Linux Kernel 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.