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

[Daniel J Blueman <daniel@quora.org>]

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/
---
 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);
+      }
+#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,
-- 
2.53.0