[PATCH] exec: Correct the permission check for unsafe exec

[Jann Horn]

On Fri, May 16, 2025 at 11:46 PM <sergeh at kernel.org> wrote:
> On Fri, May 16, 2025 at 08:06:15PM +0200, Jann Horn wrote:
> > On Fri, May 16, 2025 at 5:26 PM Eric W. Biederman <ebiederm at xmission.com> wrote:
> > > Kees Cook <kees at kernel.org> writes:
> > >
> > > > On Thu, May 15, 2025 at 11:24:47AM -0500, Eric W. Biederman wrote:
> > > >> I have condensed the logic from Linux-2.4.0-test12 to just:
> > > >>      id_changed = !uid_eq(new->euid, old->euid) || !in_group_p(new->egid);
> > > >>
> > > >> This change is userspace visible, but I don't expect anyone to care.
> > > >> [...]
> > > >> -static inline bool __is_setuid(struct cred *new, const struct cred *old)
> > > >> -{ return !uid_eq(new->euid, old->uid); }
> > > >> -
> > > >> -static inline bool __is_setgid(struct cred *new, const struct cred *old)
> > > >> -{ return !gid_eq(new->egid, old->gid); }
> > > >> -
> > > >> [...]
> > > >> -    is_setid = __is_setuid(new, old) || __is_setgid(new, old);
> > > >> +    id_changed = !uid_eq(new->euid, old->euid) || !in_group_p(new->egid);
> > > >
> > > > The core change here is testing for differing euid rather than
> > > > mismatched uid/euid. (And checking for egid in the set of all groups.)
> > >
> > > Yes.
> > >
> > > For what the code is trying to do I can't fathom what was trying to
> > > be accomplished by the "mismatched" uid/euid check.
> >
> > I remember that when I was looking at this code years ago, one case I
> > was interested in was what happens when a setuid process (running with
> > something like euid=1000,ruid=0) execve()'s a normal binary. Clearly
> > the LSM_UNSAFE_* stuff is not so interesting there, because we're
> > already coming from a privileged context; but the behavior of
> > bprm->secureexec could be important.
> >
> > Like, I think currently a setuid binary like this is probably (?) not
> > exploitable:
> >
> > int main(void) {
> >   execl("/bin/echo", "echo", "hello world");
> > }
> >
> > but after your proposed change, I think it might (?) become
> > exploitable because "echo" would not have AT_SECURE set (I think?) and
> > would therefore load libraries based on environment variables?
> >
> > To be clear, I think this would be a stupid thing for userspace to do
> > - a setuid binary just should not be running other binaries with the
> > caller-provided environment while having elevated privileges. But if
> > userspace was doing something like that, this change might make it
> > more exploitable, and I imagine that the check for mismatched uid/euid
> > was intended to catch cases like this?
>
> If the original process became privileged by executing a setuid-root
> file (and uses glibc), then LD_PRELOAD will already have been cleared,
> right?  So it would either have to add the unsafe entries back to
> LD_PRELOAD again, or it has to have been root all along, not a
> setuid-root program.  I think at that point we have to say this is what
> it intended, and possibly with good reason.

Oh, I see what you mean, glibc's loader code zaps that environment
variable on secureexec for additional safety, I didn't know that.