linux-kernel - Re: [GIT PULL] Add trusted_for(2) (was O

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Date:   Tue, 5 Apr 2022 17:38:39 +0200
From:   Mickaël Salaün <mic@...ikod.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christian Heimes <christian@...hon.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        James Morris <jmorris@...ei.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Muhammad Usama Anjum <usama.anjum@...labora.com>,
        Paul Moore <paul@...l-moore.com>,
        Philippe Trébuchet 
        <philippe.trebuchet@....gouv.fr>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Steve Dower <steve.dower@...hon.org>,
        Thibaut Sautereau <thibaut.sautereau@....gouv.fr>,
        Vincent Strubel <vincent.strubel@....gouv.fr>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-integrity <linux-integrity@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        Christian Brauner <brauner@...nel.org>
Subject: Re: [GIT PULL] Add trusted_for(2) (was O_MAYEXEC)


On 04/04/2022 23:28, Linus Torvalds wrote:
> On Mon, Apr 4, 2022 at 1:29 PM Mickaël Salaün <mic@...ikod.net> wrote:
>>
>> This initial proposal was using a new faccessat2(2) flag:
>> AT_INTERPRETED, see
>> https://lore.kernel.org/all/20200908075956.1069018-2-mic@digikod.net/
>> What do you think about that? I'm happy to get back to this version if
>> everyone is OK with it.
> 
> I'm certainly happi_er_ with that, but I find that particular patch
> odd for other reasons.
> 
> In no particular order:
> 
>   - what's with the insane non-C syntax? Double parentheses have no
> actual meaning in C:
> 
>       if ((flags & AT_INTERPRETED)) {
>           if ((mode & MAY_EXEC)) {
> 
>     so I don't understand why you'd use that strance thing.

I guess it comes from a previous version that ANDed two booleans.


> 
>   - why is this an AT_INTERPRETED flag? I don't understand the name, I
> don't understand the semantics.

I wasn't sure it was a good idea to add another mode bit, so I ended up 
using a flag to not break compatibility of other mode checks but extend 
the semantic to interpreted scripts. But I agree that a new mode makes 
sense.


> 
>     Why would that flag have the same value as AT_SYMLINK_FOLLOW?

It was a bug.


> 
>     Why isn't this just a new _mode_ bit, which is what I feel is
> sensible? We only use three bits (with no bits set meaning
> "existence"), so we have *tons* of bits left in that namespace, and it
> would make much more sense to me to have
> 
>          #define EXECVE_OK 8
> 
>      which is the same as the "group exec" bit, so it actually makes
> some kind of sense too.

Looks fine to me. The "EXECVE_" prefix is a bit weird but it will not be 
defined in the kernel like X_OK and others anyway, and as you said, it 
matches S_IXGRP.


> 
>   - related to that "I don't understand the semantics", the
> "documentation" for that thing doesn't make sense either:
> 
>      +         The
>      +    main usage is for script
>      +    interpreters to enforce a policy
>      +    consistent with the kernel's one
>      +    (through sysctl configuration or LSM
>      +    policy).  */

I'll synchronize the documentation with a next series.


> 
> Now, what I *think* you mean is
See a following email in reply to Kees.
[...]

> 
> And yes, we still need to talk details:
> 
>   - no backwards compatibility issues, because we've happily always
> checked 'mode' for being valid, so old kernels will always return
> -EINVAL.
> 
>   - POSIX namespace issues for EXECVE_OK means that the name probably
> needs some thinking (maybe we just need to call it __ACCESS_OK_EXECVE
> internally or something - the kernel actually doesn't even export the
> existing [FRWX]_OK values, because we "know" they map tho the usual
> "owner RWX" bits, with F being "no bit set")

Right, I cannot find a better name for now.

See a following email in reply to Kees.
[...]

> 
> So to recap: I'm very much ok with some access() extension, but I
> think even that very much needs clarification, and the existing patch
> is just odd in many many ways.

This v8 was kind of an early version, I'll update everything. Thanks!