From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm1-f67.google.com ([209.85.128.67]:39353 "EHLO mail-wm1-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726348AbeJHW2v (ORCPT ); Mon, 8 Oct 2018 18:28:51 -0400 Received: by mail-wm1-f67.google.com with SMTP id y144-v6so4199011wmd.4 for ; Mon, 08 Oct 2018 08:16:37 -0700 (PDT) Date: Mon, 8 Oct 2018 17:16:30 +0200 From: Christian Brauner To: Tycho Andersen Cc: Kees Cook , Jann Horn , linux-api@vger.kernel.org, containers@lists.linux-foundation.org, Akihiro Suda , Oleg Nesterov , linux-kernel@vger.kernel.org, "Eric W . Biederman" , linux-fsdevel@vger.kernel.org, Christian Brauner , Andy Lutomirski Subject: Re: [PATCH v7 3/6] seccomp: add a way to get a listener fd from ptrace Message-ID: <20181008151629.hkgzzsluevwtuclw@brauner.io> References: <20180927151119.9989-1-tycho@tycho.ws> <20180927151119.9989-4-tycho@tycho.ws> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20180927151119.9989-4-tycho@tycho.ws> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: 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? > + > + filter = get_nth_filter(task, filter_off); > + if (IS_ERR(filter)) > + return PTR_ERR(filter); > + > + fd = get_unused_fd_flags(0); > + if (fd < 0) { > + __put_seccomp_filter(filter); > + return fd; > + } > + > + listener = init_listener(task, filter); > + __put_seccomp_filter(filter); > + if (IS_ERR(listener)) { > + put_unused_fd(fd); > + return PTR_ERR(listener); > + } > + > + fd_install(fd, listener); > + return fd; > +} > #endif > diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c > index 5f4b836a6792..c6ba3ed5392e 100644 > --- a/tools/testing/selftests/seccomp/seccomp_bpf.c > +++ b/tools/testing/selftests/seccomp/seccomp_bpf.c > @@ -193,6 +193,10 @@ int seccomp(unsigned int op, unsigned int flags, void *args) > } > #endif > > +#ifndef PTRACE_SECCOMP_NEW_LISTENER > +#define PTRACE_SECCOMP_NEW_LISTENER 0x420e > +#endif > + > #if __BYTE_ORDER == __LITTLE_ENDIAN > #define syscall_arg(_n) (offsetof(struct seccomp_data, args[_n])) > #elif __BYTE_ORDER == __BIG_ENDIAN > @@ -3175,6 +3179,70 @@ TEST(get_user_notification_syscall) > EXPECT_EQ(0, WEXITSTATUS(status)); > } > > +TEST(get_user_notification_ptrace) > +{ > + pid_t pid; > + int status, listener; > + int sk_pair[2]; > + char c; > + struct seccomp_notif req = {}; > + struct seccomp_notif_resp resp = {}; > + > + ASSERT_EQ(socketpair(PF_LOCAL, SOCK_SEQPACKET, 0, sk_pair), 0); > + > + pid = fork(); > + ASSERT_GE(pid, 0); > + > + if (pid == 0) { > + EXPECT_EQ(user_trap_syscall(__NR_getpid, 0), 0); > + > + /* Test that we get ENOSYS while not attached */ > + EXPECT_EQ(syscall(__NR_getpid), -1); > + EXPECT_EQ(errno, ENOSYS); > + > + /* Signal we're ready and have installed the filter. */ > + EXPECT_EQ(write(sk_pair[1], "J", 1), 1); > + > + EXPECT_EQ(read(sk_pair[1], &c, 1), 1); > + EXPECT_EQ(c, 'H'); > + > + exit(syscall(__NR_getpid) != USER_NOTIF_MAGIC); > + } > + > + EXPECT_EQ(read(sk_pair[0], &c, 1), 1); > + EXPECT_EQ(c, 'J'); > + > + EXPECT_EQ(ptrace(PTRACE_ATTACH, pid), 0); > + EXPECT_EQ(waitpid(pid, NULL, 0), pid); > + listener = ptrace(PTRACE_SECCOMP_NEW_LISTENER, pid, 0); > + EXPECT_GE(listener, 0); > + > + /* EBUSY for second listener */ > + EXPECT_EQ(ptrace(PTRACE_SECCOMP_NEW_LISTENER, pid, 0), -1); > + EXPECT_EQ(errno, EBUSY); > + > + EXPECT_EQ(ptrace(PTRACE_DETACH, pid, NULL, 0), 0); > + > + /* Now signal we are done and respond with magic */ > + EXPECT_EQ(write(sk_pair[0], "H", 1), 1); > + > + req.len = sizeof(req); > + EXPECT_EQ(ioctl(listener, SECCOMP_NOTIF_RECV, &req), sizeof(req)); > + > + resp.len = sizeof(resp); > + resp.id = req.id; > + resp.error = 0; > + resp.val = USER_NOTIF_MAGIC; > + > + EXPECT_EQ(ioctl(listener, SECCOMP_NOTIF_SEND, &resp), sizeof(resp)); > + > + EXPECT_EQ(waitpid(pid, &status, 0), pid); > + EXPECT_EQ(true, WIFEXITED(status)); > + EXPECT_EQ(0, WEXITSTATUS(status)); > + > + close(listener); > +} > + > /* > * Check that a pid in a child namespace still shows up as valid in ours. > */ > -- > 2.17.1 > > _______________________________________________ > Containers mailing list > Containers@lists.linux-foundation.org > https://lists.linuxfoundation.org/mailman/listinfo/containers