CVE-2026-98080

In the Linux kernel, the following vulnerability has been resolved:

btrfs: do not force reloc root creation during qgroup_account_snapshot()

[BUG]
When running btrfs/252 with quota enabled through MKFS_OPTIONS="-O quota",
it has a high chance to trigger the following kernel warning and flips
the fs RO:

  BTRFS info (device dm-2): relocating block group 30408704 flags metadata|dup
  ------------[ cut here ]------------
  WARNING: fs/btrfs/extent-tree.c:879 at lookup_inline_extent_backref+0x74b/0x960 [btrfs], CPU#4: btrfs/2173
  CPU: 4 UID: 0 PID: 2173 Comm: btrfs Not tainted 7.2.0-rc6-custom+ #457 PREEMPT(full)  3adc6528fb66f7a55fe1095385818e742f200aab
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS unknown 02/02/2022
  RIP: 0010:lookup_inline_extent_backref+0x74b/0x960 [btrfs]
  Call Trace:
   <TASK>
   insert_inline_extent_backref+0x7c/0x160 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
   __btrfs_inc_extent_ref+0xa9/0x270 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
   __btrfs_run_delayed_refs+0x4af/0x11c0 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
   btrfs_run_delayed_refs+0x9d/0xf0 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
   create_pending_snapshot+0x39d/0xf00 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
   create_pending_snapshots+0x9b/0xc0 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
   btrfs_commit_transaction+0x280/0xeb0 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
   prepare_to_relocate+0x147/0x200 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
   relocate_block_group+0x6b/0x5e0 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
   btrfs_relocate_block_group+0x92c/0x2380 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
   btrfs_relocate_chunk+0x3f/0x1a0 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
   btrfs_balance+0xa2c/0x19c0 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
   btrfs_ioctl+0x2839/0x2d30 [btrfs 32f09462c54d9c922fca74a3e4866f4aa7737b72]
   __x64_sys_ioctl+0x416/0x9a0
   do_syscall_64+0xe1/0x790
   entry_SYSCALL_64_after_hwframe+0x4b/0x53
   </TASK>
  ---[ end trace 0000000000000000 ]---
  BTRFS info (device dm-2): leaf 4593991680 gen 233 total ptrs 175 free space 5953 owner 2
  BTRFS info (device dm-2): refs 3 lock_owner 2173 current 2173
  	item 0 key (166772736 METADATA_ITEM 1) itemoff 16250 itemsize 33
  		extent refs 1 gen 222 flags 2
  		ref#0: tree block backref root 266
         [ Skip the tree dump ]
  	item 174 key (263225344 METADATA_ITEM 0) itemoff 10328 itemsize 33
  		extent refs 1 gen 162 flags 258
  		ref#0: tree block backref root 267
  BTRFS error (device dm-2): extent item not found for insert, bytenr 179847168 num_bytes 16384 parent 4594335744 root_objectid 273 owner 0 offset 0
  BTRFS error (device dm-2): failed to run delayed ref for logical 179847168 num_bytes 16384 type 182 action 1 ref_mod 1: -117

[CAUSE]
The above error is showing that there is a tree reference to a metadata
extent that is no longer there.

With "ref_verify" mount option (requires CONFIG_BTRFS_DEBUG), there is
some extra debug output:

  BTRFS error (device dm-2): dumping block entry [180961280 16384], num_refs 0, metadata 1, from disk 0
  BTRFS error (device dm-2):   root entry 256, num_refs 18446744073709551615
  BTRFS error (device dm-2):   root entry 273, num_refs 18446744073709551615
  BTRFS error (device dm-2):   Ref action 3, root 273, ref_root 273, parent 0, owner 0, offset 0, num_refs 1
     btrfs_force_cow_block+0x129/0x7d0 [btrfs]
     btrfs_cow_block+0x10a/0x250 [btrfs]
     btrfs_search_slot+0x5eb/0xf40 [btrfs]
     btrfs_insert_empty_items+0x3a/0x70 [btrfs]
     insert_with_overflow+0x53/0x130 [btrfs]
     btrfs_insert_dir_item+0x125/0x290 [btrfs]
     btrfs_add_link+0xaa/0x410 [btrfs]
     btrfs_rename+0x5ea/0xcd0 [btrfs]
     btrfs_rename2+0x28/0x60 [btrfs]
     vfs_rename+0x5b2/0xe10
     filename_renameat2+0x244/0x430
     __x64_sys_rename+0x48/0x70
     do_syscall_64+0xe1/0x790
     entry_SYSCALL_64_after_hwframe+0x4b/0x53
 
---truncated---