CVE-2026-89696
Published: September 15, 2026Last modified: September 28, 2026
Description
In the Linux kernel, the following vulnerability has been resolved: nfsd: block non-SAVEFH ops after FOREIGN PUTFH to prevent NULL deref When CONFIG_NFSD_V4_2_INTER_SSC is enabled, nfsd4_putfh() can return success with fh_dentry and fh_export both NULL if fh_verify() returns nfserr_stale and putfh->no_verify is true. The NFSD4_FH_FOREIGN flag is set, but the compound dispatch loop only uses this flag to bypass the nfserr_nofilehandle check -- it does not prevent subsequent ops from running with a NULL fh_dentry. A remote client can exploit this by crafting a COMPOUND that includes an inter-SSC COPY (which causes check_if_stalefh_allowed() to set no_verify=true on the saved PUTFH) with an additional op inserted between the source PUTFH and SAVEFH. For example, SETATTR calls fh_want_write() which dereferences fh_export->ex_path.mnt without calling fh_verify() first, causing a NULL pointer dereference in the nfsd kthread. Fix this by gating the dispatch loop: when NFSD4_FH_FOREIGN is set and fh_dentry is NULL, only OP_SAVEFH (needed for the inter-SSC flow) and ops with ALLOWED_WITHOUT_FH (which don't need a resolved filehandle) may proceed. All other ops receive nfserr_stale, per RFC 7862 Section 15.2.3 which specifies that foreign filehandle validation is deferred to the consuming operation and NFS4ERR_STALE returned at that point.
Severity score breakdown
| Parameter | Value |
|---|---|
| Base score | 7.5 |
| Attack Vector | NETWORK |
| Attack complexity | LOW |
| Privileges required | NONE |
| User interaction | NONE |
| Scope | UNCHANGED |
| Confidentiality | NONE |
| Integrity impact | NONE |
| Availability impact | HIGH |
| Vector | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H |
Status
| Product | Release | Package | Status |
|---|---|---|---|
| Alpaquita Linux | 23 LTS | linux-lts | Vulnerable (6.1.182-r0) |
| 25 LTS | linux-lts | Fixed (6.12.111-r0) | |
| Stream | linux-lts | Fixed (6.18.53-r0) |
References
- https://git.kernel.org/stable/c/00c84f4eec715e501efc080a5670114189e42507
- https://git.kernel.org/stable/c/2d9846fd1c767920fddd4bde30eb35cd2969df13
- https://git.kernel.org/stable/c/311f7d926630940650447cbd1c932b076b40a6c4
- https://git.kernel.org/stable/c/35f248bd40b47b229d4999581df45b97daadd977
- https://git.kernel.org/stable/c/977e6f006a7a3ffc4216ae6034f768f8de6fd138
- https://git.kernel.org/stable/c/bf4d338dc8625d70c7f2cb0657d66851a7ac9154
- https://git.kernel.org/stable/c/c59738a00aa51b16adc1b5ceb7c80877168efb4d
- https://git.kernel.org/stable/c/ffdc844e673d2bcb2af8c8c3eef8cdea59d0bf32