tmpfs: fix race on handling dquot rbtree

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: tmpfs: fix race on handling dquot rbtree A syzkaller reproducer found a race while attempting to remove dquot information from the rb tree. Fetching the rb_tree root node must also be protected by the dqopt->dqio_sem, otherwise, giving the right timing, shmem_release_dquot() will trigger a warning because it couldn't find a node in the tree, when the real reason was the root node changing before the search starts: Thread 1 Thread 2 - shmem_release_dquot() - shmem_{acquire,release}_dquot() - fetch ROOT - Fetch ROOT - acquire dqio_sem - wait dqio_sem - do something, triger a tree rebalance - release dqio_sem - acquire dqio_sem - start searching for the node, but from the wrong location, missing the node, and triggering a warning.

References

• https://git.kernel.org/stable/c/c7077f43f30d817d10a9f8245e51576ac114b2f0
• https://git.kernel.org/stable/c/617d55b90e73c7b4aa2733ca6cc3f9b72d1124bb
• https://git.kernel.org/stable/c/f82f184874d2761ebaa60dccf577921a0dbb3810
• https://git.kernel.org/stable/c/0a69b6b3a026543bc215ccc866d0aea5579e6ce2