Re: [PATCHv4 RESEND 0/3] syscalls,x86: Add execveat() system call

[David Drysdale]

On Sun, Oct 19, 2014 at 1:20 AM, Eric W. Biederman
<ebiederm@xxxxxxxxxxxx> wrote:
> Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes:
>
>> [Added Eric Biederman, since I think your tree might be a reasonable
>> route forward for these patches.]
>>
>> On Thu, Jun 5, 2014 at 6:40 AM, David Drysdale <drysdale@xxxxxxxxxx> wrote:
>>> Resending, adding cc:linux-api.
>>>
>>> Also, it may help to add a little more background -- this patch is
>>> needed as a (small) part of implementing Capsicum in the Linux kernel.
>>>
>>> Capsicum is a security framework that has been present in FreeBSD since
>>> version 9.0 (Jan 2012), and is based on concepts from object-capability
>>> security [1].
>>>
>>> One of the features of Capsicum is capability mode, which locks down
>>> access to global namespaces such as the filesystem hierarchy.  In
>>> capability mode, /proc is thus inaccessible and so fexecve(3) doesn't
>>> work -- hence the need for a kernel-space
>>
>> I just found myself wanting this syscall for another reason: injecting
>> programs into sandboxes or otherwise heavily locked-down namespaces.
>>
>> For example, I want to be able to reliably do something like nsenter
>> --namespace-flags-here toybox sh.  Toybox's shell is unusual in that
>> it is more or less fully functional, so this should Just Work (tm),
>> except that the toybox binary might not exist in the namespace being
>> entered.  If execveat were available, I could rig nsenter or a similar
>> tool to open it with O_CLOEXEC, enter the namespace, and then call
>> execveat.
>>
>> Is there any reason that these patches can't be merged more or less as
>> is for 3.19?
>
> Yes.  There is a silliness in how it implements fexecve.  The fexecve
> case should be use the empty string "" not a NULL pointer to indication
> that.  That change will then harmonize execveat with the other ...at
> system calls and simplify the code and remove a special case.  I believe
> using the empty string "" requires implementing the AT_EMPTY_PATH flag.

Good point -- I'll shift to "" + AT_EMPTY_PATH.

> For sandboxes execveat seems to make a great deal of sense.  I can
> get the same functionality by passing in a directory file descriptor
> calling fchdir and execve so this should not introduce any new security
> holes.  And using the final file descriptor removes a race.
>
> AT_SYMLINK_NOFOLLOW seems to have some limited utility as well, although
> for exec I don't know what problems it can solve.
>
> Until I am done moving I won't have time to pick this up, and the code
> clearly needs another revision but I will be happy to work to see that
> we get a sane execveat implemented.

If it helps, I can push out another revision in the next couple of days.

> Eric
>
> p.s.  I don't believe there are any namespaces issues where doing
> something with execveat flags make sense.
>
>
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