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

[ebiederm@xmission.com (Eric W. Biederman)]

Andy Lutomirski <luto@amacapital.net> 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@google.com> 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.

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.

Eric

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