Re: [PATCH] exec: Check __FMODE_EXEC instead of in_execve for LSMs

[Linus Torvalds]

On Wed, 24 Jan 2024 at 12:15, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> Hmpf, and frustratingly Ubuntu (and Debian) still builds with
> CONFIG_USELIB, even though it was reported[2] to them almost 4 years ago.

Well, we could just remove the __FMODE_EXEC from uselib.

It's kind of wrong anyway.

Unlike a real execve(), where the target executable actually takes
control and you can't actually control it (except with ptrace, of
course), 'uselib()' really is just a wrapper around a special mmap.

And you can see it in the "acc_mode" flags: uselib already requires
MAY_READ for that reason. So you cannot uselib() a non-readable file,
unlike execve().

So I think just removing __FMODE_EXEC would just do the
RightThing(tm), and changes nothing for any sane situation.

In fact, I don't think __FMODE_EXEC really ever did anything for the
uselib() case, so removing it *really* shouldn't matter, and only fix
the new AppArmor / Tomoyo use.

Of course, as you say, not having CONFIG_USELIB enabled at all is the
_truly_ sane thing, but the only thing that used the FMODE_EXEC bit
were landlock and some special-case nfs stuff.

And at least the nfs stuff was about "don't require read permissions
for exec", which was already wrong for the uselib() case as per above.

So I think the simple oneliner is literally just

  --- a/fs/exec.c
  +++ b/fs/exec.c
  @@ -128,7 +128,7 @@ SYSCALL_DEFINE1(uselib, const char __user *, library)
        struct filename *tmp = getname(library);
        int error = PTR_ERR(tmp);
        static const struct open_flags uselib_flags = {
  -             .open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
  +             .open_flag = O_LARGEFILE | O_RDONLY,
                .acc_mode = MAY_READ | MAY_EXEC,
                .intent = LOOKUP_OPEN,
                .lookup_flags = LOOKUP_FOLLOW,

but I obviously have nothing that uses uselib(). I don't see how it
really *could* break anything, though, exactly because of that

                .acc_mode = MAY_READ | MAY_EXEC,

that means that the *regular* permission checks already require the
file to be readable. Never mind any LSM checks that might be confused.

           Linus