5.10-stable review patch. If anyone has any objections, please let me know. ------------------ From: Michael Bommarito commit 036c1b182f4da65363e79ec0ac276edc6b7296e5 upstream. nfsd4_ssc_expire_umount() walks nn->nfsd_ssc_mount_list with list_for_each_entry_safe(ni, tmp, ...). For each expired entry it sets nsui_busy = true, drops nfsd_ssc_lock to run mntput() on the source vfsmount, then reacquires the lock to list_del + kfree the entry and continue iterating via the macro's saved tmp pointer. The nsui_busy flag protects the current ni from concurrent nfsd4_ssc_setup_dul() finders during the lock-drop window, but it does not pin tmp. Another nfsd RPC thread that fails its source- server mount and reaches nfsd4_ssc_cancel_dul() will, during that same window, take nfsd_ssc_lock, list_del + kfree its own ssc_umount item, and release the lock. If that item is the saved tmp of the expire walk, the next iteration dereferences a freed nfsd4_ssc_umount_item. Restart the walk from the head after the mntput() unlock window so no saved next pointer survives the lock-drop. The list is bounded by the number of active inter-server source mounts (typically small) and the expire delayed-work runs periodically rather than per-IO, so the restart is cheap. Fixes: f4e44b393389 ("NFSD: delay unmount source's export after inter-server copy completed.") Cc: stable@vger.kernel.org Assisted-by: Claude:claude-opus-4-7 Signed-off-by: Michael Bommarito Link: https://patch.msgid.link/20260524130654.1924556-1-michael.bommarito@gmail.com Signed-off-by: Chuck Lever Signed-off-by: Greg Kroah-Hartman --- fs/nfsd/nfs4state.c | 44 +++++++++++++++++++++++++------------------- 1 file changed, 25 insertions(+), 19 deletions(-) --- a/fs/nfsd/nfs4state.c +++ b/fs/nfsd/nfs4state.c @@ -6014,30 +6014,36 @@ static void nfsd4_ssc_shutdown_umount(st static void nfsd4_ssc_expire_umount(struct nfsd_net *nn) { bool do_wakeup = false; - struct nfsd4_ssc_umount_item *ni = NULL; - struct nfsd4_ssc_umount_item *tmp; + struct nfsd4_ssc_umount_item *ni; +restart: spin_lock(&nn->nfsd_ssc_lock); - list_for_each_entry_safe(ni, tmp, &nn->nfsd_ssc_mount_list, nsui_list) { - if (time_after(jiffies, ni->nsui_expire)) { - if (refcount_read(&ni->nsui_refcnt) > 1) - continue; + list_for_each_entry(ni, &nn->nfsd_ssc_mount_list, nsui_list) { + if (!time_after(jiffies, ni->nsui_expire)) + break; + if (refcount_read(&ni->nsui_refcnt) > 1) + continue; - /* mark being unmount */ - ni->nsui_busy = true; - spin_unlock(&nn->nfsd_ssc_lock); - mntput(ni->nsui_vfsmount); - spin_lock(&nn->nfsd_ssc_lock); + /* Prevent concurrent setup during unmount */ + ni->nsui_busy = true; + spin_unlock(&nn->nfsd_ssc_lock); + mntput(ni->nsui_vfsmount); + spin_lock(&nn->nfsd_ssc_lock); - /* waiters need to start from begin of list */ - list_del(&ni->nsui_list); - kfree(ni); + /* Force concurrent scanners to restart */ + list_del(&ni->nsui_list); + kfree(ni); - /* wakeup ssc_connect waiters */ - do_wakeup = true; - continue; - } - break; + /* wakeup ssc_connect waiters */ + do_wakeup = true; + /* + * Concurrent nfsd4_ssc_cancel_dul() can free any item + * on the list under nfsd_ssc_lock while mntput() runs + * above. Restart from the head; the list is short and + * the expire worker is periodic, so this is cheap. + */ + spin_unlock(&nn->nfsd_ssc_lock); + goto restart; } if (do_wakeup) wake_up_all(&nn->nfsd_ssc_waitq);