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
