Re: [RFC PATCH v19 1/5] exec: Add a new AT_CHECK flag to execveat(2)

Mickaël Salaün
2024-07-18T15:42:41Z

On Thu, Jul 18, 2024 at 09:03:36AM -0400, James Bottomley wrote:
> On Thu, 2024-07-18 at 14:24 +0200, Mickaël Salaün wrote:
> > On Wed, Jul 17, 2024 at 07:08:17PM -0700, Jeff Xu wrote:
> > > On Wed, Jul 17, 2024 at 3:01 AM Mickaël Salaün <mic@digikod.net>
> > > wrote:
> > > > On Tue, Jul 16, 2024 at 11:33:55PM -0700, Jeff Xu wrote:
> [...]
> > > > > I'm still thinking  execveat(AT_CHECK) vs faccessat(AT_CHECK)
> > > > > in different use cases:
> > > > >
> > > > > execveat clearly has less code change, but that also means: we
> > > > > can't add logic specific to exec (i.e. logic that can't be
> > > > > applied to config) for this part (from do_execveat_common to
> > > > > security_bprm_creds_for_exec) in future.  This would require
> > > > > some agreement/sign-off, I'm not sure from whom.
> > > >
> > > > I'm not sure to follow. We could still add new flags, but for now
> > > > I don't see use cases.  This patch series is not meant to handle
> > > > all possible "trust checks", only executable code, which makes
> > > > sense for the kernel.
> > > >
> > > I guess the "configfile" discussion is where I get confused, at one
> > > point, I think this would become a generic "trust checks" api for
> > > everything related to "generating executable code", e.g.
> > > javascript, java code, and more. We will want to clearly define the
> > > scope of execveat(AT_CHECK)
> >
> > The line between data and code is blurry.  For instance, a
> > configuration file can impact the execution flow of a program.  So,
> > where to draw the line?
>
> Having a way to have config files part of the trusted envelope, either
> by signing or measurement would be really useful. The current standard
> distro IMA deployment is signed executables, but not signed config
> because it's hard to construct a policy that doesn't force the signing
> of too many extraneous files (and files which might change often).
>
> > It might makes sense to follow the kernel and interpreter semantic:
> > if a file can be executed by the kernel (e.g. ELF binary, file
> > containing a shebang, or just configured with binfmt_misc), then this
> > should be considered as executable code.  This applies to Bash,
> > Python, Javascript, NodeJS, PE, PHP...  However, we can also make a
> > picture executable with binfmt_misc.  So, again, where to draw the
> > line?
>
> Possibly by making open for config an indication executables can give?
> I'm not advocating doing it in this patch, but if we had an open for
> config indication, the LSMs could do much finer grained policy,
> especially if they knew which executable was trying to open the config
> file. It would allow things like an IMA policy saying if a signed
> executable is opening a config file, then that file must also be
> signed.

Checking configuration could be a next step, but not with this patch
series. FYI, the previous version was a (too) generic syscall:
https://lore.kernel.org/all/20220104155024.48023-1-mic@digikod.net/
One of the main concern was alignment with kernel semantic. For now,
let's focus on script execution control.

>
> James
>