# CVE-2026-31715: f2fs: fix UAF caused by decrementing sbi->nr_pages[] in f2fs_write_end_io()

CVE-2026-31715 is fixed in linux on 1 platform, first by DEBIAN-CVE-2026-31715, released May 15, 2026. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2026-31715
- Priority: High
- CVSS: 7.8 (Moderate)
- EPSS: 0.0012
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: May 1, 2026
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-31715

## 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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | linux Debian 12.15 (bookworm) | 6.1.176-1 | [DEBIAN-CVE-2026-31715](https://patchblox.com/security/data/fix/linux/debian-cve-2026-31715.md) | May 15, 2026 |
| Linux | linux Debian 13.5 (trixie) | 6.12.86-1 | [DEBIAN-CVE-2026-31715](https://patchblox.com/security/data/fix/linux/debian-cve-2026-31715.md) | May 15, 2026 |
| Linux | linux Debian 14 | 7.0.3-1 | [DEBIAN-CVE-2026-31715](https://patchblox.com/security/data/fix/linux/debian-cve-2026-31715.md) | May 15, 2026 |

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Data as of 2026-11-09T07:25:28Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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