IORING_OP_FIXED_FD_INSTALL and audit/LSM interactions

[Jens Axboe]

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.

> 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.

-- 
Jens Axboe