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

[Jann Horn]

On Fri, Jun 15, 2018 at 6:49 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>
> On Fri, Jun 15, 2018 at 05:23:35PM +0200, Jann Horn wrote:
> > As Al Viro noted in commit 128394eff343 ("sg_write()/bsg_write() is not fit
> > to be called under KERNEL_DS"), sg and bsg improperly access userspace
> > memory outside the provided buffer, permitting kernel memory corruption via
> > splice().
> > But they don't just do it on ->write(), also on ->read() and (in the case
> > of bsg) even on ->release().
> >
> > As a band-aid, make sure that the ->read() and ->write() handlers can not
> > be called in weird contexts (kernel context or credentials different from
> > file opener), like for ib_safe_file_access().
> > Also, completely prevent user memory accesses from ->release().
>
> Band-aid it is, and a bloody awful one, at that.  What the hell is going on
> in bsg_put_device() and can it _ever_ hit that call chain?  I.e.
>         bsg_release()
>                 bsg_put_device()
>                         blk_complete_sgv4_hdr_rq()
>                                 ->complete_rq()
>                                         copy_to_user()
> If it can, the whole thing is FUBAR by design - ->release() may bloody well
> be called in a context that has no userspace at all.
>
> This is completely insane; what's going on there?

Perhaps I should have split my patch into two parts; it consists of
two somewhat related changes.

The first change is that ->read() and ->write() violate the normal
contract and, as a band-aid, should not be called in uaccess_kernel()
context or with changed creds.

The second change is an actual fix: AFAICS ->release() accidentally
accessed userspace, which I've fixed using the added "cleaning_up"
parameter.