[0/2] Add further ioctl() operations for namespace discovery
diff mbox

Message ID fa9eec50-b634-51e9-fa37-4911010a1496@gmail.com
State New
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Commit Message

Michael Kerrisk (man-pages) Dec. 20, 2016, 8:55 p.m. UTC
Hi Eric,

On 12/20/2016 09:22 PM, Eric W. Biederman wrote:
> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
> 
>> Hello Eric,
>>
>> On 12/19/2016 11:53 PM, Eric W. Biederman wrote:
>>> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>>>
>>>> Eric,
>>>>
>>>> The code proposed in this patch series is pretty small. Is there any
>>>> chance we could make the 4.10 merge window, if the changes seem
>>>> acceptable to you?
>>>
>>> I see why you are asking but I am not comfortable with aiming for
>>> the merge window that is on-going and could close at any moment.
>>> I have seen recenly too many patches that should work fine have
>>> some odd minor issue.  Like an extra _ in a label used in an ifdef
>>> that resulted in memory stomps.    Linus might be more brave but i would
>>> rather wait until the next merge window, so I don't need to worry about
>>> spoiling anyone's holidays with a typo someone over looked.
>>
>> I'll just gently ask if you'll reconsider and take another look at the
>> patches. They patches are very small, and don't change any existing
>> behavior. And if we see a problem in the next weeks they could be pulled.
>> In the meantime, I'd be aiming to publicize this API somewhat, so that we
>> might get some eyeballs to spot design bugs. But, I do understand your
>> position, if the answer is still "not for this merge window".
> 
> My position is still not this merge window.  I am more than happy to
> queue up the changes for the next one.  Even on the best of days there
> is a reasonable chance Linus would not be happy to receive code
> development done in the merge window.

Okay. So, I can at least think about this at leisure! (Actually, 
I think I really do mean: thanks for saying "no" again.)

> I think there is also just a little bit of discussion that needs
> to happen with these new userspace APIs (below).  And I have seen way
> too many times user space APIs added too quickly and having to be
> repaired afterwards.

Yes, I certainly understand that.

>>> At first glance these patches seem reasonable. I don't see any problem
>>> with the ioctls you have added.
>>>
>>> That said I have a question.  Should we provide a more direct way to
>>> find the answer to your question?  Something like the access system
>>> call?
>>>
>>> I think a more direct answer would be more maintainable in the long run
>>> as it does not bind tools to specific implementation details in the
>>> future.  Which could allow us to account for LSM policies and the like.
>>
>> My thoughts:
>>
>> 1. Regarding NS_GET_NSTYPE...  It always struck me as a little odd
>>    that you could ask setns() to check if the supplied FD referred
>>    to a certain type of NS (and thus, in a round about way, setns()
>>    gives us the same information as NS_GET_NSTYPE), but you can't
>>    directly ask what the NS type is. The fact that setns() has this
>>    facility suggests that there could be other uses for the operation
>>    "tell me what type of NS this FD refers to".
> 
> Yes.  I have no problem with that one.
> 
>> 2. Regarding NS_GET_CREATOR_UID... There are defined rules about what
>>    this UID means with respect to capabilities in a namespace. It's
>>    not an implementation detail, as I understand it. Also in terms of
>>    introspecting to try to understand the structure of namespaces on
>>    a running system, knowing this UID is useful in and of itself.
> 
> I am not quite sold on the name NS_GET_CREATOR_UID.  NS_GET_OWNER_UID
> seems to match the code better.  The owner is the creator but
> the important part seems to be the ownership not the act of creation.

I actually thought about NS_GET_OWNER, but shied away from it
because it had echoes of "get owning userns". NS_GET_OWNER_UID
is better than NS_GET_OWNER, and certainly not worse than
NS_GET_CREATOR_UID.

>> 3. NS_GET_NSTYPE and NS_GET_CREATOR_UID solve my problem, but
>>    obviously your idea would make life simpler for user space.
>>    Am I correct to understand that you mean an API that takes
>>    three pieces of info: a PID, a capability, and an fd referring
>>    to a /proc/PID/ns/xxx, and tells us whether PID has the specified
>>    capability for operations in the specified namespace?
> 
> Something like that.  But yes something we can wire up to
> ns_capable_noaudit and be told the result.  

Yes, that was my line of thinking also. It seems to me that to
prevent information leaks, we also should check that the caller
has some suitable capability in the target namespace, right?
(I presume a ptrace_may_access() check.)

> That will let the
> LSMs and any future kerel changes have their say, without any extra
> maintenance burden in the kernel.

Yes.

> What I really don't want is for userspace to start depending on the
> current formula being the only factors that say if it has a capabliltiy
> in a certain situation because in practice that just isn't true.
> Permission checks just keep evoloving in the kernel.

This was the bit I hadn't really considered when I first started down 
this path, but I started to see the light a bit already today, but
didn't have it so crisply in my mind as you just said it there.

So, we have two ioctls already in 4.9, I proposed two more. And 
then we have this fifth operation. Should we have an nsctl(2)?

In the meantime, here's something I hacked together. I know it
needs work, but I just want to check whether it's the direction
that you were meaning in terms of the checks. It's done as an
ioctl() (structure hard coded in place while I play about, and
some names and types should certainly be better). Leaving aside 
the messy bits, is the below roughly the kind of checking you 
expect to be embodied in this operation?

Cheers,

Michael

Comments

Eric W. Biederman Dec. 21, 2016, 12:17 a.m. UTC | #1
"Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:

> Hi Eric,
>
> On 12/20/2016 09:22 PM, Eric W. Biederman wrote:
>> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>> 
>>> Hello Eric,
>>>
>>> On 12/19/2016 11:53 PM, Eric W. Biederman wrote:
>>>> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>>>>
>>>>> Eric,
>>>>>
>>>>> The code proposed in this patch series is pretty small. Is there any
>>>>> chance we could make the 4.10 merge window, if the changes seem
>>>>> acceptable to you?
>>>>
>>>> I see why you are asking but I am not comfortable with aiming for
>>>> the merge window that is on-going and could close at any moment.
>>>> I have seen recenly too many patches that should work fine have
>>>> some odd minor issue.  Like an extra _ in a label used in an ifdef
>>>> that resulted in memory stomps.    Linus might be more brave but i would
>>>> rather wait until the next merge window, so I don't need to worry about
>>>> spoiling anyone's holidays with a typo someone over looked.
>>>
>>> I'll just gently ask if you'll reconsider and take another look at the
>>> patches. They patches are very small, and don't change any existing
>>> behavior. And if we see a problem in the next weeks they could be pulled.
>>> In the meantime, I'd be aiming to publicize this API somewhat, so that we
>>> might get some eyeballs to spot design bugs. But, I do understand your
>>> position, if the answer is still "not for this merge window".
>> 
>> My position is still not this merge window.  I am more than happy to
>> queue up the changes for the next one.  Even on the best of days there
>> is a reasonable chance Linus would not be happy to receive code
>> development done in the merge window.
>
> Okay. So, I can at least think about this at leisure! (Actually, 
> I think I really do mean: thanks for saying "no" again.)
>
>> I think there is also just a little bit of discussion that needs
>> to happen with these new userspace APIs (below).  And I have seen way
>> too many times user space APIs added too quickly and having to be
>> repaired afterwards.
>
> Yes, I certainly understand that.
>
>>>> At first glance these patches seem reasonable. I don't see any problem
>>>> with the ioctls you have added.
>>>>
>>>> That said I have a question.  Should we provide a more direct way to
>>>> find the answer to your question?  Something like the access system
>>>> call?
>>>>
>>>> I think a more direct answer would be more maintainable in the long run
>>>> as it does not bind tools to specific implementation details in the
>>>> future.  Which could allow us to account for LSM policies and the like.
>>>
>>> My thoughts:
>>>
>>> 1. Regarding NS_GET_NSTYPE...  It always struck me as a little odd
>>>    that you could ask setns() to check if the supplied FD referred
>>>    to a certain type of NS (and thus, in a round about way, setns()
>>>    gives us the same information as NS_GET_NSTYPE), but you can't
>>>    directly ask what the NS type is. The fact that setns() has this
>>>    facility suggests that there could be other uses for the operation
>>>    "tell me what type of NS this FD refers to".
>> 
>> Yes.  I have no problem with that one.
>> 
>>> 2. Regarding NS_GET_CREATOR_UID... There are defined rules about what
>>>    this UID means with respect to capabilities in a namespace. It's
>>>    not an implementation detail, as I understand it. Also in terms of
>>>    introspecting to try to understand the structure of namespaces on
>>>    a running system, knowing this UID is useful in and of itself.
>> 
>> I am not quite sold on the name NS_GET_CREATOR_UID.  NS_GET_OWNER_UID
>> seems to match the code better.  The owner is the creator but
>> the important part seems to be the ownership not the act of creation.
>
> I actually thought about NS_GET_OWNER, but shied away from it
> because it had echoes of "get owning userns". NS_GET_OWNER_UID
> is better than NS_GET_OWNER, and certainly not worse than
> NS_GET_CREATOR_UID.
>
>>> 3. NS_GET_NSTYPE and NS_GET_CREATOR_UID solve my problem, but
>>>    obviously your idea would make life simpler for user space.
>>>    Am I correct to understand that you mean an API that takes
>>>    three pieces of info: a PID, a capability, and an fd referring
>>>    to a /proc/PID/ns/xxx, and tells us whether PID has the specified
>>>    capability for operations in the specified namespace?
>> 
>> Something like that.  But yes something we can wire up to
>> ns_capable_noaudit and be told the result.  
>
> Yes, that was my line of thinking also. It seems to me that to
> prevent information leaks, we also should check that the caller
> has some suitable capability in the target namespace, right?
> (I presume a ptrace_may_access() check.)

Well over the target process but yes.

>> That will let the
>> LSMs and any future kerel changes have their say, without any extra
>> maintenance burden in the kernel.
>
> Yes.
>
>> What I really don't want is for userspace to start depending on the
>> current formula being the only factors that say if it has a capabliltiy
>> in a certain situation because in practice that just isn't true.
>> Permission checks just keep evoloving in the kernel.
>
> This was the bit I hadn't really considered when I first started down 
> this path, but I started to see the light a bit already today, but
> didn't have it so crisply in my mind as you just said it there.
>
> So, we have two ioctls already in 4.9, I proposed two more. And 
> then we have this fifth operation. Should we have an nsctl(2)?

I would rather move the other direction and have a system call.

> In the meantime, here's something I hacked together. I know it
> needs work, but I just want to check whether it's the direction
> that you were meaning in terms of the checks. It's done as an
> ioctl() (structure hard coded in place while I play about, and
> some names and types should certainly be better). Leaving aside 
> the messy bits, is the below roughly the kind of checking you 
> expect to be embodied in this operation?

Yes.  It probably nees u32 instead of long, or eles we need to have
a compat version for 32bit OS's.

Now the question becomes who are the users of this?  Because it just
occurred to me that we now have an interesting complication.  Userspace
extending the meaning of the capability bits, and using to protect
additional things.  Ugh.  That could be a maintenance problem of another
flavor.  Definitely not my favorite.

The access system is limited to asking about yourself, and to
asking very specific questions.  If our new operation did something
similar and only allowed asking about yourself, and a capablity I would
find it less scary, and I am wondering if it would be possible to ask
not about a capability but an operation that the capability guards
such as chroot.

Implementing target operations instead of target capabilities would be a
bit trickier to implement (as it requires factoring out the permission
checks) but it may be much more useful in the long run.

So why are we asking the questions about what permissions a process has?

Eric
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Michael Kerrisk (man-pages) Dec. 21, 2016, 9:53 a.m. UTC | #2
Hi Eric,

On 12/21/2016 01:17 AM, Eric W. Biederman wrote:
> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
> 
>> Hi Eric,
>>
>> On 12/20/2016 09:22 PM, Eric W. Biederman wrote:
>>> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>>>
>>>> Hello Eric,
>>>>
>>>> On 12/19/2016 11:53 PM, Eric W. Biederman wrote:
>>>>> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>>>>>
>>>>>> Eric,

[...]

>>>> 3. NS_GET_NSTYPE and NS_GET_CREATOR_UID solve my problem, but
>>>>    obviously your idea would make life simpler for user space.
>>>>    Am I correct to understand that you mean an API that takes
>>>>    three pieces of info: a PID, a capability, and an fd referring
>>>>    to a /proc/PID/ns/xxx, and tells us whether PID has the specified
>>>>    capability for operations in the specified namespace?
>>>
>>> Something like that.  But yes something we can wire up to
>>> ns_capable_noaudit and be told the result.  
>>
>> Yes, that was my line of thinking also. It seems to me that to
>> prevent information leaks, we also should check that the caller
>> has some suitable capability in the target namespace, right?
>> (I presume a ptrace_may_access() check.)
> 
> Well over the target process but yes.
> 
>>> That will let the
>>> LSMs and any future kerel changes have their say, without any extra
>>> maintenance burden in the kernel.
>>
>> Yes.
>>
>>> What I really don't want is for userspace to start depending on the
>>> current formula being the only factors that say if it has a capabliltiy
>>> in a certain situation because in practice that just isn't true.
>>> Permission checks just keep evoloving in the kernel.
>>
>> This was the bit I hadn't really considered when I first started down 
>> this path, but I started to see the light a bit already today, but
>> didn't have it so crisply in my mind as you just said it there.
>>
>> So, we have two ioctls already in 4.9, I proposed two more. And 
>> then we have this fifth operation. Should we have an nsctl(2)?
> 
> I would rather move the other direction and have a system call.

Okay -- I'll give that a some thought.

>> In the meantime, here's something I hacked together. I know it
>> needs work, but I just want to check whether it's the direction
>> that you were meaning in terms of the checks. It's done as an
>> ioctl() (structure hard coded in place while I play about, and
>> some names and types should certainly be better). Leaving aside 
>> the messy bits, is the below roughly the kind of checking you 
>> expect to be embodied in this operation?
> 
> Yes.  It probably nees u32 instead of long, or eles we need to have
> a compat version for 32bit OS's.

Yes.

> Now the question becomes who are the users of this?  Because it just
> occurred to me that we now have an interesting complication.  Userspace
> extending the meaning of the capability bits, and using to protect
> additional things.  Ugh.  That could be a maintenance problem of another
> flavor.  Definitely not my favorite.

I don't follow you here. Could you say some more about what you mean?

> The access system is limited to asking about yourself, and to
> asking very specific questions.  If our new operation did something
> similar and only allowed asking about yourself, and a capablity I would
> find it less scary, 

Okay. But that's a less interesting operation from my point of view.
I mean: one way of knowing if we have permission to do an operation
is to try to do it.

> and I am wondering if it would be possible to ask
> not about a capability but an operation that the capability guards
> such as chroot.
> 
> Implementing target operations instead of target capabilities would be a
> bit trickier to implement (as it requires factoring out the permission
> checks) but it may be much more useful in the long run.

But, would this not mean factoring out the permission checks on a per 
operation basis? (There are of courses hundreds of such checks.)

> So why are we asking the questions about what permissions a process has?

My main interest here is monitoring/discovery/debugging on a running
system. NS_GET_PARENT, NS_GET_USERNS, NS_GET_CREATOR_UID, and 
NS_GET_NSTYPE provide most of what I'd like to see. Being able to ask
"does this process have permissions in that namespace?" would be nice 
to have in terms of understanding/debugging a system.

Cheers,

Michael
Eric W. Biederman Dec. 22, 2016, 12:27 a.m. UTC | #3
"Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:

> Hi Eric,
>
> On 12/21/2016 01:17 AM, Eric W. Biederman wrote:
>> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>> 
>>> Hi Eric,
>>>
>>> On 12/20/2016 09:22 PM, Eric W. Biederman wrote:
>>>> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>>>>
>>>>> Hello Eric,
>>>>>
>>>>> On 12/19/2016 11:53 PM, Eric W. Biederman wrote:
>>>>>> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>>>>>>

>> Now the question becomes who are the users of this?  Because it just
>> occurred to me that we now have an interesting complication.  Userspace
>> extending the meaning of the capability bits, and using to protect
>> additional things.  Ugh.  That could be a maintenance problem of another
>> flavor.  Definitely not my favorite.
>
> I don't follow you here. Could you say some more about what you mean?

I have seen user space userspace do thing such as extend CAP_SYS_REBOOT
to things such as permission to invoke "shutdown -r now".  Which
depending on what a clean reboot entails could be greately increasing
the scope of CAP_SYS_REBOOT.

I am concerned for that and similar situations that userspace
applications could lead us into situation that one wrong decision could
wind up being an unfixable mistake because fixing the mistake would
break userspsace.

>> So why are we asking the questions about what permissions a process has?
>
> My main interest here is monitoring/discovery/debugging on a running
> system. NS_GET_PARENT, NS_GET_USERNS, NS_GET_CREATOR_UID, and 
> NS_GET_NSTYPE provide most of what I'd like to see. Being able to ask
> "does this process have permissions in that namespace?" would be nice 
> to have in terms of understanding/debugging a system.

If we are just looking at explanations then I seem to have been
over-engineering things.  So let's just aim at the two ioctls.
Or at least the information in those ioctls.

With at least a comment on the ioctl returning the OWNER_UID that
describes why it is not a problem to if the owners uid is something like
((uid_t)-3).  Which overlaps with the space for error return codes.

I don't know if we are fine or not, but that review comment definitely
deserves some consideration.

Eric
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Michael Kerrisk (man-pages) Dec. 22, 2016, 7:20 a.m. UTC | #4
Hi Eric,

On 12/22/2016 01:27 AM, Eric W. Biederman wrote:
> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
> 
>> Hi Eric,
>>
>> On 12/21/2016 01:17 AM, Eric W. Biederman wrote:
>>> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>>>
>>>> Hi Eric,
>>>>
>>>> On 12/20/2016 09:22 PM, Eric W. Biederman wrote:
>>>>> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>>>>>
>>>>>> Hello Eric,
>>>>>>
>>>>>> On 12/19/2016 11:53 PM, Eric W. Biederman wrote:
>>>>>>> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>>>>>>>
> 
>>> Now the question becomes who are the users of this?  Because it just
>>> occurred to me that we now have an interesting complication.  Userspace
>>> extending the meaning of the capability bits, and using to protect
>>> additional things.  Ugh.  That could be a maintenance problem of another
>>> flavor.  Definitely not my favorite.
>>
>> I don't follow you here. Could you say some more about what you mean?
> 
> I have seen user space userspace do thing such as extend CAP_SYS_REBOOT
> to things such as permission to invoke "shutdown -r now".  Which
> depending on what a clean reboot entails could be greately increasing
> the scope of CAP_SYS_REBOOT.
> 
> I am concerned for that and similar situations that userspace
> applications could lead us into situation that one wrong decision could
> wind up being an unfixable mistake because fixing the mistake would
> break userspsace.

Okay.

>>> So why are we asking the questions about what permissions a process has?
>>
>> My main interest here is monitoring/discovery/debugging on a running
>> system. NS_GET_PARENT, NS_GET_USERNS, NS_GET_CREATOR_UID, and 
>> NS_GET_NSTYPE provide most of what I'd like to see. Being able to ask
>> "does this process have permissions in that namespace?" would be nice 
>> to have in terms of understanding/debugging a system.
> 
> If we are just looking at explanations then I seem to have been
> over-engineering things.  So let's just aim at the two ioctls.
> Or at least the information in those ioctls.

Okay.

> With at least a comment on the ioctl returning the OWNER_UID that
> describes why it is not a problem to if the owners uid is something like
> ((uid_t)-3).  Which overlaps with the space for error return codes.
>
> I don't know if we are fine or not, but that review comment definitely
> deserves some consideration.


See my reply just sent to Andrei. We should instead then just return 
the UID via a buffer pointed to by the ioctl() argument:

ioctl(fd, NS_GET_OWNER_UID, &uid);

Cheers,

Michael
Eric W. Biederman Dec. 22, 2016, 10:28 a.m. UTC | #5
"Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:

> Hi Eric,
>
> On 12/22/2016 01:27 AM, Eric W. Biederman wrote:
>> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>> 
>>> Hi Eric,
>>>
>>> On 12/21/2016 01:17 AM, Eric W. Biederman wrote:
>>>> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>>>>
>>>>> Hi Eric,
>>>>>
>>>>> On 12/20/2016 09:22 PM, Eric W. Biederman wrote:
>>>>>> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>>>>>>
>>>>>>> Hello Eric,
>>>>>>>
>>>>>>> On 12/19/2016 11:53 PM, Eric W. Biederman wrote:
>>>>>>>> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>>>>>>>>
>> 
>>>> Now the question becomes who are the users of this?  Because it just
>>>> occurred to me that we now have an interesting complication.  Userspace
>>>> extending the meaning of the capability bits, and using to protect
>>>> additional things.  Ugh.  That could be a maintenance problem of another
>>>> flavor.  Definitely not my favorite.
>>>
>>> I don't follow you here. Could you say some more about what you mean?
>> 
>> I have seen user space userspace do thing such as extend CAP_SYS_REBOOT
>> to things such as permission to invoke "shutdown -r now".  Which
>> depending on what a clean reboot entails could be greately increasing
>> the scope of CAP_SYS_REBOOT.
>> 
>> I am concerned for that and similar situations that userspace
>> applications could lead us into situation that one wrong decision could
>> wind up being an unfixable mistake because fixing the mistake would
>> break userspsace.
>
> Okay.
>
>>>> So why are we asking the questions about what permissions a process has?
>>>
>>> My main interest here is monitoring/discovery/debugging on a running
>>> system. NS_GET_PARENT, NS_GET_USERNS, NS_GET_CREATOR_UID, and 
>>> NS_GET_NSTYPE provide most of what I'd like to see. Being able to ask
>>> "does this process have permissions in that namespace?" would be nice 
>>> to have in terms of understanding/debugging a system.
>> 
>> If we are just looking at explanations then I seem to have been
>> over-engineering things.  So let's just aim at the two ioctls.
>> Or at least the information in those ioctls.
>
> Okay.
>
>> With at least a comment on the ioctl returning the OWNER_UID that
>> describes why it is not a problem to if the owners uid is something like
>> ((uid_t)-3).  Which overlaps with the space for error return codes.
>>
>> I don't know if we are fine or not, but that review comment definitely
>> deserves some consideration.
>
>
> See my reply just sent to Andrei. We should instead then just return 
> the UID via a buffer pointed to by the ioctl() argument:
>
> ioctl(fd, NS_GET_OWNER_UID, &uid);

That will work without problem.  Especially as unsigned int is the same
on both 32bit and 64bit so we won't need a compat ioctl.

Eric

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Patch
diff mbox

diff --git a/include/uapi/linux/nsfs.h b/include/uapi/linux/nsfs.h
index b3c6c78..88b7d78 100644
--- a/include/uapi/linux/nsfs.h
+++ b/include/uapi/linux/nsfs.h
@@ -14,5 +14,7 @@ 
 #define NS_GET_NSTYPE          _IO(NSIO, 0x3)
 /* Get creator UID for a user namespace */
 #define NS_GET_CREATOR_UID     _IO(NSIO, 0x4)
+/* Test whether a process has a capability in a namespace */
+#define NS_CAPABLE             _IO(NSIO, 0x5)
 
 #endif /* __LINUX_NSFS_H */
--- a/fs/nsfs.c
+++ b/fs/nsfs.c
@@ -4,6 +4,8 @@ 
 #include <linux/proc_ns.h>
 #include <linux/magic.h>
 #include <linux/ktime.h>
+#include <linux/ptrace.h>
+#include <asm/uaccess.h>
 #include <linux/seq_file.h>
 #include <linux/user_namespace.h>
 #include <linux/nsfs.h>
@@ -160,6 +162,38 @@  static int open_related_ns(struct ns_common *ns,
        return fd;
 }
 
+static long check_capable(struct user_namespace *user_ns, unsigned long arg)
+{
+       struct cc_param {
+               long pid;
+               long capability;
+       };
+       struct cc_param __user *cc_par_p = (void __user *)arg;
+       struct cc_param cc_par;
+       struct task_struct *p;
+       int retval;
+
+       retval = copy_from_user(&cc_par, cc_par_p, sizeof(cc_par));
+       if (retval)
+               return -EFAULT;
+
+       rcu_read_lock();
+
+       retval = -ESRCH;
+       p = find_task_by_vpid(cc_par.pid);
+       if (!p)
+               goto out;
+
+       retval = -EPERM;
+       if (!ptrace_may_access(p, PTRACE_MODE_READ_REALCREDS))
+               goto out;
+
+       retval = has_ns_capability_noaudit(p, user_ns, cc_par.capability);
+out:
+       rcu_read_unlock();
+       return retval;
+}
+
 static long ns_ioctl(struct file *filp, unsigned int ioctl,
                        unsigned long arg)
 {
@@ -180,6 +214,12 @@  static long ns_ioctl(struct file *filp, unsigned int ioctl,
                        return -EINVAL;
                user_ns = container_of(ns, struct user_namespace, ns);
                return from_kuid_munged(current_user_ns(), user_ns->owner);
+       case NS_CAPABLE:
+               if (ns->ops->type == CLONE_NEWUSER)
+                       user_ns = container_of(ns, struct user_namespace, ns);
+               else
+                       user_ns = (*ns->ops->owner)(ns);
+               return check_capable(user_ns, arg);
        default:
                return -ENOTTY;
        }