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. > > > which by default is 0. So C won't pass > > (get_dumpable(mm) != SUID_DUMP_USER). > > In both cases PTRACE_ATTACH shouldn't work. Now, if > > /proc/sys/fs/suid_dumpable is 1 I'd find it acceptable for this to work. > > This is an administrator choice.
