[PATCH v2 0/2] kasan: reset tag when accessing invalid data

[PATCH v2 0/2] kasan: reset tag when accessing invalid data

[Kuan-Ying Lee]

With hardware tag-based kasan enabled, we reset the tag
when we access metadata to avoid from false alarm.

Changes since v2:
 - Refine the commit message in detail.
 - Thanks Andrey's explanation about false alarm
   of kmemleak.

Kuan-Ying Lee (2):
  kasan, kmemleak: reset tags when scanning block
  kasan, slub: reset tag when printing address

 mm/kmemleak.c | 6 +++---
 mm/slub.c     | 4 ++--
 2 files changed, 5 insertions(+), 5 deletions(-)

-- 
2.18.0

[PATCH v2 1/2] kasan, kmemleak: reset tags when scanning block

[Kuan-Ying Lee]

Kmemleak need to scan kernel memory to check memory leak.
With hardware tag-based kasan enabled, when it scans on
the invalid slab and dereference, the issue will occur
as below.

Hardware tag-based KASAN doesn't use compiler instrumentation, we
can not use kasan_disable_current() to ignore tag check.

Based on the below report, there are 11 0xf7 granules, which amounts to
176 bytes, and the object is allocated from the kmalloc-256 cache. So
when kmemleak accesses the last 256-176 bytes, it causes faults, as
those are marked with KASAN_KMALLOC_REDZONE == KASAN_TAG_INVALID ==
0xfe.

Thus, we reset tags before accessing metadata to avoid from false positives.

[  151.905804] ==================================================================
[  151.907120] BUG: KASAN: out-of-bounds in scan_block+0x58/0x170
[  151.908773] Read at addr f7ff0000c0074eb0 by task kmemleak/138
[  151.909656] Pointer tag: [f7], memory tag: [fe]
[  151.910195]
[  151.910876] CPU: 7 PID: 138 Comm: kmemleak Not tainted 5.14.0-rc2-00001-g8cae8cd89f05-dirty #134
[  151.912085] Hardware name: linux,dummy-virt (DT)
[  151.912868] Call trace:
[  151.913211]  dump_backtrace+0x0/0x1b0
[  151.913796]  show_stack+0x1c/0x30
[  151.914248]  dump_stack_lvl+0x68/0x84
[  151.914778]  print_address_description+0x7c/0x2b4
[  151.915340]  kasan_report+0x138/0x38c
[  151.915804]  __do_kernel_fault+0x190/0x1c4
[  151.916386]  do_tag_check_fault+0x78/0x90
[  151.916856]  do_mem_abort+0x44/0xb4
[  151.917308]  el1_abort+0x40/0x60
[  151.917754]  el1h_64_sync_handler+0xb4/0xd0
[  151.918270]  el1h_64_sync+0x78/0x7c
[  151.918714]  scan_block+0x58/0x170
[  151.919157]  scan_gray_list+0xdc/0x1a0
[  151.919626]  kmemleak_scan+0x2ac/0x560
[  151.920129]  kmemleak_scan_thread+0xb0/0xe0
[  151.920635]  kthread+0x154/0x160
[  151.921115]  ret_from_fork+0x10/0x18
[  151.921717]
[  151.922077] Allocated by task 0:
[  151.922523]  kasan_save_stack+0x2c/0x60
[  151.923099]  __kasan_kmalloc+0xec/0x104
[  151.923502]  __kmalloc+0x224/0x3c4
[  151.924172]  __register_sysctl_paths+0x200/0x290
[  151.924709]  register_sysctl_table+0x2c/0x40
[  151.925175]  sysctl_init+0x20/0x34
[  151.925665]  proc_sys_init+0x3c/0x48
[  151.926136]  proc_root_init+0x80/0x9c
[  151.926547]  start_kernel+0x648/0x6a4
[  151.926987]  __primary_switched+0xc0/0xc8
[  151.927557]
[  151.927994] Freed by task 0:
[  151.928340]  kasan_save_stack+0x2c/0x60
[  151.928766]  kasan_set_track+0x2c/0x40
[  151.929173]  kasan_set_free_info+0x44/0x54
[  151.929568]  ____kasan_slab_free.constprop.0+0x150/0x1b0
[  151.930063]  __kasan_slab_free+0x14/0x20
[  151.930449]  slab_free_freelist_hook+0xa4/0x1fc
[  151.930924]  kfree+0x1e8/0x30c
[  151.931285]  put_fs_context+0x124/0x220
[  151.931731]  vfs_kern_mount.part.0+0x60/0xd4
[  151.932280]  kern_mount+0x24/0x4c
[  151.932686]  bdev_cache_init+0x70/0x9c
[  151.933122]  vfs_caches_init+0xdc/0xf4
[  151.933578]  start_kernel+0x638/0x6a4
[  151.934014]  __primary_switched+0xc0/0xc8
[  151.934478]
[  151.934757] The buggy address belongs to the object at ffff0000c0074e00
[  151.934757]  which belongs to the cache kmalloc-256 of size 256
[  151.935744] The buggy address is located 176 bytes inside of
[  151.935744]  256-byte region [ffff0000c0074e00, ffff0000c0074f00)
[  151.936702] The buggy address belongs to the page:
[  151.937378] page:(____ptrval____) refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x100074
[  151.938682] head:(____ptrval____) order:2 compound_mapcount:0 compound_pincount:0
[  151.939440] flags: 0xbfffc0000010200(slab|head|node=0|zone=2|lastcpupid=0xffff|kasantag=0x0)
[  151.940886] raw: 0bfffc0000010200 0000000000000000 dead000000000122 f5ff0000c0002300
[  151.941634] raw: 0000000000000000 0000000000200020 00000001ffffffff 0000000000000000
[  151.942353] page dumped because: kasan: bad access detected
[  151.942923]
[  151.943214] Memory state around the buggy address:
[  151.943896]  ffff0000c0074c00: f0 f0 f0 f0 f0 f0 f0 f0 f0 fe fe fe fe fe fe fe
[  151.944857]  ffff0000c0074d00: fe fe fe fe fe fe fe fe fe fe fe fe fe fe fe fe
[  151.945892] >ffff0000c0074e00: f7 f7 f7 f7 f7 f7 f7 f7 f7 f7 f7 fe fe fe fe fe
[  151.946407]                                                     ^
[  151.946939]  ffff0000c0074f00: fe fe fe fe fe fe fe fe fe fe fe fe fe fe fe fe
[  151.947445]  ffff0000c0075000: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
[  151.947999] ==================================================================
[  151.948524] Disabling lock debugging due to kernel taint
[  156.434569] kmemleak: 181 new suspected memory leaks (see /sys/kernel/debug/kmemleak)

Signed-off-by: Kuan-Ying Lee <Kuan-Ying.Lee@mediatek.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marco Elver <elver@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
---
 mm/kmemleak.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/kmemleak.c b/mm/kmemleak.c
index 228a2fbe0657..73d46d16d575 100644
--- a/mm/kmemleak.c
+++ b/mm/kmemleak.c
@@ -290,7 +290,7 @@ static void hex_dump_object(struct seq_file *seq,
 	warn_or_seq_printf(seq, "  hex dump (first %zu bytes):\n", len);
 	kasan_disable_current();
 	warn_or_seq_hex_dump(seq, DUMP_PREFIX_NONE, HEX_ROW_SIZE,
-			     HEX_GROUP_SIZE, ptr, len, HEX_ASCII);
+			     HEX_GROUP_SIZE, kasan_reset_tag((void *)ptr), len, HEX_ASCII);
 	kasan_enable_current();
 }
 
@@ -1171,7 +1171,7 @@ static bool update_checksum(struct kmemleak_object *object)
 
 	kasan_disable_current();
 	kcsan_disable_current();
-	object->checksum = crc32(0, (void *)object->pointer, object->size);
+	object->checksum = crc32(0, kasan_reset_tag((void *)object->pointer), object->size);
 	kasan_enable_current();
 	kcsan_enable_current();
 
@@ -1246,7 +1246,7 @@ static void scan_block(void *_start, void *_end,
 			break;
 
 		kasan_disable_current();
-		pointer = *ptr;
+		pointer = *(unsigned long *)kasan_reset_tag((void *)ptr);
 		kasan_enable_current();
 
 		untagged_ptr = (unsigned long)kasan_reset_tag((void *)pointer);
-- 
2.18.0

[PATCH v2 2/2] kasan, slub: reset tag when printing address

[Kuan-Ying Lee]

The address still includes the tags when it is printed.
With hardware tag-based kasan enabled, we will get a
false positive KASAN issue when we access metadata.

Reset the tag before we access the metadata.

Fixes: aa1ef4d7b3f6 ("kasan, mm: reset tags when accessing metadata")
Signed-off-by: Kuan-Ying Lee <Kuan-Ying.Lee@mediatek.com>
Suggested-by: Marco Elver <elver@google.com>
---
 mm/slub.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index b6c5205252eb..f77d8cd79ef7 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -576,8 +576,8 @@ static void print_section(char *level, char *text, u8 *addr,
 			  unsigned int length)
 {
 	metadata_access_enable();
-	print_hex_dump(level, kasan_reset_tag(text), DUMP_PREFIX_ADDRESS,
-			16, 1, addr, length, 1);
+	print_hex_dump(level, text, DUMP_PREFIX_ADDRESS,
+			16, 1, kasan_reset_tag((void *)addr), length, 1);
 	metadata_access_disable();
 }
 
-- 
2.18.0

Re: [PATCH v2 2/2] kasan, slub: reset tag when printing address

[Marco Elver]

On Wed, 4 Aug 2021 at 10:23, Kuan-Ying Lee <Kuan-Ying.Lee@mediatek.com> wrote:
>
> The address still includes the tags when it is printed.
> With hardware tag-based kasan enabled, we will get a
> false positive KASAN issue when we access metadata.
>
> Reset the tag before we access the metadata.
>
> Fixes: aa1ef4d7b3f6 ("kasan, mm: reset tags when accessing metadata")
> Signed-off-by: Kuan-Ying Lee <Kuan-Ying.Lee@mediatek.com>
> Suggested-by: Marco Elver <elver@google.com>

Note, in this case Suggested-by is inappropriate, because I did not
suggest the change in any way (you already had it in v1). I just
commented on the fact that it's missing a Fixes so stable can pick it
up and some clarification.

Reviewed-by: Marco Elver <elver@google.com>

> ---
>  mm/slub.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index b6c5205252eb..f77d8cd79ef7 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -576,8 +576,8 @@ static void print_section(char *level, char *text, u8 *addr,
>                           unsigned int length)
>  {
>         metadata_access_enable();
> -       print_hex_dump(level, kasan_reset_tag(text), DUMP_PREFIX_ADDRESS,
> -                       16, 1, addr, length, 1);
> +       print_hex_dump(level, text, DUMP_PREFIX_ADDRESS,
> +                       16, 1, kasan_reset_tag((void *)addr), length, 1);
>         metadata_access_disable();
>  }
>
> --
> 2.18.0

Re: [PATCH v2 2/2] kasan, slub: reset tag when printing address

[kuan.ying lee]

On Wed, 2021-08-04 at 10:41 +0200, Marco Elver wrote:
> On Wed, 4 Aug 2021 at 10:23, Kuan-Ying Lee <
> Kuan-Ying.Lee@mediatek.com> wrote:
> > 
> > The address still includes the tags when it is printed.
> > With hardware tag-based kasan enabled, we will get a
> > false positive KASAN issue when we access metadata.
> > 
> > Reset the tag before we access the metadata.
> > 
> > Fixes: aa1ef4d7b3f6 ("kasan, mm: reset tags when accessing
> > metadata")
> > Signed-off-by: Kuan-Ying Lee <Kuan-Ying.Lee@mediatek.com>
> > Suggested-by: Marco Elver <elver@google.com>
> 
> Note, in this case Suggested-by is inappropriate, because I did not
> suggest the change in any way (you already had it in v1). I just
> commented on the fact that it's missing a Fixes so stable can pick it
> up and some clarification.
> 
> Reviewed-by: Marco Elver <elver@google.com>

Got it.
I will remove it on v3.
Thanks.

> 
> > ---
> >  mm/slub.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/slub.c b/mm/slub.c
> > index b6c5205252eb..f77d8cd79ef7 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -576,8 +576,8 @@ static void print_section(char *level, char
> > *text, u8 *addr,
> >                           unsigned int length)
> >  {
> >         metadata_access_enable();
> > -       print_hex_dump(level, kasan_reset_tag(text),
> > DUMP_PREFIX_ADDRESS,
> > -                       16, 1, addr, length, 1);
> > +       print_hex_dump(level, text, DUMP_PREFIX_ADDRESS,
> > +                       16, 1, kasan_reset_tag((void *)addr),
> > length, 1);
> >         metadata_access_disable();
> >  }
> > 
> > --
> > 2.18.0