[Devel] [PATCH 18/39] mm: slub: introduce metadata_access_enable()/metadata_access_disable()

[Andrey Ryabinin]

From: Andrey Ryabinin <a.ryabinin at samsung.com>

commit a79316c6178ca419e35feef47d47f50b4e0ee9f2 upstream.

It's ok for slub to access memory that marked by kasan as inaccessible
(object's metadata).  Kasan shouldn't print report in that case because
these accesses are valid.  Disabling instrumentation of slub.c code is not
enough to achieve this because slub passes pointer to object's metadata
into external functions like memchr_inv().

We don't want to disable instrumentation for memchr_inv() because this is
quite generic function, and we don't want to miss bugs.

metadata_access_enable/metadata_access_disable used to tell KASan where
accesses to metadata starts/end, so we could temporarily disable KASan
reports.

Signed-off-by: Andrey Ryabinin <a.ryabinin at samsung.com>
Cc: Dmitry Vyukov <dvyukov at google.com>
Cc: Konstantin Serebryany <kcc at google.com>
Cc: Dmitry Chernenkov <dmitryc at google.com>
Signed-off-by: Andrey Konovalov <adech.fo at gmail.com>
Cc: Yuri Gribov <tetra2005 at gmail.com>
Cc: Konstantin Khlebnikov <koct9i at gmail.com>
Cc: Sasha Levin <sasha.levin at oracle.com>
Cc: Christoph Lameter <cl at linux.com>
Cc: Joonsoo Kim <iamjoonsoo.kim at lge.com>
Cc: Dave Hansen <dave.hansen at intel.com>
Cc: Andi Kleen <andi at firstfloor.org>
Cc: Ingo Molnar <mingo at elte.hu>
Cc: Thomas Gleixner <tglx at linutronix.de>
Cc: "H. Peter Anvin" <hpa at zytor.com>
Cc: Christoph Lameter <cl at linux.com>
Cc: Pekka Enberg <penberg at kernel.org>
Cc: David Rientjes <rientjes at google.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Andrey Ryabinin <aryabinin at odin.com>
---
 mm/slub.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/mm/slub.c b/mm/slub.c
index 306cfc4..d775ccb 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -20,6 +20,7 @@
 #include <linux/proc_fs.h>
 #include <linux/notifier.h>
 #include <linux/seq_file.h>
+#include <linux/kasan.h>
 #include <linux/kmemcheck.h>
 #include <linux/cpu.h>
 #include <linux/cpuset.h>
@@ -464,12 +465,30 @@ static char *slub_debug_slabs;
 static int disable_higher_order_debug;
 
 /*
+ * slub is about to manipulate internal object metadata.  This memory lies
+ * outside the range of the allocated object, so accessing it would normally
+ * be reported by kasan as a bounds error.  metadata_access_enable() is used
+ * to tell kasan that these accesses are OK.
+ */
+static inline void metadata_access_enable(void)
+{
+	kasan_disable_current();
+}
+
+static inline void metadata_access_disable(void)
+{
+	kasan_enable_current();
+}
+
+/*
  * Object debugging
  */
 static void print_section(char *text, u8 *addr, unsigned int length)
 {
+	metadata_access_enable();
 	print_hex_dump(KERN_ERR, text, DUMP_PREFIX_ADDRESS, 16, 1, addr,
 			length, 1);
+	metadata_access_disable();
 }
 
 static struct track *get_track(struct kmem_cache *s, void *object,
@@ -499,7 +518,9 @@ static void set_track(struct kmem_cache *s, void *object,
 		trace.max_entries = TRACK_ADDRS_COUNT;
 		trace.entries = p->addrs;
 		trace.skip = 3;
+		metadata_access_enable();
 		save_stack_trace(&trace);
+		metadata_access_disable();
 
 		/* See rant in lockdep.c */
 		if (trace.nr_entries != 0 &&
@@ -672,7 +693,9 @@ static int check_bytes_and_report(struct kmem_cache *s, struct page *page,
 	u8 *fault;
 	u8 *end;
 
+	metadata_access_enable();
 	fault = memchr_inv(start, value, bytes);
+	metadata_access_disable();
 	if (!fault)
 		return 1;
 
@@ -765,7 +788,9 @@ static int slab_pad_check(struct kmem_cache *s, struct page *page)
 	if (!remainder)
 		return 1;
 
+	metadata_access_enable();
 	fault = memchr_inv(end - remainder, POISON_INUSE, remainder);
+	metadata_access_disable();
 	if (!fault)
 		return 1;
 	while (end > fault && end[-1] == POISON_INUSE)
-- 
2.4.6