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kernfs: Drop kernfs_rwsem while invoking lookup_positive_unlocked().

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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: 5b2fabf7fe ("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>
This commit is contained in:
Sebastian Andrzej Siewior 2025-02-18 17:39:38 +01:00 • committed by Greg Kroah-Hartman
parent 2ce177e9b3
commit 6ef5b6fae3
1 changed files with 25 additions and 10 deletions
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21
fs/kernfs/mount.c
View file

@ -220,13 +220,20 @@ struct dentry *kernfs_node_dentry(struct kernfs_node *kn,
return dentry; return dentry;
root = kernfs_root(kn); root = kernfs_root(kn);
guard(rwsem_read)(&root->kernfs_rwsem); /*
* 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); knparent = find_next_ancestor(kn, NULL);
if (WARN_ON(!knparent)) { if (WARN_ON(!knparent)) {
dput(dentry); dput(dentry);
return ERR_PTR(-EINVAL); return ERR_PTR(-EINVAL);
} }
}
do { do {
struct dentry *dtmp; struct dentry *dtmp;
@ -235,14 +242,22 @@ struct dentry *kernfs_node_dentry(struct kernfs_node *kn,
if (kn == knparent) if (kn == knparent)
return dentry; return dentry;
scoped_guard(rwsem_read, &root->kernfs_rwsem) {
kntmp = find_next_ancestor(kn, knparent); kntmp = find_next_ancestor(kn, knparent);
if (WARN_ON(!kntmp)) { if (WARN_ON(!kntmp)) {
dput(dentry); dput(dentry);
return ERR_PTR(-EINVAL); return ERR_PTR(-EINVAL);
} }
name = rcu_dereference(kntmp->name); name = kstrdup(kernfs_rcu_name(kntmp), GFP_KERNEL);
}
if (!name) {
dput(dentry);
return ERR_PTR(-ENOMEM);
}
dtmp = lookup_positive_unlocked(name, dentry, strlen(name)); dtmp = lookup_positive_unlocked(name, dentry, strlen(name));
dput(dentry); dput(dentry);
kfree(name);
if (IS_ERR(dtmp)) if (IS_ERR(dtmp))
return dtmp; return dtmp;
knparent = kntmp; knparent = kntmp;