* [PATCH] btrfs: Fix BTRFS arm64 tagged KASAN false-positive
@ 2026-03-19 5:34 Daniel J Blueman
2026-03-19 8:56 ` Qu Wenruo
0 siblings, 1 reply; 6+ messages in thread
From: Daniel J Blueman @ 2026-03-19 5:34 UTC (permalink / raw)
To: Chris Mason, David Sterba, Qu Wenruo
Cc: Daniel J Blueman, linux-btrfs, linux-kernel
When booting Linux 7.0-rc4 on a Qualcomm Snapdragon X1 with KASAN
software tagging with a BTRFS filesystem, we see:
BUG: KASAN: invalid-access in xxh64_update (lib/xxhash.c:143 lib/xxhash.c:283)
Read of size 8 at addr 7bff000804fe1000 by task kworker/u49:2/138
Pointer tag: [7b], memory tag: [b2]
CPU: 0 UID: 0 PID: 138 Comm: kworker/u49:2 Not tainted 7.0.0-rc4+ #34 PREEMPTLAZY
Hardware name: LENOVO 83ED/LNVNB161216, BIOS NHCN60WW 09/11/2025
Workqueue: btrfs-endio-meta simple_end_io_work
Call trace:
show_stack (arch/arm64/kernel/stacktrace.c:501) (C)
dump_stack_lvl (lib/dump_stack.c:122)
print_report (mm/kasan/report.c:379 mm/kasan/report.c:482)
kasan_report (mm/kasan/report.c:597)
kasan_check_range (mm/kasan/sw_tags.c:86 (discriminator 1))
__hwasan_loadN_noabort (mm/kasan/sw_tags.c:158)
xxh64_update (lib/xxhash.c:143 lib/xxhash.c:283)
btrfs_csum_update (fs/btrfs/fs.c:106)
csum_tree_block (fs/btrfs/disk-io.c:103 (discriminator 3))
btrfs_validate_extent_buffer (fs/btrfs/disk-io.c:389)
end_bbio_meta_read (fs/btrfs/extent_io.c:3853 (discriminator 1))
btrfs_bio_end_io (fs/btrfs/bio.c:152)
simple_end_io_work (fs/btrfs/bio.c:388)
process_one_work (./arch/arm64/include/asm/jump_label.h:36 ./include/trace/events/workqueue.h:110 kernel/workqueue.c:3281)
worker_thread (kernel/workqueue.c:3353 (discriminator 2) kernel/workqueue.c:3440 (discriminator 2))
kthread (kernel/kthread.c:436)
ret_from_fork (arch/arm64/kernel/entry.S:861)
The buggy address belongs to the physical page:
page: refcount:3 mapcount:0 mapping:f1ff00080055dee8 index:0x2467bd pfn:0x884fe1
memcg:51ff000800e68ec0 aops:btree_aops ino:1
flags: 0x9340000000004000(private|zone=2|kasantag=0x4d)
raw: 9340000000004000 0000000000000000 dead000000000122 f1ff00080055dee8
raw: 00000000002467bd 43ff00081d0cc6f0 00000003ffffffff 51ff000800e68ec0
page dumped because: kasan: bad access detected
Memory state around the buggy address:
ffff000804fe0e00: 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b
ffff000804fe0f00: 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b
>ffff000804fe1000: b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2
^
ffff000804fe1100: b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2
ffff000804fe1200: b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2
This occurs as contiguous pages may have different KASAN tags in the upper address
bits, leading to a tag mismatch if linear addressing is used.
Fix this by treating them as discontiguous.
Signed-off-by: Daniel J Blueman <daniel@quora.org>
Fixes: 397239ed6a6c ("btrfs: allow extent buffer helpers to skip cross-page handling")
---
fs/btrfs/extent_io.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 5f97a3d2a8d7..e2b241fb6c0e 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -3517,8 +3517,16 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
* At this stage, either we allocated a large folio, thus @i
* would only be 0, or we fall back to per-page allocation.
*/
- if (i && folio_page(eb->folios[i - 1], 0) + 1 != folio_page(folio, 0))
- page_contig = false;
+ if (i > 0) {
+ struct page *prev = folio_page(eb->folios[i - 1], 0);
+ struct page *curr = folio_page(folio, 0);
+
+ /*
+ * Contiguous pages may have different tags; can't be treated as contiguous
+ */
+ if (curr != prev + 1 || page_kasan_tag(curr) != page_kasan_tag(prev))
+ page_contig = false;
+ }
if (!btrfs_meta_folio_test_uptodate(folio, eb))
uptodate = 0;
--
2.53.0
^ permalink raw reply related [flat|nested] 6+ messages in thread
* Re: [PATCH] btrfs: Fix BTRFS arm64 tagged KASAN false-positive
2026-03-19 5:34 [PATCH] btrfs: Fix BTRFS arm64 tagged KASAN false-positive Daniel J Blueman
@ 2026-03-19 8:56 ` Qu Wenruo
2026-03-20 22:01 ` David Sterba
0 siblings, 1 reply; 6+ messages in thread
From: Qu Wenruo @ 2026-03-19 8:56 UTC (permalink / raw)
To: Daniel J Blueman, Chris Mason, David Sterba; +Cc: linux-btrfs, linux-kernel
在 2026/3/19 16:04, Daniel J Blueman 写道:
> When booting Linux 7.0-rc4 on a Qualcomm Snapdragon X1 with KASAN
> software tagging with a BTRFS filesystem, we see:
>
> BUG: KASAN: invalid-access in xxh64_update (lib/xxhash.c:143 lib/xxhash.c:283)
> Read of size 8 at addr 7bff000804fe1000 by task kworker/u49:2/138
> Pointer tag: [7b], memory tag: [b2]
>
> CPU: 0 UID: 0 PID: 138 Comm: kworker/u49:2 Not tainted 7.0.0-rc4+ #34 PREEMPTLAZY
> Hardware name: LENOVO 83ED/LNVNB161216, BIOS NHCN60WW 09/11/2025
> Workqueue: btrfs-endio-meta simple_end_io_work
> Call trace:
> show_stack (arch/arm64/kernel/stacktrace.c:501) (C)
> dump_stack_lvl (lib/dump_stack.c:122)
> print_report (mm/kasan/report.c:379 mm/kasan/report.c:482)
> kasan_report (mm/kasan/report.c:597)
> kasan_check_range (mm/kasan/sw_tags.c:86 (discriminator 1))
> __hwasan_loadN_noabort (mm/kasan/sw_tags.c:158)
> xxh64_update (lib/xxhash.c:143 lib/xxhash.c:283)
> btrfs_csum_update (fs/btrfs/fs.c:106)
> csum_tree_block (fs/btrfs/disk-io.c:103 (discriminator 3))
> btrfs_validate_extent_buffer (fs/btrfs/disk-io.c:389)
> end_bbio_meta_read (fs/btrfs/extent_io.c:3853 (discriminator 1))
> btrfs_bio_end_io (fs/btrfs/bio.c:152)
> simple_end_io_work (fs/btrfs/bio.c:388)
> process_one_work (./arch/arm64/include/asm/jump_label.h:36 ./include/trace/events/workqueue.h:110 kernel/workqueue.c:3281)
> worker_thread (kernel/workqueue.c:3353 (discriminator 2) kernel/workqueue.c:3440 (discriminator 2))
> kthread (kernel/kthread.c:436)
> ret_from_fork (arch/arm64/kernel/entry.S:861)
>
> The buggy address belongs to the physical page:
> page: refcount:3 mapcount:0 mapping:f1ff00080055dee8 index:0x2467bd pfn:0x884fe1
> memcg:51ff000800e68ec0 aops:btree_aops ino:1
> flags: 0x9340000000004000(private|zone=2|kasantag=0x4d)
> raw: 9340000000004000 0000000000000000 dead000000000122 f1ff00080055dee8
> raw: 00000000002467bd 43ff00081d0cc6f0 00000003ffffffff 51ff000800e68ec0
> page dumped because: kasan: bad access detected
>
> Memory state around the buggy address:
> ffff000804fe0e00: 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b
> ffff000804fe0f00: 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b
>> ffff000804fe1000: b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2
> ^
> ffff000804fe1100: b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2
> ffff000804fe1200: b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2
>
> This occurs as contiguous pages may have different KASAN tags in the upper address
> bits, leading to a tag mismatch if linear addressing is used.
>
> Fix this by treating them as discontiguous.
>
> Signed-off-by: Daniel J Blueman <daniel@quora.org>
> Fixes: 397239ed6a6c ("btrfs: allow extent buffer helpers to skip cross-page handling")
>
> ---
> fs/btrfs/extent_io.c | 12 ++++++++++--
> 1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index 5f97a3d2a8d7..e2b241fb6c0e 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -3517,8 +3517,16 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
> * At this stage, either we allocated a large folio, thus @i
> * would only be 0, or we fall back to per-page allocation.
> */
> - if (i && folio_page(eb->folios[i - 1], 0) + 1 != folio_page(folio, 0))
> - page_contig = false;
> + if (i > 0) {
> + struct page *prev = folio_page(eb->folios[i - 1], 0);
> + struct page *curr = folio_page(folio, 0);
> +
> + /*
> + * Contiguous pages may have different tags; can't be treated as contiguous
> + */
> + if (curr != prev + 1 || page_kasan_tag(curr) != page_kasan_tag(prev))
> + page_contig = false;
I am not a fan of this solution.
Although it doesn't affect end users who don't have KASAN soft tag
enabled, I don't get what we can really get from the different tags.
I mean all those pages are already contig in physical addresses, why we
can not access the range in one go?
Maybe it will be better to set all pages with the same random tag if
page_contig is true?
Thanks,
Qu
> + }
>
> if (!btrfs_meta_folio_test_uptodate(folio, eb))
> uptodate = 0;
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] btrfs: Fix BTRFS arm64 tagged KASAN false-positive
2026-03-19 8:56 ` Qu Wenruo
@ 2026-03-20 22:01 ` David Sterba
2026-03-20 22:26 ` Qu Wenruo
0 siblings, 1 reply; 6+ messages in thread
From: David Sterba @ 2026-03-20 22:01 UTC (permalink / raw)
To: Qu Wenruo
Cc: Daniel J Blueman, Chris Mason, David Sterba, linux-btrfs,
linux-kernel
On Thu, Mar 19, 2026 at 07:26:34PM +1030, Qu Wenruo wrote:
>
>
> 在 2026/3/19 16:04, Daniel J Blueman 写道:
> > When booting Linux 7.0-rc4 on a Qualcomm Snapdragon X1 with KASAN
> > software tagging with a BTRFS filesystem, we see:
> >
> > BUG: KASAN: invalid-access in xxh64_update (lib/xxhash.c:143 lib/xxhash.c:283)
> > Read of size 8 at addr 7bff000804fe1000 by task kworker/u49:2/138
> > Pointer tag: [7b], memory tag: [b2]
> >
> > CPU: 0 UID: 0 PID: 138 Comm: kworker/u49:2 Not tainted 7.0.0-rc4+ #34 PREEMPTLAZY
> > Hardware name: LENOVO 83ED/LNVNB161216, BIOS NHCN60WW 09/11/2025
> > Workqueue: btrfs-endio-meta simple_end_io_work
> > Call trace:
> > show_stack (arch/arm64/kernel/stacktrace.c:501) (C)
> > dump_stack_lvl (lib/dump_stack.c:122)
> > print_report (mm/kasan/report.c:379 mm/kasan/report.c:482)
> > kasan_report (mm/kasan/report.c:597)
> > kasan_check_range (mm/kasan/sw_tags.c:86 (discriminator 1))
> > __hwasan_loadN_noabort (mm/kasan/sw_tags.c:158)
> > xxh64_update (lib/xxhash.c:143 lib/xxhash.c:283)
> > btrfs_csum_update (fs/btrfs/fs.c:106)
> > csum_tree_block (fs/btrfs/disk-io.c:103 (discriminator 3))
> > btrfs_validate_extent_buffer (fs/btrfs/disk-io.c:389)
> > end_bbio_meta_read (fs/btrfs/extent_io.c:3853 (discriminator 1))
> > btrfs_bio_end_io (fs/btrfs/bio.c:152)
> > simple_end_io_work (fs/btrfs/bio.c:388)
> > process_one_work (./arch/arm64/include/asm/jump_label.h:36 ./include/trace/events/workqueue.h:110 kernel/workqueue.c:3281)
> > worker_thread (kernel/workqueue.c:3353 (discriminator 2) kernel/workqueue.c:3440 (discriminator 2))
> > kthread (kernel/kthread.c:436)
> > ret_from_fork (arch/arm64/kernel/entry.S:861)
> >
> > The buggy address belongs to the physical page:
> > page: refcount:3 mapcount:0 mapping:f1ff00080055dee8 index:0x2467bd pfn:0x884fe1
> > memcg:51ff000800e68ec0 aops:btree_aops ino:1
> > flags: 0x9340000000004000(private|zone=2|kasantag=0x4d)
> > raw: 9340000000004000 0000000000000000 dead000000000122 f1ff00080055dee8
> > raw: 00000000002467bd 43ff00081d0cc6f0 00000003ffffffff 51ff000800e68ec0
> > page dumped because: kasan: bad access detected
> >
> > Memory state around the buggy address:
> > ffff000804fe0e00: 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b
> > ffff000804fe0f00: 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b
> >> ffff000804fe1000: b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2
> > ^
> > ffff000804fe1100: b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2
> > ffff000804fe1200: b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2
> >
> > This occurs as contiguous pages may have different KASAN tags in the upper address
> > bits, leading to a tag mismatch if linear addressing is used.
> >
> > Fix this by treating them as discontiguous.
> >
> > Signed-off-by: Daniel J Blueman <daniel@quora.org>
> > Fixes: 397239ed6a6c ("btrfs: allow extent buffer helpers to skip cross-page handling")
> >
> > ---
> > fs/btrfs/extent_io.c | 12 ++++++++++--
> > 1 file changed, 10 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> > index 5f97a3d2a8d7..e2b241fb6c0e 100644
> > --- a/fs/btrfs/extent_io.c
> > +++ b/fs/btrfs/extent_io.c
> > @@ -3517,8 +3517,16 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
> > * At this stage, either we allocated a large folio, thus @i
> > * would only be 0, or we fall back to per-page allocation.
> > */
> > - if (i && folio_page(eb->folios[i - 1], 0) + 1 != folio_page(folio, 0))
> > - page_contig = false;
> > + if (i > 0) {
> > + struct page *prev = folio_page(eb->folios[i - 1], 0);
> > + struct page *curr = folio_page(folio, 0);
> > +
> > + /*
> > + * Contiguous pages may have different tags; can't be treated as contiguous
> > + */
> > + if (curr != prev + 1 || page_kasan_tag(curr) != page_kasan_tag(prev))
> > + page_contig = false;
>
> I am not a fan of this solution.
>
> Although it doesn't affect end users who don't have KASAN soft tag
> enabled, I don't get what we can really get from the different tags.
>
> I mean all those pages are already contig in physical addresses, why we
> can not access the range in one go?
>
> Maybe it will be better to set all pages with the same random tag if
> page_contig is true?
I don't know if there's an interface how to change the tags but adding
one condition that enables a sanitizer to work on some platform does not
sound like a terrible thing. The contiguous pages on our side is an
optimization so it's a special case, I'd rather adapt to the sanitizers
than to let people ignore a warning or have to read a warning that that
one is harmless.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] btrfs: Fix BTRFS arm64 tagged KASAN false-positive
2026-03-20 22:01 ` David Sterba
@ 2026-03-20 22:26 ` Qu Wenruo
2026-03-21 2:39 ` Daniel J Blueman
0 siblings, 1 reply; 6+ messages in thread
From: Qu Wenruo @ 2026-03-20 22:26 UTC (permalink / raw)
To: dsterba
Cc: Daniel J Blueman, Chris Mason, David Sterba, linux-btrfs,
linux-kernel, kasan-dev
在 2026/3/21 08:31, David Sterba 写道:
> On Thu, Mar 19, 2026 at 07:26:34PM +1030, Qu Wenruo wrote:
>>
>>
>> 在 2026/3/19 16:04, Daniel J Blueman 写道:
>>> When booting Linux 7.0-rc4 on a Qualcomm Snapdragon X1 with KASAN
>>> software tagging with a BTRFS filesystem, we see:
>>>
>>> BUG: KASAN: invalid-access in xxh64_update (lib/xxhash.c:143 lib/xxhash.c:283)
>>> Read of size 8 at addr 7bff000804fe1000 by task kworker/u49:2/138
>>> Pointer tag: [7b], memory tag: [b2]
>>>
>>> CPU: 0 UID: 0 PID: 138 Comm: kworker/u49:2 Not tainted 7.0.0-rc4+ #34 PREEMPTLAZY
>>> Hardware name: LENOVO 83ED/LNVNB161216, BIOS NHCN60WW 09/11/2025
>>> Workqueue: btrfs-endio-meta simple_end_io_work
>>> Call trace:
>>> show_stack (arch/arm64/kernel/stacktrace.c:501) (C)
>>> dump_stack_lvl (lib/dump_stack.c:122)
>>> print_report (mm/kasan/report.c:379 mm/kasan/report.c:482)
>>> kasan_report (mm/kasan/report.c:597)
>>> kasan_check_range (mm/kasan/sw_tags.c:86 (discriminator 1))
>>> __hwasan_loadN_noabort (mm/kasan/sw_tags.c:158)
>>> xxh64_update (lib/xxhash.c:143 lib/xxhash.c:283)
>>> btrfs_csum_update (fs/btrfs/fs.c:106)
>>> csum_tree_block (fs/btrfs/disk-io.c:103 (discriminator 3))
>>> btrfs_validate_extent_buffer (fs/btrfs/disk-io.c:389)
>>> end_bbio_meta_read (fs/btrfs/extent_io.c:3853 (discriminator 1))
>>> btrfs_bio_end_io (fs/btrfs/bio.c:152)
>>> simple_end_io_work (fs/btrfs/bio.c:388)
>>> process_one_work (./arch/arm64/include/asm/jump_label.h:36 ./include/trace/events/workqueue.h:110 kernel/workqueue.c:3281)
>>> worker_thread (kernel/workqueue.c:3353 (discriminator 2) kernel/workqueue.c:3440 (discriminator 2))
>>> kthread (kernel/kthread.c:436)
>>> ret_from_fork (arch/arm64/kernel/entry.S:861)
>>>
>>> The buggy address belongs to the physical page:
>>> page: refcount:3 mapcount:0 mapping:f1ff00080055dee8 index:0x2467bd pfn:0x884fe1
>>> memcg:51ff000800e68ec0 aops:btree_aops ino:1
>>> flags: 0x9340000000004000(private|zone=2|kasantag=0x4d)
>>> raw: 9340000000004000 0000000000000000 dead000000000122 f1ff00080055dee8
>>> raw: 00000000002467bd 43ff00081d0cc6f0 00000003ffffffff 51ff000800e68ec0
>>> page dumped because: kasan: bad access detected
>>>
>>> Memory state around the buggy address:
>>> ffff000804fe0e00: 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b
>>> ffff000804fe0f00: 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b
>>>> ffff000804fe1000: b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2
>>> ^
>>> ffff000804fe1100: b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2
>>> ffff000804fe1200: b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2
>>>
>>> This occurs as contiguous pages may have different KASAN tags in the upper address
>>> bits, leading to a tag mismatch if linear addressing is used.
>>>
>>> Fix this by treating them as discontiguous.
>>>
>>> Signed-off-by: Daniel J Blueman <daniel@quora.org>
>>> Fixes: 397239ed6a6c ("btrfs: allow extent buffer helpers to skip cross-page handling")
>>>
>>> ---
>>> fs/btrfs/extent_io.c | 12 ++++++++++--
>>> 1 file changed, 10 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
>>> index 5f97a3d2a8d7..e2b241fb6c0e 100644
>>> --- a/fs/btrfs/extent_io.c
>>> +++ b/fs/btrfs/extent_io.c
>>> @@ -3517,8 +3517,16 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
>>> * At this stage, either we allocated a large folio, thus @i
>>> * would only be 0, or we fall back to per-page allocation.
>>> */
>>> - if (i && folio_page(eb->folios[i - 1], 0) + 1 != folio_page(folio, 0))
>>> - page_contig = false;
>>> + if (i > 0) {
>>> + struct page *prev = folio_page(eb->folios[i - 1], 0);
>>> + struct page *curr = folio_page(folio, 0);
>>> +
>>> + /*
>>> + * Contiguous pages may have different tags; can't be treated as contiguous
>>> + */
>>> + if (curr != prev + 1 || page_kasan_tag(curr) != page_kasan_tag(prev))
>>> + page_contig = false;
>>
>> I am not a fan of this solution.
>>
>> Although it doesn't affect end users who don't have KASAN soft tag
>> enabled, I don't get what we can really get from the different tags.
>>
>> I mean all those pages are already contig in physical addresses, why we
>> can not access the range in one go?
>>
>> Maybe it will be better to set all pages with the same random tag if
>> page_contig is true?
>
> I don't know if there's an interface how to change the tags but adding
> one condition that enables a sanitizer to work on some platform does not
> sound like a terrible thing. The contiguous pages on our side is an
> optimization so it's a special case, I'd rather adapt to the sanitizers
> than to let people ignore a warning or have to read a warning that that
> one is harmless.
There is the interface, page_kasan_tag_set()/page_kasan_tag_reset(), and
is already utilized inside MM.
And the deeper problem is, if this is a false alert, shouldn't we fix
the sanitizer?
Especially in this case I didn't see any problem accessing properly
allocated and physically adjacent pages.
If this is really a problem, I think a lot of bio accesses are also
going to cause problems, as one bvec can have multiple physically
adjacent pages, and if they have different tags then drivers copying a
large bvec should lead to the same tag difference.
So to the reporter/KASAN people, what's the problem of accessing
different tags in the first place?
Thanks,
Qu
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] btrfs: Fix BTRFS arm64 tagged KASAN false-positive
2026-03-20 22:26 ` Qu Wenruo
@ 2026-03-21 2:39 ` Daniel J Blueman
2026-03-21 3:50 ` Qu Wenruo
0 siblings, 1 reply; 6+ messages in thread
From: Daniel J Blueman @ 2026-03-21 2:39 UTC (permalink / raw)
To: Qu Wenruo, David Sterba, Chris Mason, dsterba
Cc: linux-btrfs, linux-kernel, kasan-dev
On Sat, 21 Mar 2026 at 06:27, Qu Wenruo <wqu@suse.com> wrote:
> 在 2026/3/21 08:31, David Sterba 写道:
> > On Thu, Mar 19, 2026 at 07:26:34PM +1030, Qu Wenruo wrote:
> >>
> >>
> >> 在 2026/3/19 16:04, Daniel J Blueman 写道:
> >>> When booting Linux 7.0-rc4 on a Qualcomm Snapdragon X1 with KASAN
> >>> software tagging with a BTRFS filesystem, we see:
> >>>
> >>> BUG: KASAN: invalid-access in xxh64_update (lib/xxhash.c:143 lib/xxhash.c:283)
> >>> Read of size 8 at addr 7bff000804fe1000 by task kworker/u49:2/138
> >>> Pointer tag: [7b], memory tag: [b2]
> >>>
> >>> CPU: 0 UID: 0 PID: 138 Comm: kworker/u49:2 Not tainted 7.0.0-rc4+ #34 PREEMPTLAZY
> >>> Hardware name: LENOVO 83ED/LNVNB161216, BIOS NHCN60WW 09/11/2025
> >>> Workqueue: btrfs-endio-meta simple_end_io_work
> >>> Call trace:
> >>> show_stack (arch/arm64/kernel/stacktrace.c:501) (C)
> >>> dump_stack_lvl (lib/dump_stack.c:122)
> >>> print_report (mm/kasan/report.c:379 mm/kasan/report.c:482)
> >>> kasan_report (mm/kasan/report.c:597)
> >>> kasan_check_range (mm/kasan/sw_tags.c:86 (discriminator 1))
> >>> __hwasan_loadN_noabort (mm/kasan/sw_tags.c:158)
> >>> xxh64_update (lib/xxhash.c:143 lib/xxhash.c:283)
> >>> btrfs_csum_update (fs/btrfs/fs.c:106)
> >>> csum_tree_block (fs/btrfs/disk-io.c:103 (discriminator 3))
> >>> btrfs_validate_extent_buffer (fs/btrfs/disk-io.c:389)
> >>> end_bbio_meta_read (fs/btrfs/extent_io.c:3853 (discriminator 1))
> >>> btrfs_bio_end_io (fs/btrfs/bio.c:152)
> >>> simple_end_io_work (fs/btrfs/bio.c:388)
> >>> process_one_work (./arch/arm64/include/asm/jump_label.h:36 ./include/trace/events/workqueue.h:110 kernel/workqueue.c:3281)
> >>> worker_thread (kernel/workqueue.c:3353 (discriminator 2) kernel/workqueue.c:3440 (discriminator 2))
> >>> kthread (kernel/kthread.c:436)
> >>> ret_from_fork (arch/arm64/kernel/entry.S:861)
> >>>
> >>> The buggy address belongs to the physical page:
> >>> page: refcount:3 mapcount:0 mapping:f1ff00080055dee8 index:0x2467bd pfn:0x884fe1
> >>> memcg:51ff000800e68ec0 aops:btree_aops ino:1
> >>> flags: 0x9340000000004000(private|zone=2|kasantag=0x4d)
> >>> raw: 9340000000004000 0000000000000000 dead000000000122 f1ff00080055dee8
> >>> raw: 00000000002467bd 43ff00081d0cc6f0 00000003ffffffff 51ff000800e68ec0
> >>> page dumped because: kasan: bad access detected
> >>>
> >>> Memory state around the buggy address:
> >>> ffff000804fe0e00: 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b
> >>> ffff000804fe0f00: 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b 7b
> >>>> ffff000804fe1000: b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2
> >>> ^
> >>> ffff000804fe1100: b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2
> >>> ffff000804fe1200: b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2 b2
> >>>
> >>> This occurs as contiguous pages may have different KASAN tags in the upper address
> >>> bits, leading to a tag mismatch if linear addressing is used.
> >>>
> >>> Fix this by treating them as discontiguous.
> >>>
> >>> Signed-off-by: Daniel J Blueman <daniel@quora.org>
> >>> Fixes: 397239ed6a6c ("btrfs: allow extent buffer helpers to skip cross-page handling")
> >>>
> >>> ---
> >>> fs/btrfs/extent_io.c | 12 ++++++++++--
> >>> 1 file changed, 10 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> >>> index 5f97a3d2a8d7..e2b241fb6c0e 100644
> >>> --- a/fs/btrfs/extent_io.c
> >>> +++ b/fs/btrfs/extent_io.c
> >>> @@ -3517,8 +3517,16 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
> >>> * At this stage, either we allocated a large folio, thus @i
> >>> * would only be 0, or we fall back to per-page allocation.
> >>> */
> >>> - if (i && folio_page(eb->folios[i - 1], 0) + 1 != folio_page(folio, 0))
> >>> - page_contig = false;
> >>> + if (i > 0) {
> >>> + struct page *prev = folio_page(eb->folios[i - 1], 0);
> >>> + struct page *curr = folio_page(folio, 0);
> >>> +
> >>> + /*
> >>> + * Contiguous pages may have different tags; can't be treated as contiguous
> >>> + */
> >>> + if (curr != prev + 1 || page_kasan_tag(curr) != page_kasan_tag(prev))
> >>> + page_contig = false;
> >>
> >> I am not a fan of this solution.
> >>
> >> Although it doesn't affect end users who don't have KASAN soft tag
> >> enabled, I don't get what we can really get from the different tags.
> >>
> >> I mean all those pages are already contig in physical addresses, why we
> >> can not access the range in one go?
> >>
> >> Maybe it will be better to set all pages with the same random tag if
> >> page_contig is true?
> >
> > I don't know if there's an interface how to change the tags but adding
> > one condition that enables a sanitizer to work on some platform does not
> > sound like a terrible thing. The contiguous pages on our side is an
> > optimization so it's a special case, I'd rather adapt to the sanitizers
> > than to let people ignore a warning or have to read a warning that that
> > one is harmless.
>
> There is the interface, page_kasan_tag_set()/page_kasan_tag_reset(), and
> is already utilized inside MM.
>
> And the deeper problem is, if this is a false alert, shouldn't we fix
> the sanitizer?
>
> Especially in this case I didn't see any problem accessing properly
> allocated and physically adjacent pages.
I agree with this, however BTRFS doesn't have multi-page folios, thus
these pages are from different allocations; from extent_io.c:
/* For now, we should only have single-page folios for btree inode. */
ASSERT(folio_nr_pages(existing_folio) == 1);
The goal of the differing KASAN tags per allocation is to catch UAF
and other lifecycle issues. If preferred, I can test and submit a
patch to set the same KASAN tag in alloc_eb_folio_array()? This would
be removed when multi-page folio support is added and is along the
lines of:
for (int i = 1; i < num_pages; i++)
page_kasan_tag_set(page_array[i], page_kasan_tag(page_array[0]));
It will be less minimal as won't implicitly be compiled out as per my
previous patch.
Thanks,
Dan
--
Daniel J Blueman
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] btrfs: Fix BTRFS arm64 tagged KASAN false-positive
2026-03-21 2:39 ` Daniel J Blueman
@ 2026-03-21 3:50 ` Qu Wenruo
0 siblings, 0 replies; 6+ messages in thread
From: Qu Wenruo @ 2026-03-21 3:50 UTC (permalink / raw)
To: Daniel J Blueman, Qu Wenruo, David Sterba, Chris Mason, dsterba
Cc: linux-btrfs, linux-kernel, kasan-dev
在 2026/3/21 13:09, Daniel J Blueman 写道:
> On Sat, 21 Mar 2026 at 06:27, Qu Wenruo <wqu@suse.com> wrote:
>> 在 2026/3/21 08:31, David Sterba 写道:
>>> On Thu, Mar 19, 2026 at 07:26:34PM +1030, Qu Wenruo wrote:
[...]
>>
>> There is the interface, page_kasan_tag_set()/page_kasan_tag_reset(), and
>> is already utilized inside MM.
>>
>> And the deeper problem is, if this is a false alert, shouldn't we fix
>> the sanitizer?
>>
>> Especially in this case I didn't see any problem accessing properly
>> allocated and physically adjacent pages.
>
> I agree with this, however BTRFS doesn't have multi-page folios, thus
> these pages are from different allocations; from extent_io.c:
>
> /* For now, we should only have single-page folios for btree inode. */
> ASSERT(folio_nr_pages(existing_folio) == 1);
>
> The goal of the differing KASAN tags per allocation is to catch UAF
> and other lifecycle issues. If preferred, I can test and submit a
> patch to set the same KASAN tag in alloc_eb_folio_array()? This would
> be removed when multi-page folio support is added and is along the
> lines of:
>
> for (int i = 1; i < num_pages; i++)
> page_kasan_tag_set(page_array[i], page_kasan_tag(page_array[0]));
That's exactly what I expect.
>
> It will be less minimal as won't implicitly be compiled out as per my
> previous patch.
To be honest, I think the above change is the minimal fix, as it doesn't
inject kasan code into the main loop of folio allocation.
And it will be compiled out, as page_kasan_tag_set() is no-op if neither
software nor hardware tag is selected, and I believe the compiler is
definitely clever enough to remove a loop with no-op in it.
Thanks,
Qu
>
> Thanks,
> Dan
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2026-03-21 3:50 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2026-03-19 5:34 [PATCH] btrfs: Fix BTRFS arm64 tagged KASAN false-positive Daniel J Blueman
2026-03-19 8:56 ` Qu Wenruo
2026-03-20 22:01 ` David Sterba
2026-03-20 22:26 ` Qu Wenruo
2026-03-21 2:39 ` Daniel J Blueman
2026-03-21 3:50 ` Qu Wenruo
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