linux-kernel - ubifs: fix use-after-free on symlink traversal

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Date:   Tue, 26 Mar 2019 01:40:38 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     syzbot <syzbot+7a8ba368b47fdefca61e@...kaller.appspotmail.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: ubifs: fix use-after-free on symlink traversal

free the symlink body after the same RCU delay we have for freeing the 
struct inode itself, so that traversal during RCU pathwalk wouldn't step 
into freed memory.

Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
---
diff --git a/fs/ubifs/super.c b/fs/ubifs/super.c
index 8dc2818fdd84..12628184772c 100644
--- a/fs/ubifs/super.c
+++ b/fs/ubifs/super.c
@@ -276,14 +276,12 @@ static void ubifs_i_callback(struct rcu_head *head)
 {
 	struct inode *inode = container_of(head, struct inode, i_rcu);
 	struct ubifs_inode *ui = ubifs_inode(inode);
+	kfree(ui->data);
 	kmem_cache_free(ubifs_inode_slab, ui);
 }
 
 static void ubifs_destroy_inode(struct inode *inode)
 {
-	struct ubifs_inode *ui = ubifs_inode(inode);
-
-	kfree(ui->data);
 	call_rcu(&inode->i_rcu, ubifs_i_callback);
 }