# CVE-2026-99174: ext4: free the sbi after a grace period

CVE-2026-99174 is fixed in linux on 1 platform, first by DEBIAN-CVE-2026-99174, released November 3, 2026. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2026-99174
- Priority: High
- CVSS: 7.1 (Moderate)
- EPSS: 0.0018
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Oct 28, 2026
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-99174

## Description

In the Linux kernel, the following vulnerability has been resolved:  ext4: free the sbi after a grace period  ext4_get_link() follows a symlink that keeps its target in a data block in rcu pathwalk mode with ext4_getblk(EXT4_GET_BLOCKS_CACHED_NOWAIT). That reads s_mount_state in the ASSERT() of ext4_getblk() and again in ext4_es_lookup_extent(), and the lookup bumps the extent status hit or miss counter, all through sb->s_fs_info.  ext4_put_super() destroys those counters, clears sb->s_fs_info and frees the sbi without waiting for a grace period. A pathwalk that is still in ext4_get_link() when the last reference to the mount goes away then dereferences a NULL s_fs_info, the freed sbi or the destroyed percpu counters.  With a pathwalk held up in ext4_get_link() while the last reference goes away:    KASAN: null-ptr-deref in range [0x00000000000000a8-0x00000000000000af]   RIP: 0010:ext4_getblk+0xf0/0x640   Call Trace:    <TASK>    ext4_get_link+0x26c/0x410    pick_link+0xf96/0x1780    step_into_slowpath+0x9b8/0x1250    path_lookupat+0x484/0xb00    filename_lookup+0x1f1/0x5b0    vfs_statx+0xd7/0x3b0    vfs_fstatat+0x4d/0x90    __do_sys_newfstatat+0xa0/0x100    do_syscall_64+0x10e/0x5e0    entry_SYSCALL_64_after_hwframe+0x77/0x7f  So far that has been hard to hit. umount(2) waits for a grace period in namespace_unlock() before it drops the mount, so only a mount whose last reference goes away elsewhere, like the close of a file or a chdir out of a lazily unmounted directory, can race a walker.  Keep sb->s_fs_info and free the sbi together with the extent status counters after a grace period. Split the counters out of ext4_es_unregister_shrinker() for that. The shrinker still gets wasted in ext4_put_super(), the counters go with the sbi.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | linux Debian 13.5 (trixie) | 6.12.115-2 | [DEBIAN-CVE-2026-99174](https://patchblox.com/security/data/fix/linux/debian-cve-2026-99174.md) | Nov 3, 2026 |
| Linux | linux Debian 14 | 7.2.13-1 | [DEBIAN-CVE-2026-99174](https://patchblox.com/security/data/fix/linux/debian-cve-2026-99174.md) | Nov 3, 2026 |

---

Data as of 2026-11-08T06:59:42Z 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

Free to use with attribution: show "Data: Patchblox Patch Radar" with a link to https://patchblox.com/security/patch-radar. Terms, including what needs a written agreement: https://patchblox.com/security/patch-radar/use-the-data

Product names, logos and trademarks shown on Patch Radar belong to their respective owners. They are used only to identify the software each security fix applies to and to help readers find their platform. Their use does not imply any affiliation with, or endorsement by, those owners, and Patchblox is not affiliated with any of them.