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

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Date:   Tue, 5 Apr 2022 18:09:03 +0200
From:   Mickaël Salaün <mic@...ikod.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>
Cc:     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>,
        Theodore Ts'o <tytso@....edu>
Subject: Re: [GIT PULL] Add trusted_for(2) (was O_MAYEXEC)


On 05/04/2022 01:26, Linus Torvalds wrote:
> On Mon, Apr 4, 2022 at 3:25 PM Kees Cook <keescook@...omium.org> wrote:

[...]

> 
>> I think this already exists as AT_EACCESS? It was added with
>> faccessat2() itself, if I'm reading the history correctly.
> 
> Yeah, I noticed myself, I just hadn't looked (and I don't do enough
> user-space programming to be aware of if that way).

I think AT_EACCESS should be usable with the new EXECVE_OK too.


> 
>>>      (a) "what about suid bits that user space cannot react to"
>>
>> What do you mean here? Do you mean setid bits on the file itself?
> 
> Right.
> 
> Maybe we don't care.

I think we don't. I think the only corner case that could be different 
is for files that are executable, SUID and non-readable. In this case it 
wouldn't matter because userspace could not read the file, which is 
required for interpretation/execution. Anyway, S[GU]ID bits in scripts 
are just ignored by execve and we want to follow the same semantic.


> 
> Maybe we do.
> 
> Is the user-space loader going to honor them? Is it going to ignore
> them? I don't know. And it actually interacts with things like
> 'nosuid', which the kernel does know about, and user space has a hard
> time figuring out.
> 
> So if the point is "give me an interface so that I can do the same
> thing a kernel execve() loader would do", then those sgid/suid bits
> actually may be exactly the kind of thing that user space wants the
> kernel to react to - should it ignore them, or should it do something
> special when it sees that they are set?
> 
> I'm not saying that they *should* be something we care about. All I'm
> saying is that I want that *discussion* to happen.
> 
>                 Linus