kernfs: link kn to its parent before the LSM init hook

After commit 12e9e3cd03b5 ("simpe_xattr: use per-sb cache"),
kernfs_xattr_set() and kernfs_xattr_get() compute the cache via
kernfs_root(kn) before any other check.  kernfs_root(kn) walks
kn->__parent first and falls back to kn->dir.root, both of which are
NULL on a freshly kmem_cache_zalloc()'d kn. kn->__parent was being set
in kernfs_new_node() after __kernfs_new_node() returned, and kn->dir.root
is set even later by kernfs_create_dir_ns() / kernfs_create_empty_dir().

The LSM kernfs_init_security hook is invoked from inside
__kernfs_new_node(), before either field has been initialized.
selinux_kernfs_init_security() ends with kernfs_xattr_set(kn,
XATTR_NAME_SELINUX, ...).  kernfs_root(kn) then returns NULL, and
&((struct kernfs_root *)NULL)->xa_cache evaluates to
offsetof(struct kernfs_root, xa_cache) which faults:

  BUG: kernel NULL pointer dereference, address: 00000000000000e0
  RIP: 0010:simple_xattr_set+0x27/0x8b0
  Call Trace:
   kernfs_xattr_set+0x63/0xb0
   selinux_kernfs_init_security+0x13b/0x270
   security_kernfs_init_security+0x36/0xc0
   __kernfs_new_node+0x182/0x290
   kernfs_new_node+0x80/0xc0
   kernfs_create_dir_ns+0x2b/0xa0
   cgroup_create+0x116/0x380
   cgroup_mkdir+0x7c/0x1a0

Reproduces deterministically at PID 1 (systemd) on an SELinux-enabled
distro. The first cgroup mkdir under /sys/fs/cgroup with a labelled
parent panics the kernel.

The LSM hook's contract is that the kn_dir argument is the parent of
the new kn, so kn->__parent should already point at kn_dir when the
hook runs.  Move kernfs_get(parent) and rcu_assign_pointer of
kn->__parent from kernfs_new_node() into __kernfs_new_node() right
before the security hook, and unwind the parent reference on the
err_out4 path.  kernfs_root(kn) then takes its parent branch during
the hook and returns parent->dir.root, which is the correct root.

This also closes the same-shape latent bug in kernfs_xattr_get() (which
today is hidden only by kernfs_iattrs_noalloc() returning NULL on a
fresh kn).

Fixes: 12e9e3cd03b5 ("simpe_xattr: use per-sb cache")
Reported-by: Calum Mackay <calum.mackay@oracle.com>
Closes: https://lore.kernel.org/all/5386153f-9112-4971-98fc-de90d7aae2c6@oracle.com/
Link: https://patch.msgid.link/20260526-ablief-demut-wehen-aef8446ef5c9@brauner
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

1 files changed, 5 insertions, 4 deletions

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 8ba2f2f3da9e..6d47b8469642 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -698,6 +698,9 @@ static struct kernfs_node *__kernfs_new_node(struct kernfs_root *root,
 	}
 
 	if (parent) {
+		kernfs_get(parent);
+		rcu_assign_pointer(kn->__parent, parent);
+
 		ret = security_kernfs_init_security(parent, kn);
 		if (ret)
 			goto err_out4;
@@ -706,6 +709,8 @@ static struct kernfs_node *__kernfs_new_node(struct kernfs_root *root,
 	return kn;
 
  err_out4:
+	RCU_INIT_POINTER(kn->__parent, NULL);
+	kernfs_put(parent);
 	if (kn->iattr) {
 		simple_xattrs_free(&root->xa_cache, &kn->iattr->xattrs, NULL);
 		kmem_cache_free(kernfs_iattrs_cache, kn->iattr);
@@ -742,10 +747,6 @@ struct kernfs_node *kernfs_new_node(struct kernfs_node *parent,
 
 	kn = __kernfs_new_node(kernfs_root(parent), parent,
 			       name, mode, uid, gid, flags);
-	if (kn) {
-		kernfs_get(parent);
-		rcu_assign_pointer(kn->__parent, parent);
-	}
 	return kn;
 }