linux-kernel - Re: [PATCH] net: always initialize sysctl ownership

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Date: Tue, 19 Mar 2024 14:21:58 +0100
From: Joel Granados <j.granados@...sung.com>
To: Kuniyuki Iwashima <kuniyu@...zon.com>
CC: <linux@...ssschuh.net>, <davem@...emloft.net>,
	<dmitry.torokhov@...il.com>, <ebiederm@...ssion.com>, <edumazet@...gle.com>,
	<kuba@...nel.org>, <linux-kernel@...r.kernel.org>, <mcgrof@...nel.org>,
	<netdev@...r.kernel.org>, <pabeni@...hat.com>
Subject: Re: [PATCH] net: always initialize sysctl ownership

On Fri, Mar 15, 2024 at 05:39:58PM -0700, Kuniyuki Iwashima wrote:
> From: Thomas Weißschuh <linux@...ssschuh.net>
> Date: Fri, 15 Mar 2024 17:20:31 +0100
> > The sysctl core does not initialize these fields when the set_ownership
> > callback is present.
> > So always do it in the callback.
> 
> Could you add few more words here to explain what the problem is
> like commit 5ec27ec735ba ("fs/proc/proc_sysctl.c: fix the default
> values of i_uid/i_gid on /proc/sys inodes.").
I second this proposal. There is more to the story than a simple setting
of default values. IMO, you should mention how the missbehavior was
introduced in 5ec27ec735ba0 (fs/proc/proc_sysctl.c: fix the default
values of i_uid/i_gid on /proc/sys inodes.)

> 
> BTW, it seems that we can change the value even if the uid/gid is
> invalid unlike the issue mentioned in 5ec27ec735ba.
Indeed and I think it is because net_ctl_permissions set the mode in the
same way as test_perm does. Therefore regardless of the tests against
GLOBAL_ROOT_[U|G]ID being true or false mode gets right shifted by 6 and
3.

> 
> So, the problem is just the invalid uid/gid exposed to userspace
> or am I missing something ?
That seems to be the case. I think that should just be
GLOBAL_ROOT_[G|U]ID. Please correct me if I'm wrong.

> 
> ---8<---
> [root@...alhost ~]# unshare --user --keep-caps
> [nobody@...alhost ~]$ id
> uid=65534(nobody) gid=65534(nobody) groups=65534(nobody)
> [nobody@...alhost ~]$ unshare --net --keep-caps
> [nobody@...alhost ~]$ stat /proc/sys/net/core/somaxconn
>   File: /proc/sys/net/core/somaxconn
>   Size: 0         	Blocks: 0          IO Block: 1024   regular empty file
> Device: 13h/19d	Inode: 3726        Links: 1
> Access: (0644/-rw-r--r--)  Uid: (65534/  nobody)   Gid: (65534/  nobody)
> Access: 2024-03-16 00:28:51.937789000 +0000
> Modify: 2024-03-16 00:28:51.937789000 +0000
> Change: 2024-03-16 00:28:51.937789000 +0000
>  Birth: -
> [nobody@...alhost ~]$ cat /proc/sys/net/core/somaxconn
> 4096
> [nobody@...alhost ~]$ echo 2048 > /proc/sys/net/core/somaxconn
> [nobody@...alhost ~]$ cat /proc/sys/net/core/somaxconn
> 2048
> ---8<---
> 
> 
> > 
> > Fixes: e79c6a4fc923 ("net: make net namespace sysctls belong to container's owner")
> > Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>
> > ---
> >  net/sysctl_net.c | 6 ++----
> >  1 file changed, 2 insertions(+), 4 deletions(-)
> > 
> > diff --git a/net/sysctl_net.c b/net/sysctl_net.c
> > index 051ed5f6fc93..03e320ddacc9 100644
> > --- a/net/sysctl_net.c
> > +++ b/net/sysctl_net.c
> > @@ -62,12 +62,10 @@ static void net_ctl_set_ownership(struct ctl_table_header *head,
> >  	kgid_t ns_root_gid;
> >  
> >  	ns_root_uid = make_kuid(net->user_ns, 0);
> > -	if (uid_valid(ns_root_uid))
> > -		*uid = ns_root_uid;
> > +	*uid = uid_valid(ns_root_uid) ? ns_root_uid : GLOBAL_ROOT_UID;
> >  
> >  	ns_root_gid = make_kgid(net->user_ns, 0);
> > -	if (gid_valid(ns_root_gid))
> > -		*gid = ns_root_gid;
> > +	*gid = gid_valid(ns_root_gid) ? ns_root_gid : GLOBAL_ROOT_GID;
> >  }
> 
> How about setting the default in proc_sys_make_inode() instead ?
> because the default value configured by new_inode() is not
> appropriate for procfs, IIUC.
Are you proposing something like this in proc_sys_make_inode/

inode->i_uid = GLOBAL_ROOT_UID;
inode->i_gid = GLOBAL_ROOT_GID;
if (root->set_ownership)
    root->set_ownership(head, table, &indoe->i_uid, &inode->i_gid)

?

> 
> Thanks!
> 
> >  
> >  static struct ctl_table_root net_sysctl_root = {
> > 
> > ---
> > base-commit: e5eb28f6d1afebed4bb7d740a797d0390bd3a357
> > change-id: 20240315-sysctl-net-ownership-bc4e17eaeea6
> > 
> > Best regards,
> > -- 
> > Thomas Weißschuh <linux@...ssschuh.net>

-- 

Joel Granados

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