Re: [PATCH 0/4] Relocate execve() sanity checks

[Kees Cook <keescook@chromium.org>]

On Tue, May 19, 2020 at 10:06:32AM -0500, Eric W. Biederman wrote:
> Kees Cook <keescook@chromium.org> writes:
> 
> > Hi,
> >
> > While looking at the code paths for the proposed O_MAYEXEC flag, I saw
> > some things that looked like they should be fixed up.
> >
> >   exec: Change uselib(2) IS_SREG() failure to EACCES
> > 	This just regularizes the return code on uselib(2).
> >
> >   exec: Relocate S_ISREG() check
> > 	This moves the S_ISREG() check even earlier than it was already.
> >
> >   exec: Relocate path_noexec() check
> > 	This adds the path_noexec() check to the same place as the
> > 	S_ISREG() check.
> >
> >   fs: Include FMODE_EXEC when converting flags to f_mode
> > 	This seemed like an oversight, but I suspect there is some
> > 	reason I couldn't find for why FMODE_EXEC doesn't get set in
> > 	f_mode and just stays in f_flags.
> 
> So I took a look at this series.
> 
> I think the belt and suspenders approach of adding code in open and then
> keeping it in exec and uselib is probably wrong.  My sense of the
> situation is a belt and suspenders approach is more likely to be
> confusing and result in people making mistakes when maintaining the code
> than to actually be helpful.

This is why I added the comments in fs/exec.c's redundant checks. When I
was originally testing this series, I had entirely removed the checks in
fs/exec.c, but then had nightmares about some kind of future VFS paths
that would somehow bypass do_open() and result in execve() working on
noexec mounts, there by allowing for the introduction of a really nasty
security bug.

The S_ISREG test is demonstrably too late (as referenced in the series),
and given the LSM hooks, I think the noexec check is too late as well.
(This is especially true for the coming O_MAYEXEC series, which will
absolutely need those tests earlier as well[1] -- the permission checking
is then in the correct place: during open, not exec.) I think the only
question is about leaving the redundant checks in fs/exec.c, which I
think are a cheap way to retain a sense of robustness.

-Kees

[1] https://lore.kernel.org/lkml/202005142343.D580850@keescook/

-- 
Kees Cook