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Date: Mon, 11 Oct 2021 14:07:55 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Mickaël Salaün <mic@...ikod.net>
Cc: Al Viro <viro@...iv.linux.org.uk>, Aleksa Sarai <cyphar@...har.com>,
Andy Lutomirski <luto@...nel.org>, Arnd Bergmann <arnd@...db.de>, Casey
Schaufler <casey@...aufler-ca.com>, Christian Brauner
<christian.brauner@...ntu.com>, Christian Heimes <christian@...hon.org>,
Deven Bowers <deven.desai@...ux.microsoft.com>, Dmitry Vyukov
<dvyukov@...gle.com>, Eric Biggers <ebiggers@...nel.org>, Eric Chiang
<ericchiang@...gle.com>, Florian Weimer <fweimer@...hat.com>, Geert
Uytterhoeven <geert@...ux-m68k.org>, James Morris <jmorris@...ei.org>, Jan
Kara <jack@...e.cz>, Jann Horn <jannh@...gle.com>, Jonathan Corbet
<corbet@....net>, Kees Cook <keescook@...omium.org>, Lakshmi
Ramasubramanian <nramas@...ux.microsoft.com>, "Madhavan T . Venkataraman"
<madvenka@...ux.microsoft.com>, Matthew Garrett <mjg59@...gle.com>, Matthew
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Trébuchet <philippe.trebuchet@....gouv.fr>, Scott Shell
<scottsh@...rosoft.com>, Shuah Khan <shuah@...nel.org>, Steve Dower
<steve.dower@...hon.org>, Steve Grubb <sgrubb@...hat.com>, Thibaut
Sautereau <thibaut.sautereau@....gouv.fr>, Vincent Strubel
<vincent.strubel@....gouv.fr>, kernel-hardening@...ts.openwall.com,
linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-security-module@...r.kernel.org
Subject: Re: [PATCH v14 0/3] Add trusted_for(2) (was O_MAYEXEC)
On Mon, 11 Oct 2021 10:47:04 +0200 Mickaël Salaün <mic@...ikod.net> wrote:
>
> On 10/10/2021 23:48, Andrew Morton wrote:
> > On Fri, 8 Oct 2021 12:48:37 +0200 Mickaël Salaün <mic@...ikod.net> wrote:
> >
> >> The final goal of this patch series is to enable the kernel to be a
> >> global policy manager by entrusting processes with access control at
> >> their level. To reach this goal, two complementary parts are required:
> >> * user space needs to be able to know if it can trust some file
> >> descriptor content for a specific usage;
> >> * and the kernel needs to make available some part of the policy
> >> configured by the system administrator.
> >
> > Apologies if I missed this...
> >
> > It would be nice to see a description of the proposed syscall interface
> > in these changelogs! Then a few questions I have will be answered...
>
> I described this syscall and it's semantic in the first patch in
> Documentation/admin-guide/sysctl/fs.rst
Well, kinda. It didn't explain why the `usage' and `flags' arguments
exist and what are the plans for them.
> Do you want me to copy-paste this content in the cover letter?
That would be best please. It's basically the most important thing
when reviewing the implementation.
> >
> > long trusted_for(const int fd,
> > const enum trusted_for_usage usage,
> > const u32 flags)
> >
> > - `usage' must be equal to TRUSTED_FOR_EXECUTION, so why does it
> > exist? Some future modes are planned? Please expand on this.
>
> Indeed, the current use case is to check if the kernel would allow
> execution of a file. But as Florian pointed out, we may want to add more
> context in the future, e.g. to enforce signature verification, to check
> if this is a legitimate (system) library, to check if the file is
> allowed to be used as (trusted) configuration…
>
> >
> > - `flags' is unused (must be zero). So why does it exist? What are
> > the plans here?
>
> This is mostly to follow syscall good practices for extensibility. It
> could be used in combination with the usage argument (which defines the
> user space semantic), e.g. to check for extra properties such as
> cryptographic or integrity requirements, origin of the file…
>
> >
> > - what values does the syscall return and what do they mean?
> >
>
> It returns 0 on success, or -EACCES if the kernel policy denies the
> specified usage.
And please document all of this in the changelog also.
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