((unsigned long)addr & KASAN_GRANULE_MASK))
 		return;
+	/*
+	 * Explicitly initialize the memory with the precise object size to
+	 * avoid overwriting the SLAB redzone. This disables initialization in
+	 * the arch code and may thus lead to performance penalty. The penalty
+	 * is accepted since SLAB redzones aren't enabled in production builds.
+	 */
+	if (slub_debug_enabled_unlikely() &&
+	    init && ((unsigned long)size & KASAN_GRANULE_MASK)) {
+		init = false;
+		memzero_explicit((void *)addr, size);
+	}
 	size = round_up(size, KASAN_GRANULE_SIZE);
 
 	hw_set_mem_tag_range((void *)addr, size, tag, init);
-- 
2.18.0
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