[PATCH] kernfs: fix UAF race condition in __kernfs_remove()

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

syzbot is reporting use-after-free read at __kernfs_remove() [1], for
commit 35beab0635f3cdd4 ("kernfs: restructure removal path to fix possible
premature return") missed that we need to keep a ref on "kn" as well as
"pos".

This race condition happens when two concurrent removers "T1" and "T2"
interfere due to kernfs_drain() temporarily dropping kernfs_rwsem.

  T1:                     T2:
  down_write(&root->kernfs_rwsem);
  do {
    pos = kernfs_leftmost_descendant(kn);
    kernfs_get(pos);
    kernfs_drain(pos) {
      up_write(&root->kernfs_rwsem);
                          down_write(&root->kernfs_rwsem);
                          do {
                            // Removes all children and "kn", but won't
                            // free T1's "pos" and "kn", for T1 has a ref
                            // on T1's "pos", and T1's "pos" in turn keeps
                            // a ref on "kn".
                            pos = kernfs_leftmost_descendant(kn);
                            kernfs_put(pos);
                          } while (pos != kn) // Will break.
                          up_write(&root->kernfs_rwsem);
      down_write(&root->kernfs_rwsem);
    }
    // Frees "pos" because this was the last ref, and also frees "kn"
    // because a ref by "pos" was gone (i.e. "kn" no longer has ref)
    // via "goto repeat;" inside kernfs_put().
    kernfs_put(pos);
  } while (pos != kn) // Will continue, despite "kn" already freed.

Link: https://syzkaller.appspot.com/bug?extid=8bee3285b9e190f1509e [1]
Reported-by: syzbot+8bee3285b9e190f1509e@syzkaller.appspotmail.com
Fixes: 35beab0635f3cdd4 ("kernfs: restructure removal path to fix possible premature return")
Tested-by: syzbot+8bee3285b9e190f1509e@syzkaller.appspotmail.com
Co-developed-by: Hillf Danton <hdanton@sina.com>
Signed-off-by: Hillf Danton <hdanton@sina.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
 fs/kernfs/dir.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 1cc88ba6de90..effb461d34fa 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -1365,6 +1365,11 @@ static void __kernfs_remove(struct kernfs_node *kn)
 			atomic_add(KN_DEACTIVATED_BIAS, &pos->active);
 
 	/* deactivate and unlink the subtree node-by-node */
+	/*
+	 * kernfs_put(pos) will invoke kernfs_put(kn) if @pos was the last
+	 * reference to @kn. Make sure @kn doesn't go away underneath us.
+	 */
+	kernfs_get(kn);
 	do {
 		pos = kernfs_leftmost_descendant(kn);
 
@@ -1406,6 +1411,7 @@ static void __kernfs_remove(struct kernfs_node *kn)
 
 		kernfs_put(pos);
 	} while (pos != kn);
+	kernfs_put(kn);
 }
 
 /**
-- 
2.34.1