Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH] Allow restricting permissions in /proc/sys
To:     Andy Lutomirski <luto@kernel.org>,
        Christian Brauner <christian.brauner@ubuntu.com>
Cc:     Luis Chamberlain <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "open list:FILESYSTEMS (VFS and infrastructure)" 
        <linux-fsdevel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
References: <74a91362-247c-c749-5200-7bdce704ed9e@gmail.com>
 <20191112232239.yevpeemgxz4wy32b@wittgenstein>
 <CALCETrUEQMdugz1t6HfK5MvDq_kOw13yuF+98euqVJgZ4WR1VA@mail.gmail.com>
From:   Topi Miettinen <toiwoton@gmail.com>
Message-ID: <0ba24187-caf6-c851-baaa-f768885cda47@gmail.com>
Date:   Wed, 13 Nov 2019 12:52:02 +0200
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.9.0
MIME-Version: 1.0
In-Reply-To: <CALCETrUEQMdugz1t6HfK5MvDq_kOw13yuF+98euqVJgZ4WR1VA@mail.gmail.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 13.11.2019 6.50, Andy Lutomirski wrote:
> On Tue, Nov 12, 2019 at 3:22 PM Christian Brauner
> <christian.brauner@ubuntu.com> wrote:
>>
>> [Cc+ linux-api@vger.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@gmail.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