How about just O_EXEC? (was Re: [PATCH v5 3/6] fs: Enable to enforce noexec mounts or file exec through O_MAYEXEC)

[Kees Cook]

On Thu, May 14, 2020 at 09:16:13PM +0200, Mickaël Salaün wrote:
> On 14/05/2020 18:10, Stephen Smalley wrote:
> > On Thu, May 14, 2020 at 11:45 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> >> So, it looks like adding FMODE_EXEC into f_flags in do_open() is needed in
> >> addition to injecting MAY_EXEC into acc_mode in do_open()? Hmmm
> > 
> > Just do both in build_open_flags() and be done with it? Looks like he
> > was already setting FMODE_EXEC in patch 1 so we just need to teach
> > AppArmor/TOMOYO to check for it and perform file execute checking in
> > that case if !current->in_execve?
> 
> I can postpone the file permission check for another series to make this
> one simpler (i.e. mount noexec only). Because it depends on the sysctl
> setting, it is OK to add this check later, if needed. In the meantime,
> AppArmor and Tomoyo could be getting ready for this.

So, after playing around with this series, investigating Stephen's
comments, digging through the existing FMODE_EXEC uses, and spending a
bit more time thinking about Lev and Aleksa's dislike of the sysctls, I've
got a much more radically simplified solution that I think could work.

Maybe I've missed some earlier discussion that ruled this out, but I
couldn't find it: let's just add O_EXEC and be done with it. It actually
makes the execve() path more like openat2() and is much cleaner after
a little refactoring. Here are the results, though I haven't emailed it
yet since I still want to do some more testing:
https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/log/?h=kspp/o_exec/v1

I look forward to flames! ;)

-- 
Kees Cook