Re: [PATCH v14 0/3] Add trusted_for(2) (was O_MAYEXEC)

Andrew Morton
2021-10-11T21:08:02Z

On Mon, 11 Oct 2021 10:47:04 +0200 Mickaël Salaün <mic@digikod.net> wrote:

>
> On 10/10/2021 23:48, Andrew Morton wrote:
> > On Fri, 8 Oct 2021 12:48:37 +0200 Mickaël Salaün <mic@digikod.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.