CVE-2026-89686nfsd: fix BUG_ON in nfsd4_alloc_layout_stateid on racing delegation revoke

Official description

In the Linux kernel, the following vulnerability has been resolved: nfsd: fix BUG_ON in nfsd4_alloc_layout_stateid on racing delegation revoke nfsd4_alloc_layout_stateid reads fp->fi_deleg_file without holding fi_lock when the parent stateid is a delegation. A concurrent delegation revoke via the laundromat can clear fi_deleg_file under fi_lock, causing nfsd_file_get() to return NULL and triggering the BUG_ON. This race is client-reachable: two NFS clients can trigger it by having one hold a delegation while another opens the same file to force a recall. When the first client doesn't respond to the recall, the laundromat revokes it. A concurrent LAYOUTGET from any client using the delegation stateid hits the race window. Fix this by taking fi_lock around the fi_deleg_file read in the SC_TYPE_DELEG path, matching the locking discipline of the find_any_file() arm, and replacing the BUG_ON with a graceful error return that cleans up the partially-initialized layout stateid.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/f5011c26246d1abed3dc750c9744e33cb79ff9bc
• https://git.kernel.org/stable/c/d12af460df27876661707efab3a8532146cb961f
• https://git.kernel.org/stable/c/c517f27498757e616d2a8fe6d16caad1fee422e6
• https://git.kernel.org/stable/c/607a56fea772c1f4f4989d8e255dd4f7192d9604
• https://git.kernel.org/stable/c/97bda8b4284d90897a1f1922e5082ff9e35d7c7e
• https://git.kernel.org/stable/c/ca94ba36172046be6a694a7986f6931e47ed4d51