IORING_OP_FIXED_FD_INSTALL and audit/LSM interactions

[Jens Axboe]

On 1/19/24 10:20 AM, Paul Moore wrote:
> On Fri, Jan 19, 2024 at 12:02?PM Jens Axboe <axboe at kernel.dk> wrote:
>>
>> On 1/19/24 9:33 AM, Paul Moore wrote:
>>> Hello all,
>>>
>>> I just noticed the recent addition of IORING_OP_FIXED_FD_INSTALL and I
>>> see that it is currently written to skip the io_uring auditing.
>>> Assuming I'm understanding the patch correctly, and I'll admit that
>>> I've only looked at it for a short time today, my gut feeling is that
>>> we want to audit the FIXED_FD_INSTALL opcode as it could make a
>>> previously io_uring-only fd generally accessible to userspace.
>>
>> We can certainly remove the audit skip, it was mostly done as we're
>> calling into the security parts anyway later on. But it's not like doing
>> the extra audit here would cause any concerns on the io_uring front.
> 
> Great.  Do you want to put a patch together for that, or should I?

Either way - I'd say if you have time to do it, please do! Probably just
include the REQ_F_CREDS change too. FWIW, I'd add that in
io_uring/openclose.c:io_install_fixed_fd_prep() - just check for
REQ_F_CREDS in there and return -EPERM (I think that would be
appropriate?) and that should disallow any IORING_OP_FIXED_FD_INSTALL if
creds have been reassigned.

>>> I'm also trying to determine how worried we should be about
>>> io_install_fixed_fd() potentially happening with the current task's
>>> credentials overridden by the io_uring's personality.  Given that this
>>> io_uring operation inserts a fd into the current process, I believe
>>> that we should be checking to see if the current task's credentials,
>>> and not the io_uring's credentials/personality, are allowed to receive
>>> the fd in receive_fd()/security_file_receive().  I don't see an
>>> obvious way to filter/block credential overrides on a per-opcode
>>> basis, but if we don't want to add a mask for io_kiocb::flags in
>>> io_issue_defs (or something similar), perhaps we can forcibly mask out
>>> REQ_F_CREDS in io_install_fixed_fd_prep()?  I'm very interested to
>>> hear what others think about this.
>>>
>>> Of course if I'm reading the commit or misunderstanding the
>>> IORING_OP_FIXED_FD_INSTALL operation, corrections are welcome :)
>>
>> I think if there are concerns for that, the easiest solution would be to
>> just fail IORING_OP_FIXED_INSTALL if REQ_F_CREDS is set. I don't really
>> see a good way to have the security side know about the old creds, as
>> the task itself is running with the assigned creds.
> 
> The more I've been thinking about it, yes, I believe there are
> concerns around FIXED_FD_INSTALL and io_uring personalities for LSMs.
> Assuming an io_uring with stored credentials for task A, yet
> accessible via task B, task B could submit an IORING_OP_OPENAT command
> to open a file using task A's creds and then FIXED_FD_INSTALL that fd
> into its own (task B's) file descriptor table without a problem as the
> installer's creds (the io_uring creds, or task A) match the file's
> creds (also task A since the io_uring opened the file).  Following
> code paths in task B that end up going through
> security_file_permission() and similar hooks may very well end up
> catching the mismatch between the file's creds and task B (depending
> on the LSM), but arguably it is something that should have been caught
> at receive_fd() time.

If there are any concerns, then I say let's just explicitly disable it
rather than rely on maybe something in the security checking catching
it. Especially because I don't think there's a valid use case for doing
this, other than perhaps trying to bypass checks you'd normally hit.
Better to err on the side of caution then.

See above for a HOWTO, if in doubt.

Thanks for looking into this!

-- 
Jens Axboe