CVE-2026-31715

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

f2fs: fix UAF caused by decrementing sbi->nr_pages[] in f2fs_write_end_io()

The xfstests case "generic/107" and syzbot have both reported a NULL
pointer dereference.

The concurrent scenario that triggers the panic is as follows:

F2FS_WB_CP_DATA write callback          umount
                                        - f2fs_write_checkpoint
                                         - f2fs_wait_on_all_pages(sbi, F2FS_WB_CP_DATA)
- blk_mq_end_request
 - bio_endio
  - f2fs_write_end_io
   : dec_page_count(sbi, F2FS_WB_CP_DATA)
   : wake_up(&sbi->cp_wait)
                                        - kill_f2fs_super
                                         - kill_block_super
                                          - f2fs_put_super
                                           : iput(sbi->node_inode)
                                           : sbi->node_inode = NULL
   : f2fs_in_warm_node_list
    - is_node_folio // sbi->node_inode is NULL and panic

The root cause is that f2fs_put_super() calls iput(sbi->node_inode) and
sets sbi->node_inode to NULL after sbi->nr_pages[F2FS_WB_CP_DATA] is
decremented to zero. As a result, f2fs_in_warm_node_list() may
dereference a NULL node_inode when checking whether a folio belongs to
the node inode, leading to a panic.

This patch fixes the issue by calling f2fs_in_warm_node_list() before
decrementing sbi->nr_pages[F2FS_WB_CP_DATA], thus preventing the
use-after-free condition.