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 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:

+	/* Should execution permissions be checked on open? */
+	if (flags & O_EXEC) {
+		flags |= __FMODE_EXEC;
+		acc_mode |= MAY_EXEC;
+	}

-- 
Kees Cook