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

[Al Viro <viro@ZenIV.linux.org.uk>]

On Fri, Jun 15, 2018 at 06:44:51PM +0200, Jann Horn wrote:
> On Fri, Jun 15, 2018 at 6:40 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > On Fri, Jun 15, 2018 at 05:23:35PM +0200, Jann Horn wrote:
> >
> > > I've mostly copypasted ib_safe_file_access() over as
> > > scsi_safe_file_access() because I couldn't find a good common header -
> > > please tell me if you know a better way.
> > > The duplicate pr_err_once() calls are so that each of them fires once;
> > > otherwise, this would probably have to be a macro.
> > >
> > > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> > > Cc: <stable@vger.kernel.org>
> > > Signed-off-by: Jann Horn <jannh@google.com>
> > > ---
> >
> > WTF do you mean, in ->release()?  That makes no sense whatsoever -
> > what kind of copy_{to,from}_user() would be possible in there?
> 
> bsg_release -> bsg_put_device -> bsg_complete_all_commands ->
> blk_complete_sgv4_hdr_rq -> bsg_scsi_complete_rq -> copy_to_user.
> I don't think that was intentional.
> 
> Basically, the sense buffer is copied to a userspace address supplied
> in the previous ->write() when you ->read() the reply. But when you
> ->release() the file without reading the reply, they have to clean it
> up, and for that, they reuse the same code they use for ->read() - so
> the sense buffer is written to userspace on ->release().

Pardon me, that has only one fix - git rm.  This is too broken for words -
if your reading is correct, the interface is unsalvagable.  I hope you
*are* misreading it, but if not... how did that insanity get through
review at merge time?