Fix KASAN identified use-after-free issue.

[  612.157429] ==================================================================
[  612.158275] BUG: KASAN: use-after-free in process_one_work+0x90/0x9b0
[  612.158801] Read of size 8 at addr ffff88810a31ca60 by task kworker/2:9/2382

[  612.159611] CPU: 2 PID: 2382 Comm: kworker/2:9 Tainted: G
OE     5.13.0-rc2+ #98
[  612.159623] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS 1.14.0-1.fc33 04/01/2014
[  612.159640] Workqueue:  0x0 (deferredclose)
[  612.159669] Call Trace:
[  612.159685]  dump_stack+0xbb/0x107
[  612.159711]  print_address_description.constprop.0+0x18/0x140
[  612.159733]  ? process_one_work+0x90/0x9b0
[  612.159743]  ? process_one_work+0x90/0x9b0
[  612.159754]  kasan_report.cold+0x7c/0xd8
[  612.159778]  ? lock_is_held_type+0x80/0x130
[  612.159789]  ? process_one_work+0x90/0x9b0
[  612.159812]  kasan_check_range+0x145/0x1a0
[  612.159834]  process_one_work+0x90/0x9b0
[  612.159877]  ? pwq_dec_nr_in_flight+0x110/0x110
[  612.159914]  ? spin_bug+0x90/0x90
[  612.159967]  worker_thread+0x3b6/0x6c0
[  612.160023]  ? process_one_work+0x9b0/0x9b0
[  612.160038]  kthread+0x1dc/0x200
[  612.160051]  ? kthread_create_worker_on_cpu+0xd0/0xd0
[  612.160092]  ret_from_fork+0x1f/0x30

[  612.160399] Allocated by task 2358:
[  612.160757]  kasan_save_stack+0x1b/0x40
[  612.160768]  __kasan_kmalloc+0x9b/0xd0
[  612.160778]  cifs_new_fileinfo+0xb0/0x960 [cifs]
[  612.161170]  cifs_open+0xadf/0xf20 [cifs]
[  612.161421]  do_dentry_open+0x2aa/0x6b0
[  612.161432]  path_openat+0xbd9/0xfa0
[  612.161441]  do_filp_open+0x11d/0x230
[  612.161450]  do_sys_openat2+0x115/0x240
[  612.161460]  __x64_sys_openat+0xce/0x140

When mod_delayed_work is called to modify the delay of pending work,
it might return false and queue a new work when pending work is
already scheduled or when try to grab pending work failed.

So, Increase the reference count when new work is scheduled to
avoid use-after-free.
Signed-off-by: Rohith Surabattula <rohiths@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/file.c

@@ -874,10 +874,6 @@ void smb2_deferred_work_close(struct work_struct *work)
 			struct cifsFileInfo, deferred.work);
 
 	spin_lock(&CIFS_I(d_inode(cfile->dentry))->deferred_lock);
-	if (!cfile->deferred_close_scheduled) {
-		spin_unlock(&CIFS_I(d_inode(cfile->dentry))->deferred_lock);
-		return;
-	}
 	cifs_del_deferred_close(cfile);
 	cfile->deferred_close_scheduled = false;
 	spin_unlock(&CIFS_I(d_inode(cfile->dentry))->deferred_lock);
@@ -904,8 +900,13 @@ int cifs_close(struct inode *inode, struct file *file)
 			cifs_add_deferred_close(cfile, dclose);
 			if (cfile->deferred_close_scheduled &&
 			    delayed_work_pending(&cfile->deferred)) {
-				mod_delayed_work(deferredclose_wq,
-						&cfile->deferred, cifs_sb->ctx->acregmax);
+				/*
+				 * If there is no pending work, mod_delayed_work queues new work.
+				 * So, Increase the ref count to avoid use-after-free.
+				 */
+				if (!mod_delayed_work(deferredclose_wq,
+						&cfile->deferred, cifs_sb->ctx->acregmax))
+					cifsFileInfo_get(cfile);
 			} else {
 				/* Deferred close for files */
 				queue_delayed_work(deferredclose_wq,
@@ -4879,7 +4880,12 @@ void cifs_oplock_break(struct work_struct *work)
 	if (is_deferred &&
 	    cfile->deferred_close_scheduled &&
 	    delayed_work_pending(&cfile->deferred)) {
-		mod_delayed_work(deferredclose_wq, &cfile->deferred, 0);
+		/*
+		 * If there is no pending work, mod_delayed_work queues new work.
+		 * So, Increase the ref count to avoid use-after-free.
+		 */
+		if (!mod_delayed_work(deferredclose_wq, &cfile->deferred, 0))
+			cifsFileInfo_get(cfile);
 	}
 	spin_unlock(&CIFS_I(inode)->deferred_lock);
 	_cifsFileInfo_put(cfile, false /* do not wait for ourself */, false);

fs/cifs/misc.c

@@ -674,6 +674,8 @@ cifs_add_pending_open(struct cifs_fid *fid, struct tcon_link *tlink,
 
 /*
  * Critical section which runs after acquiring deferred_lock.
+ * As there is no reference count on cifs_deferred_close, pdclose
+ * should not be used outside deferred_lock.
  */
 bool
 cifs_is_deferred_close(struct cifsFileInfo *cfile, struct cifs_deferred_close **pdclose)
@@ -752,8 +754,14 @@ cifs_close_all_deferred_files(struct cifs_tcon *tcon)
 	spin_lock(&tcon->open_file_lock);
 	list_for_each(tmp, &tcon->openFileList) {
 		cfile = list_entry(tmp, struct cifsFileInfo, tlist);
-		if (delayed_work_pending(&cfile->deferred))
-			mod_delayed_work(deferredclose_wq, &cfile->deferred, 0);
+		if (delayed_work_pending(&cfile->deferred)) {
+			/*
+			 * If there is no pending work, mod_delayed_work queues new work.
+			 * So, Increase the ref count to avoid use-after-free.
+			 */
+			if (!mod_delayed_work(deferredclose_wq, &cfile->deferred, 0))
+				cifsFileInfo_get(cfile);
+		}
 	}
 	spin_unlock(&tcon->open_file_lock);
 }