Re: Re: [GIT PULL] SCSI fixes for 4.18-rc3

Linus Torvalds
2018-07-10T22:24:35Z

On Tue, Jul 10, 2018 at 2:53 PM Tony Battersby <tonyb@cybernetics.com> wrote:
>
> At my job (https://www.cybernetics.com/), I use the write()/read()
> interface to the SCSI generic driver for access to tape drives and tape
> medium changers. For example, the write()/read() interface is useful
> for implementing RAID-like functionality for tape drives since a single
> thread can send commands to multiple tape drives at once and poll() for
> command completion.

Ugh.

> We have a lot of code invested in this interface,
> so it would be a huge pain for us if it were removed. But in our case,
> everything runs as root (as the firmware of an embedded storage
> appliance), so extra permission checks should be OK.

I wonder if we could make the warning literally about not having
CAP_SYS_RAWIO on the file descriptor.

Because the /dev/sg interfaces don't even do the kind of command
filtering that the SG_IO code does.

The SG_IO code (well, at least the block/scsi_ioctl.c code - again
/dev/sg does NOT get it right!) actually has a whitelist of commands
that have known behavior, and that can be done by normal users when
you open a device.

To actually do _arbitrary_ commands, you have to have CAP_SYS_RAWIO.
And there we check the _current_ capabilities (not the file descriptor
one), because an ioctl isn't something you just fool a random suid
program into doing for you.

So /dev/sg really has serious issues. Not just the read/write part,
but even the SG_IO part is broken right now (sg_new_write() actually
does do blk_verify_command(), but only for read-only opens for some
reason).

Linus