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Date: Mon, 19 May 2014 12:54:34 -0700
From: Kees Cook <keescook@...omium.org>
To: Andrew Morton <akpm@...ux-foundation.org>,
Greg KH <gregkh@...uxfoundation.org>
Cc: Oleg Nesterov <oleg@...hat.com>, mcgrathr@...omium.org,
Julien Tinnes <jln@...omium.org>, jan.kratochvil@...hat.com,
Matthew Dempsky <mdempsky@...omium.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: + ptrace-fix-fork-event-messages-across-pid-namespaces.patch
added to -mm tree
On Wed, Apr 30, 2014 at 1:17 PM, <akpm@...ux-foundation.org> wrote:
> Subject: + ptrace-fix-fork-event-messages-across-pid-namespaces.patch added to -mm tree
> To: mdempsky@...omium.org,jan.kratochvil@...hat.com,jln@...omium.org,keescook@...omium.org,mcgrathr@...omium.org,oleg@...hat.com
> From: akpm@...ux-foundation.org
> Date: Wed, 30 Apr 2014 13:17:17 -0700
>
>
> The patch titled
> Subject: ptrace: fix fork event messages across pid namespaces
> has been added to the -mm tree. Its filename is
> ptrace-fix-fork-event-messages-across-pid-namespaces.patch
>
> This patch should soon appear at
> http://ozlabs.org/~akpm/mmots/broken-out/ptrace-fix-fork-event-messages-across-pid-namespaces.patch
> and later at
> http://ozlabs.org/~akpm/mmotm/broken-out/ptrace-fix-fork-event-messages-across-pid-namespaces.patch
>
> Before you just go and hit "reply", please:
> a) Consider who else should be cc'ed
> b) Prefer to cc a suitable mailing list as well
> c) Ideally: find the original patch on the mailing list and do a
> reply-to-all to that, adding suitable additional cc's
>
> *** Remember to use Documentation/SubmitChecklist when testing your code ***
>
> The -mm tree is included into linux-next and is updated
> there every 3-4 working days
>
> ------------------------------------------------------
> From: Matthew Dempsky <mdempsky@...omium.org>
> Subject: ptrace: fix fork event messages across pid namespaces
>
> When tracing a process in another pid namespace, it's important for fork
> event messages to contain the child's pid as seen from the tracer's pid
> namespace, not the parent's. Otherwise, the tracer won't be able to
> correlate the fork event with later SIGTRAP signals it receives from the
> child.
>
> We still risk a race condition if a ptracer from a different pid namespace
> attaches after we compute the pid_t value. However, sending a bogus fork
> event message in this unlikely scenario is still a vast improvement over
> the status quo where we always send bogus fork event messages to debuggers
> in a different pid namespace than the forking process.
>
> Signed-off-by: Matthew Dempsky <mdempsky@...omium.org>
> Acked-by: Oleg Nesterov <oleg@...hat.com>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Julien Tinnes <jln@...omium.org>
> Cc: Roland McGrath <mcgrathr@...omium.org>
> Cc: Jan Kratochvil <jan.kratochvil@...hat.com>
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> ---
>
> include/linux/ptrace.h | 32 ++++++++++++++++++++++++++++++++
> kernel/fork.c | 10 +++++++---
> 2 files changed, 39 insertions(+), 3 deletions(-)
>
> diff -puN include/linux/ptrace.h~ptrace-fix-fork-event-messages-across-pid-namespaces include/linux/ptrace.h
> --- a/include/linux/ptrace.h~ptrace-fix-fork-event-messages-across-pid-namespaces
> +++ a/include/linux/ptrace.h
> @@ -5,6 +5,7 @@
> #include <linux/sched.h> /* For struct task_struct. */
> #include <linux/err.h> /* for IS_ERR_VALUE */
> #include <linux/bug.h> /* For BUG_ON. */
> +#include <linux/pid_namespace.h> /* For task_active_pid_ns. */
> #include <uapi/linux/ptrace.h>
>
> /*
> @@ -129,6 +130,37 @@ static inline void ptrace_event(int even
> }
>
> /**
> + * ptrace_event_pid - possibly stop for a ptrace event notification
> + * @event: %PTRACE_EVENT_* value to report
> + * @pid: process identifier for %PTRACE_GETEVENTMSG to return
> + *
> + * Check whether @event is enabled and, if so, report @event and @pid
> + * to the ptrace parent. @pid is reported as the pid_t seen from the
> + * the ptrace parent's pid namespace.
> + *
> + * Called without locks.
> + */
> +static inline void ptrace_event_pid(int event, struct pid *pid)
> +{
> + /*
> + * FIXME: There's a potential race if a ptracer in a different pid
> + * namespace than parent attaches between computing message below and
> + * when we acquire tasklist_lock in ptrace_stop(). If this happens,
> + * the ptracer will get a bogus pid from PTRACE_GETEVENTMSG.
> + */
> + unsigned long message = 0;
> + struct pid_namespace *ns;
> +
> + rcu_read_lock();
> + ns = task_active_pid_ns(rcu_dereference(current->parent));
> + if (ns)
> + message = pid_nr_ns(pid, ns);
> + rcu_read_unlock();
> +
> + ptrace_event(event, message);
> +}
> +
> +/**
> * ptrace_init_task - initialize ptrace state for a new child
> * @child: new child task
> * @ptrace: true if child should be ptrace'd by parent's tracer
> diff -puN kernel/fork.c~ptrace-fix-fork-event-messages-across-pid-namespaces kernel/fork.c
> --- a/kernel/fork.c~ptrace-fix-fork-event-messages-across-pid-namespaces
> +++ a/kernel/fork.c
> @@ -1606,10 +1606,12 @@ long do_fork(unsigned long clone_flags,
> */
> if (!IS_ERR(p)) {
> struct completion vfork;
> + struct pid *pid;
>
> trace_sched_process_fork(current, p);
>
> - nr = task_pid_vnr(p);
> + pid = get_task_pid(p, PIDTYPE_PID);
> + nr = pid_vnr(pid);
>
> if (clone_flags & CLONE_PARENT_SETTID)
> put_user(nr, parent_tidptr);
> @@ -1624,12 +1626,14 @@ long do_fork(unsigned long clone_flags,
>
> /* forking complete and child started to run, tell ptracer */
> if (unlikely(trace))
> - ptrace_event(trace, nr);
> + ptrace_event_pid(trace, pid);
>
> if (clone_flags & CLONE_VFORK) {
> if (!wait_for_vfork_done(p, &vfork))
> - ptrace_event(PTRACE_EVENT_VFORK_DONE, nr);
> + ptrace_event_pid(PTRACE_EVENT_VFORK_DONE, pid);
> }
> +
> + put_pid(pid);
> } else {
> nr = PTR_ERR(p);
> }
> _
>
> Patches currently in -mm which might be from mdempsky@...omium.org are
>
> ptrace-fix-fork-event-messages-across-pid-namespaces.patch
>
Could this be considered for -stable as well? It fixes an old bug in
ptracing processes in pid namespaces.
-Kees
--
Kees Cook
Chrome OS Security
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