2018-12-20 06:42:11

by Dexuan Cui

[permalink] [raw]

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

---

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 kernel config for next-20181218, and my dmesg are attached.
I can always reproduce the panic every time I boot up the kernel.

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:

int security_sb_set_mnt_opts(struct super_block *sb,
void *mnt_opts,
unsigned long kern_flags,
unsigned long *set_kern_flags)
{
return call_int_hook(sb_set_mnt_opts,
mnt_opts ? -EOPNOTSUPP : 0, sb,
mnt_opts, kern_flags, set_kern_flags);
}

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,
struct fs_parameter *param)
{
struct apparmor_fs_context *afc = fc->security;
const char *value;
size_t space = 0, k_len = strlen(param->key), len = k_len, v_len;
char *p, *q;

if (!afc) {
afc = kzalloc(sizeof(*afc), GFP_KERNEL);
fc->security = afc;
}

apparmor_fs_context_parse_param() is added recently in:
c36d02347290 ("apparmor: Implement security hooks for the new mount API")

Unluckily I know nothing about LSM, so I'm not sure if the bug is in the VFS or
LSM. Here let me Cc the related people. I suppose somebody would give a quick fix.

Thanks!
-- Dexuan

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Attachments:

config.txt.tar.gz (33.72 kB)
config.txt.tar.gz dmesg.txt (11.96 kB)
dmesg.txt Download all attachments

2018-12-21 06:57:56

by Dexuan Cui

[permalink] [raw]

Subject: 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 <[email protected]> has fixed the issue in
the updated version of the patch by adding a dummy apparmor_set_mnt_opts():

From 8c5887f849301a5e00fa4b5e5ea7a3feae16b02d
From: David Howells <[email protected]>
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