# CVE-2026-31715: f2fs: fix UAF caused by decrementing sbi-\>nr\_pages\[\] in f2fs\_write\_end\_io()

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Published: 2026-05-01 · Updated: 2026-06-19

## Affected

- Linux: version range not reported
- Linux: 4.19

## Description

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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, no fixed version is listed yet; follow the vendor advisory per the references below.
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## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/7dbdab4430e4654db9aacef12b9b3b8b29ca25cb
- https://git.kernel.org/stable/c/ffb94770dbdfb5411be5d9f44a960b010ec890ad
- https://git.kernel.org/stable/c/0d40b26377f891e6dcb6efaf8ef9374c99be1b1d
- https://git.kernel.org/stable/c/1171f329cf1c175321251ac40fd126150d7ad1e8
- https://git.kernel.org/stable/c/7be222de96c0f9eee6e65eeb017ef855ee185cfa
- https://git.kernel.org/stable/c/963d2e24d9d92a31e6773b0f642214f10013ebf7
- https://git.kernel.org/stable/c/188bb65f247a7a7c62f287c9a263aee3cad96fa5
- https://git.kernel.org/stable/c/2d9c4a4ed4eef1f82c5b16b037aee8bad819fd53

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