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 Fri, May 15, 2020 at 04:43:37PM +0200, Florian Weimer wrote:
> * Kees Cook:
> 
> > On Fri, May 15, 2020 at 10:43:34AM +0200, Florian Weimer wrote:
> >> * Kees Cook:
> >> 
> >> > 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 think POSIX specifies O_EXEC in such a way that it does not confer
> >> read permissions.  This seems incompatible with what we are trying to
> >> achieve here.
> >
> > I was trying to retain this behavior, since we already make this
> > distinction between execve() and uselib() with the MAY_* flags:
> >
> > execve():
> >         struct open_flags open_exec_flags = {
> >                 .open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
> >                 .acc_mode = MAY_EXEC,
> >
> > uselib():
> >         static const struct open_flags uselib_flags = {
> >                 .open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
> >                 .acc_mode = MAY_READ | MAY_EXEC,
> >
> > I tried to retain this in my proposal, in the O_EXEC does not imply
> > MAY_READ:
> 
> That doesn't quite parse for me, sorry.
> 
> The point is that the script interpreter actually needs to *read* those
> files in order to execute them.

I think I misunderstood what you meant (Mickaël got me sorted out
now). If O_EXEC is already meant to be "EXEC and _not_ READ nor WRITE",
then yes, this new flag can't be O_EXEC. I was reading the glibc
documentation (which treats it as a permission bit flag, not POSIX,
which treats it as a complete mode description).

-- 
Kees Cook