AZL-40322

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 rbtree root node must also be protected by the dqopt->dqiosem, otherwise, giving the right timing, shmemreleasedquot() 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 - shmemreleasedquot() - 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.