On Tue, Sep 07, 2021 at 08:00:26PM -0700, Andrew Morton wrote: > From: Zhen Lei <thunder.leizhen@xxxxxxxxxx> > Subject: nilfs2: use refcount_dec_and_lock() to fix potential UAF > > When the refcount is decreased to 0, the resource reclamation branch is > entered. Before CPU0 reaches the race point (1), CPU1 may obtain the > spinlock and traverse the rbtree to find 'root', see nilfs_lookup_root(). > Although CPU1 will call refcount_inc() to increase the refcount, it is > obviously too late. CPU0 will release 'root' directly, CPU1 then accesses > 'root' and triggers UAF. > > Use refcount_dec_and_lock() to ensure that both the operations of decrease > refcount to 0 and link deletion are lock protected eliminates this risk. > > CPU0 CPU1 > nilfs_put_root(): > <-------- (1) > spin_lock(&nilfs->ns_cptree_lock); > rb_erase(&root->rb_node, &nilfs->ns_cptree); > spin_unlock(&nilfs->ns_cptree_lock); > > kfree(root); > <-------- use-after-free I don't know where this happened, but the leading whitespace has been eaten at some point, making this description of the race completely unreadable as everything appears to be done by CPU 0.
