linux-kernel - Re: [PATCH] Allow restricting permissions in /proc/sys

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Date:   Wed, 13 Nov 2019 18:19:17 +0200
From:   Topi Miettinen <toiwoton@...il.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "open list:FILESYSTEMS (VFS and infrastructure)" 
        <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Christian Brauner <christian.brauner@...ntu.com>
Subject: Re: [PATCH] Allow restricting permissions in /proc/sys

On 13.11.2019 18.00, Jann Horn wrote:
> On Wed, Nov 13, 2019 at 12:22 AM Christian Brauner
> <christian.brauner@...ntu.com> wrote:
>> On Sun, Nov 03, 2019 at 04:55:48PM +0200, Topi Miettinen wrote:
>>> Several items in /proc/sys need not be accessible to unprivileged
>>> tasks. Let the system administrator change the permissions, but only
>>> to more restrictive modes than what the sysctl tables allow.
>>>
>>> Signed-off-by: Topi Miettinen <toiwoton@...il.com>
>>> ---
>>>   fs/proc/proc_sysctl.c | 41 +++++++++++++++++++++++++++++++----------
>>>   1 file changed, 31 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
>>> index d80989b6c344..88c4ca7d2782 100644
>>> --- a/fs/proc/proc_sysctl.c
>>> +++ b/fs/proc/proc_sysctl.c
>>> @@ -818,6 +818,10 @@ static int proc_sys_permission(struct inode *inode, int
>>> mask)
>>>          if ((mask & MAY_EXEC) && S_ISREG(inode->i_mode))
>>>                  return -EACCES;
>>>
>>> +       error = generic_permission(inode, mask);
>>> +       if (error)
>>> +               return error;
> 
> In kernel/ucount.c, the ->permissions handler set_permissions() grants
> access based on whether the caller has CAP_SYS_RESOURCE. And in
> net/sysctl_net.c, the handler net_ctl_permissions() grants access
> based on whether the caller has CAP_NET_ADMIN. This added check is
> going to break those, right?
> 

Right. The comment above seems then a bit misleading:
	/*
	 * sysctl entries that are not writeable,
	 * are _NOT_ writeable, capabilities or not.
	 */

-Topi