nfsd: fix UAF when access ex_uuid or ex_stats

We can access exp->ex_stats or exp->ex_uuid in rcu context(c_show and
e_show). All these resources should be released using kfree_rcu. Fix this
by using call_rcu, clean them all after a rcu grace period.

==================================================================
BUG: KASAN: slab-use-after-free in svc_export_show+0x362/0x430 [nfsd]
Read of size 1 at addr ff11000010fdc120 by task cat/870

CPU: 1 UID: 0 PID: 870 Comm: cat Not tainted 6.12.0-rc3+ #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
1.16.1-2.fc37 04/01/2014
Call Trace:
 <TASK>
 dump_stack_lvl+0x53/0x70
 print_address_description.constprop.0+0x2c/0x3a0
 print_report+0xb9/0x280
 kasan_report+0xae/0xe0
 svc_export_show+0x362/0x430 [nfsd]
 c_show+0x161/0x390 [sunrpc]
 seq_read_iter+0x589/0x770
 seq_read+0x1e5/0x270
 proc_reg_read+0xe1/0x140
 vfs_read+0x125/0x530
 ksys_read+0xc1/0x160
 do_syscall_64+0x5f/0x170
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

Allocated by task 830:
 kasan_save_stack+0x20/0x40
 kasan_save_track+0x14/0x30
 __kasan_kmalloc+0x8f/0xa0
 __kmalloc_node_track_caller_noprof+0x1bc/0x400
 kmemdup_noprof+0x22/0x50
 svc_export_parse+0x8a9/0xb80 [nfsd]
 cache_do_downcall+0x71/0xa0 [sunrpc]
 cache_write_procfs+0x8e/0xd0 [sunrpc]
 proc_reg_write+0xe1/0x140
 vfs_write+0x1a5/0x6d0
 ksys_write+0xc1/0x160
 do_syscall_64+0x5f/0x170
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

Freed by task 868:
 kasan_save_stack+0x20/0x40
 kasan_save_track+0x14/0x30
 kasan_save_free_info+0x3b/0x60
 __kasan_slab_free+0x37/0x50
 kfree+0xf3/0x3e0
 svc_export_put+0x87/0xb0 [nfsd]
 cache_purge+0x17f/0x1f0 [sunrpc]
 nfsd_destroy_serv+0x226/0x2d0 [nfsd]
 nfsd_svc+0x125/0x1e0 [nfsd]
 write_threads+0x16a/0x2a0 [nfsd]
 nfsctl_transaction_write+0x74/0xa0 [nfsd]
 vfs_write+0x1a5/0x6d0
 ksys_write+0xc1/0x160
 do_syscall_64+0x5f/0x170
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

Fixes: ae74136 ("SUNRPC: Allow cache lookups to use RCU protection rather than the r/w spinlock")
Signed-off-by: Yang Erkun <[email protected]>
Reviewed-by: NeilBrown <[email protected]>
Reviewed-by: Jeff Layton <[email protected]>
Signed-off-by: Chuck Lever <[email protected]>

Author: Yang Erkun

fs/nfsd/export.c

@@ -355,16 +355,25 @@ static void export_stats_destroy(struct export_stats *stats)
 					    EXP_STATS_COUNTERS_NUM);
 }
 
-static void svc_export_put(struct kref *ref)
+static void svc_export_release(struct rcu_head *rcu_head)
 {
-	struct svc_export *exp = container_of(ref, struct svc_export, h.ref);
-	path_put(&exp->ex_path);
-	auth_domain_put(exp->ex_client);
+	struct svc_export *exp = container_of(rcu_head, struct svc_export,
+			ex_rcu);
+
 	nfsd4_fslocs_free(&exp->ex_fslocs);
 	export_stats_destroy(exp->ex_stats);
 	kfree(exp->ex_stats);
 	kfree(exp->ex_uuid);
-	kfree_rcu(exp, ex_rcu);
+	kfree(exp);
+}
+
+static void svc_export_put(struct kref *ref)
+{
+	struct svc_export *exp = container_of(ref, struct svc_export, h.ref);
+
+	path_put(&exp->ex_path);
+	auth_domain_put(exp->ex_client);
+	call_rcu(&exp->ex_rcu, svc_export_release);
 }
 
 static int svc_export_upcall(struct cache_detail *cd, struct cache_head *h)