Re: [PATCH] kernel/signal: Signal-based pre-coredump notification

[Jann Horn]

On Mon, Oct 15, 2018 at 8:36 PM Enke Chen <enkechen@xxxxxxxxx> wrote:
> On 10/13/18 11:27 AM, Jann Horn wrote:
> > On Sat, Oct 13, 2018 at 2:33 AM Enke Chen <enkechen@xxxxxxxxx> wrote:
> >> For simplicity and consistency, this patch provides an implementation
> >> for signal-based fault notification prior to the coredump of a child
> >> process. A new prctl command, PR_SET_PREDUMP_SIG, is defined that can
> >> be used by an application to express its interest and to specify the
> >> signal (SIGCHLD or SIGUSR1 or SIGUSR2) for such a notification. A new
> >> signal code (si_code), CLD_PREDUMP, is also defined for SIGCHLD.
> >
> > Your suggested API looks vaguely similar to PR_SET_PDEATHSIG, but with
> > some important differences:
> >
> >  - You don't reset the signal on setuid execution.
[...]
> >
> > For both of these: Are these differences actually necessary, and if
> > so, can you provide a specific rationale? From a security perspective,
> > I would very much prefer it if this API had semantics closer to
> > PR_SET_PDEATHSIG.
>
[...]
>
> Regarding the impact of "setuid", this property "PR_SET_PREDUMP_SIG" has to
> do with the application/process whether the signal handler is set for receiving
> such a notification.  If it is set, the "uid" should not matter.

If an attacker's process first calls PR_SET_PREDUMP_SIG, then forks
off a child, then calls execve() on a setuid binary, the setuid binary
calls setuid(0), and the attacker-controlled child then crashes, the
privileged process will receive an unexpected signal that the attacker
wouldn't have been allowed to send otherwise. For similar reasons, the
parent death signal is reset when a setuid binary is executed:

void setup_new_exec(struct linux_binprm * bprm)
{
        /*
         * Once here, prepare_binrpm() will not be called any more, so
         * the final state of setuid/setgid/fscaps can be merged into the
         * secureexec flag.
         */
        bprm->secureexec |= bprm->cap_elevated;

        if (bprm->secureexec) {
                /* Make sure parent cannot signal privileged process. */
                current->pdeath_signal = 0;
[...]
        }
[...]
}

int commit_creds(struct cred *new)
{
[...]
        /* dumpability changes */
        if (!uid_eq(old->euid, new->euid) ||
            !gid_eq(old->egid, new->egid) ||
            !uid_eq(old->fsuid, new->fsuid) ||
            !gid_eq(old->fsgid, new->fsgid) ||
            !cred_cap_issubset(old, new)) {
                if (task->mm)
                        set_dumpable(task->mm, suid_dumpable);
                task->pdeath_signal = 0;
                smp_wmb();
        }
[...]
}

AppArmor and SELinux also do related changes:

static void apparmor_bprm_committing_creds(struct linux_binprm *bprm)
{
[...]
        /* bail out if unconfined or not changing profile */
        if ((new_label->proxy == label->proxy) ||
            (unconfined(new_label)))
                return;

        aa_inherit_files(bprm->cred, current->files);

        current->pdeath_signal = 0;
[...]
}

static void selinux_bprm_committing_creds(struct linux_binprm *bprm)
{
[...]
        new_tsec = bprm->cred->security;
        if (new_tsec->sid == new_tsec->osid)
                return;

        /* Close files for which the new task SID is not authorized. */
        flush_unauthorized_files(bprm->cred, current->files);

        /* Always clear parent death signal on SID transitions. */
        current->pdeath_signal = 0;
[...]
}

You should probably reset the coredump signal in the same places - or
even better, add a new helper for resetting the parent death signal,
and then add code for resetting the coredump signal in there.