From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ot1-f66.google.com ([209.85.210.66]:37710 "EHLO mail-ot1-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727747AbeJHWqG (ORCPT ); Mon, 8 Oct 2018 18:46:06 -0400 Received: by mail-ot1-f66.google.com with SMTP id o14so4724327oth.4 for ; Mon, 08 Oct 2018 08:33:49 -0700 (PDT) MIME-Version: 1.0 References: <20180927151119.9989-1-tycho@tycho.ws> <20180927151119.9989-4-tycho@tycho.ws> <20181008151629.hkgzzsluevwtuclw@brauner.io> In-Reply-To: <20181008151629.hkgzzsluevwtuclw@brauner.io> From: Jann Horn Date: Mon, 8 Oct 2018 17:33:22 +0200 Message-ID: Subject: Re: [PATCH v7 3/6] seccomp: add a way to get a listener fd from ptrace To: christian@brauner.io Cc: Tycho Andersen , Kees Cook , Linux API , containers@lists.linux-foundation.org, suda.akihiro@lab.ntt.co.jp, Oleg Nesterov , kernel list , "Eric W. Biederman" , linux-fsdevel@vger.kernel.org, Christian Brauner , Andy Lutomirski Content-Type: text/plain; charset="UTF-8" Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Mon, Oct 8, 2018 at 5:16 PM Christian Brauner wrote: > > On Thu, Sep 27, 2018 at 09:11:16AM -0600, Tycho Andersen wrote: > > As an alternative to SECCOMP_FILTER_FLAG_GET_LISTENER, perhaps a ptrace() > > version which can acquire filters is useful. There are at least two reasons > > this is preferable, even though it uses ptrace: > > > > 1. You can control tasks that aren't cooperating with you > > 2. You can control tasks whose filters block sendmsg() and socket(); if the > > task installs a filter which blocks these calls, there's no way with > > SECCOMP_FILTER_FLAG_GET_LISTENER to get the fd out to the privileged task. > > So for the slow of mind aka me: > I'm not sure I completely understand this problem. Can you outline how > sendmsg() and socket() are involved in this? > > I'm also not sure that this holds (but I might misunderstand the > problem) afaict, you could do try to get the fd out via CLONE_FILES and > other means so something like: > > // let's pretend the libc wrapper for clone actually has sane semantics > pid = clone(CLONE_FILES); > if (pid == 0) { > fd = seccomp(SECCOMP_SET_MODE_FILTER, SECCOMP_FILTER_FLAG_NEW_LISTENER, &prog); > > // Now this fd will be valid in both parent and child. > // If you haven't blocked it you can inform the parent what > // the fd number is via pipe2(). If you have blocked it you can > // use dup2() and dup to a known fd number. > } > > > > > v2: fix a bug where listener mode was not unset when an unused fd was not > > available > > v3: fix refcounting bug (Oleg) > > v4: * change the listener's fd flags to be 0 > > * rename GET_LISTENER to NEW_LISTENER (Matthew) > > v5: * add capable(CAP_SYS_ADMIN) requirement > > v7: * point the new listener at the right filter (Jann) > > > > Signed-off-by: Tycho Andersen > > CC: Kees Cook > > CC: Andy Lutomirski > > CC: Oleg Nesterov > > CC: Eric W. Biederman > > CC: "Serge E. Hallyn" > > CC: Christian Brauner > > CC: Tyler Hicks > > CC: Akihiro Suda > > --- > > include/linux/seccomp.h | 7 ++ > > include/uapi/linux/ptrace.h | 2 + > > kernel/ptrace.c | 4 ++ > > kernel/seccomp.c | 31 +++++++++ > > tools/testing/selftests/seccomp/seccomp_bpf.c | 68 +++++++++++++++++++ > > 5 files changed, 112 insertions(+) > > > > diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h > > index 017444b5efed..234c61b37405 100644 > > --- a/include/linux/seccomp.h > > +++ b/include/linux/seccomp.h > > @@ -83,6 +83,8 @@ static inline int seccomp_mode(struct seccomp *s) > > #ifdef CONFIG_SECCOMP_FILTER > > extern void put_seccomp_filter(struct task_struct *tsk); > > extern void get_seccomp_filter(struct task_struct *tsk); > > +extern long seccomp_new_listener(struct task_struct *task, > > + unsigned long filter_off); > > #else /* CONFIG_SECCOMP_FILTER */ > > static inline void put_seccomp_filter(struct task_struct *tsk) > > { > > @@ -92,6 +94,11 @@ static inline void get_seccomp_filter(struct task_struct *tsk) > > { > > return; > > } > > +static inline long seccomp_new_listener(struct task_struct *task, > > + unsigned long filter_off) > > +{ > > + return -EINVAL; > > +} > > #endif /* CONFIG_SECCOMP_FILTER */ > > > > #if defined(CONFIG_SECCOMP_FILTER) && defined(CONFIG_CHECKPOINT_RESTORE) > > diff --git a/include/uapi/linux/ptrace.h b/include/uapi/linux/ptrace.h > > index d5a1b8a492b9..e80ecb1bd427 100644 > > --- a/include/uapi/linux/ptrace.h > > +++ b/include/uapi/linux/ptrace.h > > @@ -73,6 +73,8 @@ struct seccomp_metadata { > > __u64 flags; /* Output: filter's flags */ > > }; > > > > +#define PTRACE_SECCOMP_NEW_LISTENER 0x420e > > + > > /* Read signals from a shared (process wide) queue */ > > #define PTRACE_PEEKSIGINFO_SHARED (1 << 0) > > > > diff --git a/kernel/ptrace.c b/kernel/ptrace.c > > index 21fec73d45d4..289960ac181b 100644 > > --- a/kernel/ptrace.c > > +++ b/kernel/ptrace.c > > @@ -1096,6 +1096,10 @@ int ptrace_request(struct task_struct *child, long request, > > ret = seccomp_get_metadata(child, addr, datavp); > > break; > > > > + case PTRACE_SECCOMP_NEW_LISTENER: > > + ret = seccomp_new_listener(child, addr); > > + break; > > + > > default: > > break; > > } > > diff --git a/kernel/seccomp.c b/kernel/seccomp.c > > index 44a31ac8373a..17685803a2af 100644 > > --- a/kernel/seccomp.c > > +++ b/kernel/seccomp.c > > @@ -1777,4 +1777,35 @@ static struct file *init_listener(struct task_struct *task, > > > > return ret; > > } > > + > > +long seccomp_new_listener(struct task_struct *task, > > + unsigned long filter_off) > > +{ > > + struct seccomp_filter *filter; > > + struct file *listener; > > + int fd; > > + > > + if (!capable(CAP_SYS_ADMIN)) > > + return -EACCES; > > I know this might have been discussed a while back but why exactly do we > require CAP_SYS_ADMIN in init_userns and not in the target userns? What > if I want to do a setns()fd, CLONE_NEWUSER) to the target process and > use ptrace from in there? See https://lore.kernel.org/lkml/CAG48ez3R+ZJ1vwGkDfGzKX2mz6f=jjJWsO5pCvnH68P+RKO8Ow@mail.gmail.com/ . Basically, the problem is that this doesn't just give you capability over the target task, but also over every other task that has the same filter installed; you need some sort of "is the caller capable over the filter and anyone who uses it" check.