On Tue, Oct 9, 2018 at 6:20 PM Christian Brauner <christian@xxxxxxxxxx> wrote: > On Tue, Oct 09, 2018 at 05:26:26PM +0200, Jann Horn wrote: > > On Tue, Oct 9, 2018 at 4:09 PM Christian Brauner <christian@xxxxxxxxxx> wrote: > > > On Tue, Oct 09, 2018 at 03:50:53PM +0200, Jann Horn wrote: > > > > On Tue, Oct 9, 2018 at 3:49 PM Christian Brauner <christian@xxxxxxxxxx> wrote: > > > > > On Tue, Oct 09, 2018 at 03:36:04PM +0200, Jann Horn wrote: > > > > > > On Tue, Oct 9, 2018 at 3:29 PM Christian Brauner <christian@xxxxxxxxxx> wrote: > > > > > > > One more thing. Citing from [1] > > > > > > > > > > > > > > > I think there's a security problem here. Imagine the following scenario: > > > > > > > > > > > > > > > > 1. task A (uid==0) sets up a seccomp filter that uses SECCOMP_RET_USER_NOTIF > > > > > > > > 2. task A forks off a child B > > > > > > > > 3. task B uses setuid(1) to drop its privileges > > > > > > > > 4. task B becomes dumpable again, either via prctl(PR_SET_DUMPABLE, 1) > > > > > > > > or via execve() > > > > > > > > 5. task C (the attacker, uid==1) attaches to task B via ptrace > > > > > > > > 6. task C uses PTRACE_SECCOMP_NEW_LISTENER on task B > > > > > > > > > > > > > > Sorry, to be late to the party but would this really pass > > > > > > > __ptrace_may_access() in ptrace_attach()? It doesn't seem obvious to me > > > > > > > that it would... Doesn't look like it would get past: > > > > > > > > > > > > > > tcred = __task_cred(task); > > > > > > > if (uid_eq(caller_uid, tcred->euid) && > > > > > > > uid_eq(caller_uid, tcred->suid) && > > > > > > > uid_eq(caller_uid, tcred->uid) && > > > > > > > gid_eq(caller_gid, tcred->egid) && > > > > > > > gid_eq(caller_gid, tcred->sgid) && > > > > > > > gid_eq(caller_gid, tcred->gid)) > > > > > > > goto ok; > > > > > > > if (ptrace_has_cap(tcred->user_ns, mode)) > > > > > > > goto ok; > > > > > > > rcu_read_unlock(); > > > > > > > return -EPERM; > > > > > > > ok: > > > > > > > rcu_read_unlock(); > > > > > > > mm = task->mm; > > > > > > > if (mm && > > > > > > > ((get_dumpable(mm) != SUID_DUMP_USER) && > > > > > > > !ptrace_has_cap(mm->user_ns, mode))) > > > > > > > return -EPERM; > > > > > > > > > > > > Which specific check would prevent task C from attaching to task B? If > > > > > > the UIDs match, the first "goto ok" executes; and you're dumpable, so > > > > > > you don't trigger the second "return -EPERM". > > > > > > > > > > You'd also need CAP_SYS_PTRACE in the mm->user_ns which you shouldn't > > > > > have if you did a setuid to an unpriv user. (But I always find that code > > > > > confusing.) > > > > > > > > Only if the target hasn't gone through execve() since setuid(). > > > > > > Sorry if I want to know this in excessive detail but I'd like to > > > understand this properly so bear with me :) > > > - If task B has setuid()ed and prctl(PR_SET_DUMPABLE, 1)ed but not > > > execve()ed then C won't pass ptrace_has_cap(mm->user_ns, mode). > > > > Yeah. > > > > > - If task B has setuid()ed, exeved()ed it will get its dumpable flag set > > > to /proc/sys/fs/suid_dumpable > > > > Not if you changed all UIDs (e.g. by calling setuid() as root). In > > that case, setup_new_exec() calls "set_dumpable(current->mm, > > SUID_DUMP_USER)". > > Actually, looking at this when C is trying to PTRACE_ATTACH to B as an > unprivileged user even if B execve()ed and it is dumpable C still > wouldn't have CAP_SYS_PTRACE in the mm->user_ns unless it already is > privileged over mm->user_ns which means it must be in an ancestor > user_ns. Huh? Why would you need CAP_SYS_PTRACE for anything here? You can ptrace another process running under your UID just fine, no matter what the namespaces are. I'm not sure what you're saying. _______________________________________________ Selinux mailing list Selinux@xxxxxxxxxxxxx To unsubscribe, send email to Selinux-leave@xxxxxxxxxxxxx. To get help, send an email containing "help" to Selinux-request@xxxxxxxxxxxxx.
