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

Jann Horn
2018-06-15T17:13:52Z

On Fri, Jun 15, 2018 at 7:10 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> On Fri, Jun 15, 2018 at 05:53:10PM +0100, Al Viro wrote:
> > 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?
>
> AFAICS, it went in as part of commit 3d6392cfbd7d "bsg: support for full
> generic block layer SG v3", so your 2.6.12-rc2 is too old...

I picked 2.6.12-rc2 for the Fixes tag because the bad copy_to_user()
in sg_new_read() is at least that old.
Do you think I should split this up into two patches or so - one for
the creds/uaccess_kernel checks, one for the ->release() bug?