From: Sebastian Andrzej Siewior bigeasy@linutronix.de
[ Upstream commit 6ef5b6fae304091593956be59065c0c8633ad9e8 ]
syzbot reported two warnings: - kernfs_node::name was accessed outside of a RCU section so it created warning. The kernfs_rwsem was held so it was okay but it wasn't seen.
- While kernfs_rwsem was held invoked lookup_positive_unlocked()-> kernfs_dop_revalidate() which acquired kernfs_rwsem.
kernfs_rwsem was both acquired as a read lock so it can be acquired twice. However if a writer acquires the lock after the first reader then neither the writer nor the second reader can obtain the lock so it deadlocks.
The reason for the lock is to ensure that kernfs_node::name remain stable during lookup_positive_unlocked()'s invocation. The function can not be invoked within a RCU section because it may sleep.
Make a temporary copy of the kernfs_node::name under the lock so GFP_KERNEL can be used and use this instead.
Reported-by: syzbot+ecccecbc636b455f9084@syzkaller.appspotmail.com Fixes: 5b2fabf7fe8f ("kernfs: Acquire kernfs_rwsem in kernfs_node_dentry().") Signed-off-by: Sebastian Andrzej Siewior bigeasy@linutronix.de Acked-by: Tejun Heo tj@kernel.org Link: https://lore.kernel.org/r/20250218163938.xmvjlJ0K@linutronix.de Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org Signed-off-by: Sasha Levin sashal@kernel.org --- fs/kernfs/mount.c | 33 ++++++++++++++++++++++++--------- 1 file changed, 24 insertions(+), 9 deletions(-)
diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c index d1f512b7bf867..f1cea282aae32 100644 --- a/fs/kernfs/mount.c +++ b/fs/kernfs/mount.c @@ -220,12 +220,19 @@ struct dentry *kernfs_node_dentry(struct kernfs_node *kn, return dentry;
root = kernfs_root(kn); - guard(rwsem_read)(&root->kernfs_rwsem); - - knparent = find_next_ancestor(kn, NULL); - if (WARN_ON(!knparent)) { - dput(dentry); + /* + * As long as kn is valid, its parent can not vanish. This is cgroup's + * kn so it not have its parent replaced. Therefore it is safe to use + * the ancestor node outside of the RCU or locked section. + */ + if (WARN_ON_ONCE(!(root->flags & KERNFS_ROOT_INVARIANT_PARENT))) return ERR_PTR(-EINVAL); + scoped_guard(rcu) { + knparent = find_next_ancestor(kn, NULL); + if (WARN_ON(!knparent)) { + dput(dentry); + return ERR_PTR(-EINVAL); + } }
do { @@ -235,14 +242,22 @@ struct dentry *kernfs_node_dentry(struct kernfs_node *kn,
if (kn == knparent) return dentry; - kntmp = find_next_ancestor(kn, knparent); - if (WARN_ON(!kntmp)) { + + scoped_guard(rwsem_read, &root->kernfs_rwsem) { + kntmp = find_next_ancestor(kn, knparent); + if (WARN_ON(!kntmp)) { + dput(dentry); + return ERR_PTR(-EINVAL); + } + name = kstrdup(kernfs_rcu_name(kntmp), GFP_KERNEL); + } + if (!name) { dput(dentry); - return ERR_PTR(-EINVAL); + return ERR_PTR(-ENOMEM); } - name = rcu_dereference(kntmp->name); dtmp = lookup_positive_unlocked(name, dentry, strlen(name)); dput(dentry); + kfree(name); if (IS_ERR(dtmp)) return dtmp; knparent = kntmp;