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CVE-2026-89696

nfsd: block non-SAVEFH ops after FOREIGN PUTFH to prevent NULL deref

Published
Last modified
published
Not listed in CISA KEV
Fix
Fixed in 5.10.270, 5.15.221, 6.1.188, 6.6.157, 6.12.109, and 2 more
None on Exploit-DB, Metasploit or GitHub

Summary

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.

Timeline

  1. CVE published
  2. Record last updated

Affected versions

  • Linux Linux

    From b9e8638e3d9ed8334f1f7071e081860aac37e83e · fixed in ffdc844e673d2bcb2af8c8c3eef8cdea59d0bf32; From b9e8638e3d9ed8334f1f7071e081860aac37e83e · fixed in 00c84f4eec715e501efc080a5670114189e42507; From b9e8638e3d9ed8334f1f7071e081860aac37e83e · fixed in 2d9846fd1c767920fddd4bde30eb35cd2969df13; From b9e8638e3d9ed8334f1f7071e081860aac37e83e · fixed in 977e6f006a7a3ffc4216ae6034f768f8de6fd138; From b9e8638e3d9ed8334f1f7071e081860aac37e83e · fixed in 35f248bd40b47b229d4999581df45b97daadd977; From b9e8638e3d9ed8334f1f7071e081860aac37e83e · fixed in 311f7d926630940650447cbd1c932b076b40a6c4; From b9e8638e3d9ed8334f1f7071e081860aac37e83e · fixed in bf4d338dc8625d70c7f2cb0657d66851a7ac9154; From b9e8638e3d9ed8334f1f7071e081860aac37e83e · fixed in c59738a00aa51b16adc1b5ceb7c80877168efb4d

  • Linux Linux

    5.6

  • KernelLinux

    From 5.6.0 · fixed in 5.10.270; From 5.11.0 · fixed in 5.15.221; From 5.16.0 · fixed in 6.1.188; From 6.2.0 · fixed in 6.6.157; From 6.7.0 · fixed in 6.12.109; From 6.13.0 · fixed in 6.18.50; From 6.19.0 · fixed in 7.2.4

References 8

git.kernel.org