Re: [PATCH] sg, bsg: mitigate read/write abuse, block uaccess in release

[Christoph Hellwig <hch@infradead.org>]

On Thu, Jun 21, 2018 at 08:07:23AM -0600, Jens Axboe wrote:
> I'd be fine with that, if we knew that nobody uses it. But that's
> really hard to figure out. I did see Jann's source code scan, which
> even if non-exhaustive, still shows at least one user of it.

One is an example, and the other looks very close to an example,
as far as I can tell it was Nic doing a bsg read/write WIP for a
tgt module without anyone every picking up on it.  I did add the tgt
list to Cc and no one seemed to care about the bsg read/write support.
Adding the tgt list back, but I doubt anyone ever actually used it.

> How about we just make the write interface sync? Then any copy can
> happen while the we block the task, and the read side is just
> copying the header info back, or dumping it if the task didn't
> read it before it went away.

How is that going to work?  As far as I can tell each I/O using
bsg read/write needs a write and a read, so they need to pair
and thus can't be a purely sync interface.

It also doesn't help with the issue that bsg_write may possible
write to user memory, which is highly unusal and asking for security
issues itself.

Either way, we should probably at very least apply a respun version
of the patch from Jann to 4.18-rc and -stable while we keep discussing
this.

Jann, can you respin the bsg patch with the same changes as the now
included sg one?