[PATCH] configfs: Delay the fop setting to prevent noise generated by open

[Edward Adam Davis <eadavis@sina.com>]

mkdir() triggers:
  configfs_mkdir() ->
   configfs_attach_group() ->
    populate_groups() ->
     create_default_group("ana_groups") ->
      configfs_attach_group() ->
       populate_groups() ->
	create_default_group("1")

The creation of an inode for the subdentry 1 under ana_groups failed
due to fault injection. [1]

Because helper threads are racing open() on ana_groups, ana_groups has
dentry refcount > 1. d_delete() skips dentry_unlink_inode(), leaving
d_inode non-NULL. create_default_group() hits BUG_ON(d_inode(child)) at
fs/configfs/dir.c:734! [2]

[1]
FAULT_INJECTION: forcing a failure.
name failslab, interval 1, probability 0, space 0, times -1
Call Trace:
 should_failslab+0xa8/0x100
 kmem_cache_alloc_noprof+0xa1/0x600
 security_inode_alloc+0x39/0x310
 inode_init_always_gfp+0x97e/0xd30
 alloc_inode+0x82/0x1b0
 new_inode+0x1f/0x170
 configfs_new_inode+0x27/0x410
 configfs_create+0xb5/0x270
 configfs_attach_item+0x25f/0xd60
 configfs_attach_group+0x2a/0x370
 create_default_group+0x87/0x180
 configfs_attach_group+0x169/0x370
 configfs_mkdir+0x6f0/0x9e0
 vfs_mkdir+0x40c/0x620

[2]
 kernel BUG at fs/configfs/dir.c:734!
 Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI
 RIP: 0010:create_default_group+0x17c/0x180
 Call Trace:
  configfs_attach_group+0x169/0x370
  configfs_mkdir+0x6f0/0x9e0
  vfs_mkdir+0x40c/0x620

The root cause is a race condition between open and mkdir; delaying the
assignment of the fop ensures that directories are not opened before they
are ready. 

Fixes: 7063fbf22611 ("[PATCH] configfs: User-driven configuration filesystem")
Reported-by: syzbot+4fae162886e497f72228@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=4fae162886e497f72228
Signed-off-by: Edward Adam Davis <eadavis@sina.com>
---
 fs/configfs/dir.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/fs/configfs/dir.c b/fs/configfs/dir.c
index eda80c2a2d38..45a955b8ab36 100644
--- a/fs/configfs/dir.c
+++ b/fs/configfs/dir.c
@@ -313,7 +313,6 @@ static int configfs_create_dir(struct config_item *item, struct dentry *dentry,
 		goto out_remove;
 
 	inode->i_op = &configfs_dir_inode_operations;
-	inode->i_fop = &configfs_dir_operations;
 	/* directory inodes start off with i_nlink == 2 (for "." entry) */
 	inc_nlink(inode);
 	d_make_persistent(dentry, inode);
@@ -930,6 +929,10 @@ static int configfs_attach_group(struct config_item *item,
 		inode_unlock(d_inode(dentry));
 		if (ret)
 			d_delete(dentry);
+		else {
+			struct inode *inode = d_inode(dentry);
+			inode->i_fop = &configfs_dir_operations;
+		}
 	}
 
 	return ret;
@@ -1430,8 +1433,13 @@ static struct dentry *configfs_mkdir(struct mnt_idmap *idmap, struct inode *dir,
 
 	if (group)
 		ret = configfs_attach_group(item, dentry, frag);
-	else
+	else {
 		ret = configfs_attach_item(item, dentry, frag);
+		if (!ret) {
+			struct inode *inode = d_inode(dentry);
+			inode->i_fop = &configfs_dir_operations;
+		}
+	}
 
 	spin_lock(&configfs_dirent_lock);
 	sd->s_type &= ~CONFIGFS_USET_IN_MKDIR;
-- 
2.43.0