CVE-2025-37871

In the Linux kernel, the following vulnerability has been resolved:

nfsd: decrease sc_count directly if fail to queue dl_recall

A deadlock warning occurred when invoking nfs4_put_stid following a failed
dl_recall queue operation:
            T1                            T2
                                nfs4_laundromat
                                 nfs4_get_client_reaplist
                                  nfs4_anylock_blockers
__break_lease
 spin_lock // ctx->flc_lock
                                   spin_lock // clp->cl_lock
                                   nfs4_lockowner_has_blockers
                                    locks_owner_has_blockers
                                     spin_lock // flctx->flc_lock
 nfsd_break_deleg_cb
  nfsd_break_one_deleg
   nfs4_put_stid
    refcount_dec_and_lock
     spin_lock // clp->cl_lock

When a file is opened, an nfs4_delegation is allocated with sc_count
initialized to 1, and the file_lease holds a reference to the delegation.
The file_lease is then associated with the file through kernel_setlease.

The disassociation is performed in nfsd4_delegreturn via the following
call chain:
nfsd4_delegreturn --> destroy_delegation --> destroy_unhashed_deleg -->
nfs4_unlock_deleg_lease --> kernel_setlease --> generic_delete_lease
The corresponding sc_count reference will be released after this
disassociation.

Since nfsd_break_one_deleg executes while holding the flc_lock, the
disassociation process becomes blocked when attempting to acquire flc_lock
in generic_delete_lease. This means:
1) sc_count in nfsd_break_one_deleg will not be decremented to 0;
2) The nfs4_put_stid called by nfsd_break_one_deleg will not attempt to
acquire cl_lock;
3) Consequently, no deadlock condition is created.

Given that sc_count in nfsd_break_one_deleg remains non-zero, we can
safely perform refcount_dec on sc_count directly. This approach
effectively avoids triggering deadlock warnings.