On Wed, Oct 10, 2018 at 03:10:11PM +0200, Jann Horn wrote: > On Wed, Oct 10, 2018 at 2:54 PM Christian Brauner <christian@xxxxxxxxxx> wrote: > > On Tue, Oct 09, 2018 at 06:26:47PM +0200, Jann Horn wrote: > > > 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. > > > > Sorry, I was out the door yesterday when answering this and was too > > brief. I forgot to mention: /proc/sys/kernel/yama/ptrace_scope. It > > should be enabled by default on nearly all distros > > "nearly all distros"? AFAIK it's off on Debian, for starters. And Yama > still doesn't help you if one of the tasks enters a new user namespace > or whatever. > > Yama is a little bit of extra, heuristic, **opt-in** hardening enabled > in some configurations. It is **not** a fundamental building block you > can rely on. > > > and even if not - > > which is an administrators choice - you can usually easily enable it via > > sysctl. > > Opt-in security isn't good enough. Kernel interfaces should still be > safe to use even on a system that has all the LSM stuff disabled in > the kernel config. Then ptrace() isn't, I guess? But see https://lists.linuxfoundation.org/pipermail/containers/2018-October/039567.html I don't care as long as we have a way of getting the fd without the CAP_SYS_ADMIN requirement throught seccomp(). > > > 1 ("restricted ptrace") [default value] > > When performing an operation that requires a PTRACE_MODE_ATTACH check, > > the calling process must either have the CAP_SYS_PTRACE capability in > > the user namespace of the target process or it must have a prede‐ fined > > relationship with the target process. By default, the predefined > > relationship is that the target process must be a descendant of the > > caller. > > > > If you don't have it set you're already susceptible to all kinds of > > other attacks > > Oh? Can you be more specific, please? I was referring to attacks where you attach to processes that run as your user but might expose in-memory credentials or other sensitive information, (essentially what the manpage is outlining). > > > and I'm still not convinced we need to bring out the big > > capable(CAP_SYS_ADMIN) gun here.
