[RFC PATCH v8 1/3] fs: Introduce AT_INTERPRETED flag for faccessat2(2)

[Mickaël Salaün]

On 08/09/2020 15:42, Stephen Smalley wrote:
> On Tue, Sep 8, 2020 at 9:29 AM Mimi Zohar <zohar at linux.ibm.com> wrote:
>>
>> On Tue, 2020-09-08 at 08:52 -0400, Stephen Smalley wrote:
>>> On Tue, Sep 8, 2020 at 8:50 AM Stephen Smalley
>>> <stephen.smalley.work at gmail.com> wrote:
>>>>
>>>> On Tue, Sep 8, 2020 at 8:43 AM Mickaël Salaün <mic at digikod.net> wrote:
>>>>>
>>>>>
>>>>> On 08/09/2020 14:28, Mimi Zohar wrote:
>>>>>> Hi Mickael,
>>>>>>
>>>>>> On Tue, 2020-09-08 at 09:59 +0200, Mickaël Salaün wrote:
>>>>>>> +                    mode |= MAY_INTERPRETED_EXEC;
>>>>>>> +                    /*
>>>>>>> +                     * For compatibility reasons, if the system-wide policy
>>>>>>> +                     * doesn't enforce file permission checks, then
>>>>>>> +                     * replaces the execute permission request with a read
>>>>>>> +                     * permission request.
>>>>>>> +                     */
>>>>>>> +                    mode &= ~MAY_EXEC;
>>>>>>> +                    /* To be executed *by* user space, files must be readable. */
>>>>>>> +                    mode |= MAY_READ;
>>>>>>
>>>>>> After this change, I'm wondering if it makes sense to add a call to
>>>>>> security_file_permission().  IMA doesn't currently define it, but
>>>>>> could.
>>>>>
>>>>> Yes, that's the idea. We could replace the following inode_permission()
>>>>> with file_permission(). I'm not sure how this will impact other LSMs though.
>>
>> I wasn't suggesting replacing the existing security_inode_permission
>> hook later, but adding a new security_file_permission hook here.
>>
>>>>
>>>> They are not equivalent at least as far as SELinux is concerned.
>>>> security_file_permission() was only to be used to revalidate
>>>> read/write permissions previously checked at file open to support
>>>> policy changes and file or process label changes.  We'd have to modify
>>>> the SELinux hook if we wanted to have it check execute access even if
>>>> nothing has changed since open time.
>>>
>>> Also Smack doesn't appear to implement file_permission at all, so it
>>> would skip Smack checking.
>>
>> My question is whether adding a new security_file_permission call here
>> would break either SELinux or Apparmor?
> 
> selinux_inode_permission() has special handling for MAY_ACCESS so we'd
> need to duplicate that into selinux_file_permission() ->
> selinux_revalidate_file_permission().  Also likely need to adjust
> selinux_file_permission() to explicitly check whether the mask
> includes any permissions not checked at open time.  So some changes
> would be needed here.  By default, it would be a no-op unless there
> was a policy reload or the file was relabeled between the open(2) and
> the faccessat(2) call.
> 

We could create a new hook path_permission(struct path *path, int mask)
as a superset of inode_permission(). To be more convenient, his new hook
could then just call inode_permission() for every LSMs not implementing
path_permission().