Re: [PATCH 2/2] sys, seccomp: add PR_SECCOMP_EXT and SECCOMP_EXT_ACT_TSYNC

[Andy Lutomirski]

On Tue, Jan 14, 2014 at 10:59 AM, Will Drewry <wad@xxxxxxxxxxxx> wrote:
> On Mon, Jan 13, 2014 at 5:36 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxx>
>>> Signed-off-by: Will Drewry <wad@xxxxxxxxxxxx>
>>> ---
>>>  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.)

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.

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.

--Andy
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