RE: Recent VFS/LSM patches cause Kernel panic - not syncing: Can't create rootfs

From: Dexuan Cui
Sent: Wednesday, December 19, 2018 8:30 PM

Hi,
We started to see a "Can't create rootfs" panic with linux-next's
next-20181218 and next-20181219. Note: next-20181217 is good.

Our test team found the first bad commit by git-bisect:
013c7af575e5 ("vfs: Implement a filesystem superblock creation/configuration
context")

I had a look and I think another patch also helped to cause the panic:
c36d02347290 ("apparmor: Implement security hooks for the new mount API")

My finding is: the panic happens because
start_kernel() -> vfs_caches_init() -> mnt_init() ->
sysfs_init() -> register_filesystem() -> init_mount_tree() ->
vfs_kern_mount(type, 0, "rootfs", NULL) -> vfs_get_tree() ->
security_sb_set_mnt_opts(sb, fc->security, 0, NULL) returns -EOPNOTSUPP:

This means: fc->security is not NULL in
security_sb_set_mnt_opts(sb, fc->security, 0, NULL), and the
security_hook_heads.FUNC is empty in call_int_hook().

The fc->security is assigned in this function (i.e. the line "fc->security = afc;" ):

static int apparmor_fs_context_parse_param(struct fs_context *fc,

Well, obviously David Howells [off-list ref] has fixed the issue in
the updated version of the patch by adding a dummy apparmor_set_mnt_opts():

From 8c5887f849301a5e00fa4b5e5ea7a3feae16b02d
From: David Howells <dhowells@redhat.com>
Date: Thu, 1 Nov 2018 23:07:24 +0000
Subject: apparmor: Implement security hooks for the new mount API
(https://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs.git/commit/?h=for-next&id=8c5887f849301a5e00fa4b5e5ea7a3feae16b02d )

I expect the fix will be in linux-next soon, probably tomorrow.

Thanks!
-- Dexuan