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

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Date:   Wed, 13 Nov 2019 12:52:02 +0200
From:   Topi Miettinen <toiwoton@...il.com>
To:     Andy Lutomirski <luto@...nel.org>,
        Christian Brauner <christian.brauner@...ntu.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>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] Allow restricting permissions in /proc/sys

On 13.11.2019 6.50, Andy Lutomirski wrote:
> On Tue, Nov 12, 2019 at 3:22 PM Christian Brauner
> <christian.brauner@...ntu.com> wrote:
>>
>> [Cc+ linux-api@...r.kernel.org]
>>
>> since that's potentially relevant to quite a few people.
>>
>> 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;
>>> +
>>>          head = grab_header(inode);
>>>          if (IS_ERR(head))
>>>                  return PTR_ERR(head);
>>> @@ -837,9 +841,35 @@ static int proc_sys_setattr(struct dentry *dentry,
>>> struct iattr *attr)
>>>          struct inode *inode = d_inode(dentry);
>>>          int error;
>>>
>>> -       if (attr->ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID))
>>> +       if (attr->ia_valid & (ATTR_UID | ATTR_GID))
>>>                  return -EPERM;
> 
> Supporting at least ATTR_GID would make this much more useful.

Yes, also XATTR/ACL support would be useful. But so far I've tried to 
allow only tightening of permissions.

>>>
>>> +       if (attr->ia_valid & ATTR_MODE) {
>>> +               struct ctl_table_header *head = grab_header(inode);
>>> +               struct ctl_table *table = PROC_I(inode)->sysctl_entry;
>>> +               umode_t max_mode = 0777; /* Only these bits may change */
>>> +
>>> +               if (IS_ERR(head))
>>> +                       return PTR_ERR(head);
>>> +
>>> +               if (!table) /* global root - r-xr-xr-x */
>>> +                       max_mode &= ~0222;
>>> +               else /*
>>> +                     * Don't allow permissions to become less
>>> +                     * restrictive than the sysctl table entry
>>> +                     */
>>> +                       max_mode &= table->mode;
> 
> Style nit: please put braces around multi-line if and else branches,
> even if they're only multi-line because of comments.

OK, thanks.

>>> +
>>> +               sysctl_head_finish(head);
>>> +
>>> +               /* Execute bits only allowed for directories */
>>> +               if (!S_ISDIR(inode->i_mode))
>>> +                       max_mode &= ~0111;
> 
> Why is this needed?
> 

In general, /proc/sys does not allow executable permissions for the 
files, so I've continued this policy.

-Topi