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Message-ID: <CABqD9hbCt97rW72b6pf1Gst-A9uN2Ap+q2fZgjqaYn3pg8toeQ@mail.gmail.com>
Date: Tue, 14 Jan 2014 14:59:26 -0600
From: Will Drewry <wad@...omium.org>
To: Andy Lutomirski <luto@...capital.net>
Cc: LKML <linux-kernel@...r.kernel.org>,
Nicolas Schichan <nschichan@...ebox.fr>,
Kees Cook <keescook@...omium.org>,
James Morris <james.l.morris@...cle.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Oleg Nesterov <oleg@...hat.com>,
Alexander Viro <viro@...iv.linux.org.uk>,
Julien Tinnes <jln@...omium.org>
Subject: Re: [PATCH 2/2] sys, seccomp: add PR_SECCOMP_EXT and SECCOMP_EXT_ACT_TSYNC
On Tue, Jan 14, 2014 at 2:24 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Tue, Jan 14, 2014 at 10:59 AM, Will Drewry <wad@...omium.org> wrote:
>> On Mon, Jan 13, 2014 at 5:36 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>> On 01/13/2014 12:30 PM, Will Drewry wrote:
>>>> Applying restrictive seccomp filter programs to large or diverse
>>>> codebases often requires handling threads which may be started early in
>>>> the process lifetime (e.g., by code that is linked in). While it is
>>>> possible to apply permissive programs prior to process start up, it is
>>>> difficult to further restrict the kernel ABI to those threads after that
>>>> point.
>>>>
>>>> This change adds a new seccomp "extension" for synchronizing thread
>>>> group seccomp filters and a prctl() for accessing that functionality.
>>>> The need for the added prctl() is due to the lack of reserved arguments
>>>> in PR_SET_SECCOMP.
>>>>
>>>> When prctl(PR_SECCOMP_EXT, SECCOMP_EXT_ACT_TSYNC, 0, 0) is called, it
>>>> will attempt to synchronize all threads in current's threadgroup to its
>>>> seccomp filter program. This is possible iff all threads are using a
>>>> filter that is an ancestor to the filter current is attempting to
>>>> synchronize to. NULL filters (where the task is running as
>>>> SECCOMP_MODE_NONE) are also treated as ancestors allowing threads to be
>>>> transitioned into SECCOMP_MODE_FILTER. On success, 0 is returned. On
>>>> failure, the pid of one of the failing threads will be returned.
>>>>
>>>> Suggested-by: Julien Tinnes <jln@...omium.org>
>>>> Signed-off-by: Will Drewry <wad@...omium.org>
>>>> ---
>>>> include/linux/seccomp.h | 7 +++
>>>> include/uapi/linux/prctl.h | 6 ++
>>>> include/uapi/linux/seccomp.h | 6 ++
>>>> kernel/seccomp.c | 128 ++++++++++++++++++++++++++++++++++++++++++
>>>> kernel/sys.c | 3 +
>>>> 5 files changed, 150 insertions(+)
>>>>
>>>> diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
>>>> index 85c0895..3163db6 100644
>>>> --- a/include/linux/seccomp.h
>>>> +++ b/include/linux/seccomp.h
>>>> @@ -77,6 +77,8 @@ static inline int seccomp_mode(struct seccomp *s)
>>>> extern void put_seccomp_filter(struct task_struct *tsk);
>>>> extern void get_seccomp_filter(struct task_struct *tsk);
>>>> extern u32 seccomp_bpf_load(int off);
>>>> +extern long prctl_seccomp_ext(unsigned long, unsigned long,
>>>> + unsigned long, unsigned long);
>>>> #else /* CONFIG_SECCOMP_FILTER */
>>>> static inline void put_seccomp_filter(struct task_struct *tsk)
>>>> {
>>>> @@ -86,5 +88,10 @@ static inline void get_seccomp_filter(struct task_struct *tsk)
>>>> {
>>>> return;
>>>> }
>>>> +static inline long prctl_seccomp_ext(unsigned long arg2, unsigned long arg3,
>>>> + unsigned long arg4, unsigned long arg5)
>>>> +{
>>>> + return -EINVAL;
>>>> +}
>>>> #endif /* CONFIG_SECCOMP_FILTER */
>>>> #endif /* _LINUX_SECCOMP_H */
>>>> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
>>>> index 289760f..5dcd5d3 100644
>>>> --- a/include/uapi/linux/prctl.h
>>>> +++ b/include/uapi/linux/prctl.h
>>>> @@ -149,4 +149,10 @@
>>>>
>>>> #define PR_GET_TID_ADDRESS 40
>>>>
>>>> +/*
>>>> + * Access seccomp extensions
>>>> + * See Documentation/prctl/seccomp_filter.txt for more details.
>>>> + */
>>>> +#define PR_SECCOMP_EXT 41
>>>> +
>>>> #endif /* _LINUX_PRCTL_H */
>>>> diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
>>>> index ac2dc9f..49b5279 100644
>>>> --- a/include/uapi/linux/seccomp.h
>>>> +++ b/include/uapi/linux/seccomp.h
>>>> @@ -10,6 +10,12 @@
>>>> #define SECCOMP_MODE_STRICT 1 /* uses hard-coded filter. */
>>>> #define SECCOMP_MODE_FILTER 2 /* uses user-supplied filter. */
>>>>
>>>> +/* Valid extension types as arg2 for prctl(PR_SECCOMP_EXT) */
>>>> +#define SECCOMP_EXT_ACT 1
>>>> +
>>>> +/* Valid extension actions as arg3 to prctl(PR_SECCOMP_EXT, SECCOMP_EXT_ACT) */
>>>> +#define SECCOMP_EXT_ACT_TSYNC 1 /* attempt to synchronize thread filters */
>>>> +
>>>> /*
>>>> * All BPF programs must return a 32-bit value.
>>>> * The bottom 16-bits are for optional return data.
>>>> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
>>>> index 71512e4..8a0de7b 100644
>>>> --- a/kernel/seccomp.c
>>>> +++ b/kernel/seccomp.c
>>>> @@ -24,6 +24,7 @@
>>>> #ifdef CONFIG_SECCOMP_FILTER
>>>> #include <asm/syscall.h>
>>>> #include <linux/filter.h>
>>>> +#include <linux/pid.h>
>>>> #include <linux/ptrace.h>
>>>> #include <linux/security.h>
>>>> #include <linux/slab.h>
>>>> @@ -220,6 +221,108 @@ static u32 seccomp_run_filters(int syscall)
>>>> return ret;
>>>> }
>>>>
>>>> +/* Returns 1 if the candidate is an ancestor. */
>>>> +static int is_ancestor(struct seccomp_filter *candidate,
>>>> + struct seccomp_filter *child)
>>>> +{
>>>> + /* NULL is the root ancestor. */
>>>> + if (candidate == NULL)
>>>> + return 1;
>>>> + for (; child; child = child->prev)
>>>> + if (child == candidate)
>>>> + return 1;
>>>> + return 0;
>>>> +}
>>>> +
>>>> +/**
>>>> + * seccomp_sync_threads: sets all threads to use current's filter
>>>> + *
>>>> + * Returns 0 on success or the pid of a thread which was either not
>>>> + * in the correct seccomp mode or it did not have an ancestral
>>>> + * seccomp filter. current must be in seccomp.mode=2 already.
>>>> + */
>>>> +static pid_t seccomp_sync_threads(void)
>>>> +{
>>>> + struct task_struct *thread, *caller;
>>>> + pid_t failed = 0;
>>>> + thread = caller = current;
>>>> +
>>>> + read_lock(&tasklist_lock);
>>>> + if (thread_group_empty(caller))
>>>> + goto done;
>>>> + while_each_thread(caller, thread) {
>>>> + task_lock(thread);
>>>> + /*
>>>> + * All threads must not be in SECCOMP_MODE_STRICT to
>>>> + * be eligible for synchronization.
>>>> + */
>>>> + if ((thread->seccomp.mode == SECCOMP_MODE_DISABLED ||
>>>> + thread->seccomp.mode == SECCOMP_MODE_FILTER) &&
>>>> + is_ancestor(thread->seccomp.filter,
>>>> + caller->seccomp.filter)) {
>>>> + /* Get a task reference for the new leaf node. */
>>>> + get_seccomp_filter(caller);
>>>> + /*
>>>> + * Drop the task reference to the shared ancestor since
>>>> + * current's path will hold a reference. (This also
>>>> + * allows a put before the assignment.)
>>>> + */
>>>> + put_seccomp_filter(thread);
>>>> + thread->seccomp.filter = caller->seccomp.filter;
>>>> + /* Opt the other thread into seccomp if needed.
>>>> + * As threads are considered to be trust-realm
>>>> + * equivalent (see ptrace_may_access), it is safe to
>>>> + * allow one thread to transition the other.
>>>> + */
>>>> + if (thread->seccomp.mode == SECCOMP_MODE_DISABLED) {
>>>> + thread->seccomp.mode = SECCOMP_MODE_FILTER;
>>>> + /*
>>>> + * Don't let an unprivileged task work around
>>>> + * the no_new_privs restriction by creating
>>>> + * a thread that sets it up, enters seccomp,
>>>> + * then dies.
>>>> + */
>>>> + if (caller->no_new_privs)
>>>> + thread->no_new_privs = 1;
>>>> + set_tsk_thread_flag(thread, TIF_SECCOMP);
>>>
>>> no_new_privs is a bitfield, and some of the other bits in there look
>>> like things that might not want to be read and written back from another
>>> thread.
>>
>> Ah :/ Good catch!
>>
>>> Would it be too annoying to require that the other threads already have
>>> no_new_privs set?
>>
>> Hrm, it's pretty painful in the edge cases where you don't control the
>> process initialization which might setup threads you need to ensnare.
>>
>> Would it be crazy to do something like below in sched.h?
>> - unsigned no_new_privs:1;
>> + unsigned no_new_privs;
>
> set_bit, etc. would also work. (Although there isn't a 32-bit set_bit
> AFAIK, or at least there isn't one that works on 64-bit BE archs.)
I wasn't sure if I could use set_bit() in a way that wouldn't get me
banned from submitting patches forever :)
> Also, is 'unsigned' actually safe for this purpose, on all supported
> archs/compilers? I'm pretty sure it's okay by C++11 rules, but those
> don't apply here. Maybe some day the kernel will move to C11 and life
> will be good.
>
>>
>> It feels like a big hammer though, but it also seems weird to wrap those
>> bitfields with task_lock. Any suggestions are welcome! I'll think about
>> this a bit more and see if there is a good way to do this transition
>> safely and cheaply.
>
> Hmm. I bet you could move no_new_privs somewhere else in task_lock
> where there's a bit free. It could also go in 'struct creds', but I
> think that's even worse from your perspective.
>
> Here's another dumb idea: Add an accessor task_no_new_privs(struct
> task_struct *) and move no_new_privs into struct seccomp (i.e. make it
> a bit in the seccomp mode). It kind of sucks on !CONFIG_SECCOMP, but
> it's free if CONFIG_SECCOMP.
That'd certainly be fine with me. I was considering adding a
"needs_transition" bit to struct seccomp, but moving nnp there could be tidy.
I'd need to make sure reading it locklessly still makes sense, but I really
don't want to put a lock on the syscall path...
> P.S. Have you seen the Linux Capsicum port? It fiddles with seccomp
> mode, too, and I suspect it needs a fair amount of work, but I really
> like the idea.
Yup - that's a whole other thread to be had! It sets a new mode as short-hand,
but what it needs really is a seccomp<->LSM interaction mechanism. That's on
my todo list next to help sort out.
thanks!
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