nfsd: fix race between laundromat and free_stateid

There is a race between laundromat handling of revoked delegations
and a client sending free_stateid operation. Laundromat thread
finds that delegation has expired and needs to be revoked so it
marks the delegation stid revoked and it puts it on a reaper list
but then it unlock the state lock and the actual delegation revocation
happens without the lock. Once the stid is marked revoked a racing
free_stateid processing thread does the following (1) it calls
list_del_init() which removes it from the reaper list and (2) frees
the delegation stid structure. The laundromat thread ends up not
calling the revoke_delegation() function for this particular delegation
but that means it will no release the lock lease that exists on
the file.

Now, a new open for this file comes in and ends up finding that
lease list isn't empty and calls nfsd_breaker_owns_lease() which ends
up trying to derefence a freed delegation stateid. Leading to the
followint use-after-free KASAN warning:

kernel: ==================================================================
kernel: BUG: KASAN: slab-use-after-free in nfsd_breaker_owns_lease+0x140/0x160 [nfsd]
kernel: Read of size 8 at addr ffff0000e73cd0c8 by task nfsd/6205
kernel:
kernel: CPU: 2 UID: 0 PID: 6205 Comm: nfsd Kdump: loaded Not tainted 6.11.0-rc7+ #9
kernel: Hardware name: Apple Inc. Apple Virtualization Generic Platform, BIOS 2069.0.0.0.0 08/03/2024
kernel: Call trace:
kernel: dump_backtrace+0x98/0x120
kernel: show_stack+0x1c/0x30
kernel: dump_stack_lvl+0x80/0xe8
kernel: print_address_description.constprop.0+0x84/0x390
kernel: print_report+0xa4/0x268
kernel: kasan_report+0xb4/0xf8
kernel: __asan_report_load8_noabort+0x1c/0x28
kernel: nfsd_breaker_owns_lease+0x140/0x160 [nfsd]
kernel: nfsd_file_do_acquire+0xb3c/0x11d0 [nfsd]
kernel: nfsd_file_acquire_opened+0x84/0x110 [nfsd]
kernel: nfs4_get_vfs_file+0x634/0x958 [nfsd]
kernel: nfsd4_process_open2+0xa40/0x1a40 [nfsd]
kernel: nfsd4_open+0xa08/0xe80 [nfsd]
kernel: nfsd4_proc_compound+0xb8c/0x2130 [nfsd]
kernel: nfsd_dispatch+0x22c/0x718 [nfsd]
kernel: svc_process_common+0x8e8/0x1960 [sunrpc]
kernel: svc_process+0x3d4/0x7e0 [sunrpc]
kernel: svc_handle_xprt+0x828/0xe10 [sunrpc]
kernel: svc_recv+0x2cc/0x6a8 [sunrpc]
kernel: nfsd+0x270/0x400 [nfsd]
kernel: kthread+0x288/0x310
kernel: ret_from_fork+0x10/0x20

This patch proposes a fixed that's based on adding 2 new additional
stid's sc_status values that help coordinate between the laundromat
and other operations (nfsd4_free_stateid() and nfsd4_delegreturn()).

First to make sure, that once the stid is marked revoked, it is not
removed by the nfsd4_free_stateid(), the laundromat take a reference
on the stateid. Then, coordinating whether the stid has been put
on the cl_revoked list or we are processing FREE_STATEID and need to
make sure to remove it from the list, each check that state and act
accordingly. If laundromat has added to the cl_revoke list before
the arrival of FREE_STATEID, then nfsd4_free_stateid() knows to remove
it from the list. If nfsd4_free_stateid() finds that operations arrived
before laundromat has placed it on cl_revoke list, it marks the state
freed and then laundromat will no longer add it to the list.

Also, for nfsd4_delegreturn() when looking for the specified stid,
we need to access stid that are marked removed or freeable, it means
the laundromat has started processing it but hasn't finished and this
delegreturn needs to return nfserr_deleg_revoked and not
nfserr_bad_stateid. The latter will not trigger a FREE_STATEID and the
lack of it will leave this stid on the cl_revoked list indefinitely.

Fixes: 2d4a532 ("nfsd: ensure that clp->cl_revoked list is protected by clp->cl_lock")
CC: [email protected]
Signed-off-by: Olga Kornievskaia <[email protected]>
Signed-off-by: Chuck Lever <[email protected]>

fs/nfsd/nfs4state.c

@@ -1359,21 +1359,47 @@ static void destroy_delegation(struct nfs4_delegation *dp)
 		destroy_unhashed_deleg(dp);
 }
 
+/**
+ * revoke_delegation - perform nfs4 delegation structure cleanup
+ * @dp: pointer to the delegation
+ *
+ * This function assumes that it's called either from the administrative
+ * interface (nfsd4_revoke_states()) that's revoking a specific delegation
+ * stateid or it's called from a laundromat thread (nfsd4_landromat()) that
+ * determined that this specific state has expired and needs to be revoked
+ * (both mark state with the appropriate stid sc_status mode). It is also
+ * assumed that a reference was taken on the @dp state.
+ *
+ * If this function finds that the @dp state is SC_STATUS_FREED it means
+ * that a FREE_STATEID operation for this stateid has been processed and
+ * we can proceed to removing it from recalled list. However, if @dp state
+ * isn't marked SC_STATUS_FREED, it means we need place it on the cl_revoked
+ * list and wait for the FREE_STATEID to arrive from the client. At the same
+ * time, we need to mark it as SC_STATUS_FREEABLE to indicate to the
+ * nfsd4_free_stateid() function that this stateid has already been added
+ * to the cl_revoked list and that nfsd4_free_stateid() is now responsible
+ * for removing it from the list. Inspection of where the delegation state
+ * in the revocation process is protected by the clp->cl_lock.
+ */
 static void revoke_delegation(struct nfs4_delegation *dp)
 {
 	struct nfs4_client *clp = dp->dl_stid.sc_client;
 
 	WARN_ON(!list_empty(&dp->dl_recall_lru));
+	WARN_ON_ONCE(!(dp->dl_stid.sc_status &
+		     (SC_STATUS_REVOKED | SC_STATUS_ADMIN_REVOKED)));
 
 	trace_nfsd_stid_revoke(&dp->dl_stid);
 
-	if (dp->dl_stid.sc_status &
-	    (SC_STATUS_REVOKED | SC_STATUS_ADMIN_REVOKED)) {
-		spin_lock(&clp->cl_lock);
-		refcount_inc(&dp->dl_stid.sc_count);
-		list_add(&dp->dl_recall_lru, &clp->cl_revoked);
-		spin_unlock(&clp->cl_lock);
+	spin_lock(&clp->cl_lock);
+	if (dp->dl_stid.sc_status & SC_STATUS_FREED) {
+		list_del_init(&dp->dl_recall_lru);
+		goto out;
 	}
+	list_add(&dp->dl_recall_lru, &clp->cl_revoked);
+	dp->dl_stid.sc_status |= SC_STATUS_FREEABLE;
+out:
+	spin_unlock(&clp->cl_lock);
 	destroy_unhashed_deleg(dp);
 }
 
@@ -1780,6 +1806,7 @@ void nfsd4_revoke_states(struct net *net, struct super_block *sb)
 					mutex_unlock(&stp->st_mutex);
 					break;
 				case SC_TYPE_DELEG:
+					refcount_inc(&stid->sc_count);
 					dp = delegstateid(stid);
 					spin_lock(&state_lock);
 					if (!unhash_delegation_locked(
@@ -6545,6 +6572,7 @@ nfs4_laundromat(struct nfsd_net *nn)
 		dp = list_entry (pos, struct nfs4_delegation, dl_recall_lru);
 		if (!state_expired(&lt, dp->dl_time))
 			break;
+		refcount_inc(&dp->dl_stid.sc_count);
 		unhash_delegation_locked(dp, SC_STATUS_REVOKED);
 		list_add(&dp->dl_recall_lru, &reaplist);
 	}
@@ -7157,7 +7185,9 @@ nfsd4_free_stateid(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 			s->sc_status |= SC_STATUS_CLOSED;
 			spin_unlock(&s->sc_lock);
 			dp = delegstateid(s);
-			list_del_init(&dp->dl_recall_lru);
+			if (s->sc_status & SC_STATUS_FREEABLE)
+				list_del_init(&dp->dl_recall_lru);
+			s->sc_status |= SC_STATUS_FREED;
 			spin_unlock(&cl->cl_lock);
 			nfs4_put_stid(s);
 			ret = nfs_ok;
@@ -7487,7 +7517,9 @@ nfsd4_delegreturn(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 	if ((status = fh_verify(rqstp, &cstate->current_fh, S_IFREG, 0)))
 		return status;
 
-	status = nfsd4_lookup_stateid(cstate, stateid, SC_TYPE_DELEG, 0, &s, nn);
+	status = nfsd4_lookup_stateid(cstate, stateid, SC_TYPE_DELEG,
+				      SC_STATUS_REVOKED | SC_STATUS_FREEABLE,
+				      &s, nn);
 	if (status)
 		goto out;
 	dp = delegstateid(s);

fs/nfsd/state.h

@@ -114,6 +114,8 @@ struct nfs4_stid {
 /* For a deleg stateid kept around only to process free_stateid's: */
 #define SC_STATUS_REVOKED	BIT(1)
 #define SC_STATUS_ADMIN_REVOKED	BIT(2)
+#define SC_STATUS_FREEABLE	BIT(3)
+#define SC_STATUS_FREED		BIT(4)
 	unsigned short		sc_status;
 
 	struct list_head	sc_cp_list;