Re: [PATCH v3] btrfs: Fix BTRFS arm64 tagged KASAN false-positive

[Qu Wenruo <wqu@suse.com>]

在 2026/3/27 18:54, Daniel J Blueman 写道:
> When booting Linux 7.0-rc5 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 allocation in btrfs_alloc_page_array is from multiple
> discrete pages thus different KASAN tags by design, leading to a tag
> mismatch when linear access is used where the pages are physically
> contiguous.
> 
> Fix this by retagging all the EB pages with the same KASAN tag.
> 
> Cc: stable@vger.kernel.org
> Signed-off-by: Daniel J Blueman <daniel@quora.org>
> Fixes: 397239ed6a6c ("btrfs: allow extent buffer helpers to skip cross-page handling")
> Changelog:
>   v3: Retag only when contiguous; fix build failure when generic KASAN configured
>   v2: https://lore.kernel.org/lkml/20260323061827.22903-1-daniel@quora.org/
>    - Retag pages rather than bypass linear access optimisation
>   v1: https://lore.kernel.org/lkml/20260319053413.14771-1-daniel@quora.org/

This one looks good to me for btrfs.

But still a question related to kasan_unpoison_range().

> ---
>   fs/btrfs/extent_io.c | 17 ++++++++++++++++-
>   1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index 5f97a3d2a8d7..141092da871b 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -3533,8 +3533,23 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
>   	if (uptodate)
>   		set_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags);
>   	/* All pages are physically contiguous, can skip cross page handling. */
> -	if (page_contig)
> +	if (page_contig) {
> +#if defined(CONFIG_KASAN_SW_TAGS) || defined(CONFIG_KASAN_HW_TAGS)
> +		struct page *page = folio_page(eb->folios[0], 0);
> +		u8 tag = page_kasan_tag(page);
> +
> +		/*
> +		 * Since pages are from multiple allocations and physically
> +		 * contiguous allowing linear access, prevent KASAN warnings
> +		 * by retagging with the first tag
> +		 */
> +		for (int i = 1; i < num_extent_pages(eb); i++) {
> +			page_kasan_tag_set(page + i, tag);
> +			kasan_unpoison_range(page_address(page + i), PAGE_SIZE);

There is a LKP report that __kasan_unpoison_range() is not properly 
exported thus if btrfs is compiled as a module, we can not properly link 
the module.

   https://lore.kernel.org/linux-btrfs/202603240559.BNndaqHO-lkp@intel.com/

You may need to export __kasan_unpoison_range() to fix it, or explain 
why kasan_unpoison_range() is needed here.

AFAIK those pages are already unpoisoned at allocation, thus I'm not 
sure why we need to call kasan_unpoison_range() here again.

Thanks,
Qu

> +      }
> +#endif
>   		eb->addr = folio_address(eb->folios[0]) + offset_in_page(eb->start);
> +	}
>   again:
>   	xa_lock_irq(&fs_info->buffer_tree);
>   	existing_eb = __xa_cmpxchg(&fs_info->buffer_tree,