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

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Message-ID: <CAG48ez3BWeZkW5XG814K4o7p0KWXVm9Gpzh2rTthNei=4fhH-A@mail.gmail.com>
Date:   Thu, 21 Jun 2018 14:51:16 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     axboe@...nel.dk, dgilbert@...erlog.com,
        Al Viro <viro@...iv.linux.org.uk>,
        fujita.tomonori@....ntt.co.jp, jejb@...ux.vnet.ibm.com,
        martin.petersen@...cle.com, linux-block@...r.kernel.org,
        linux-scsi@...r.kernel.org,
        kernel list <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        security@...nel.org
Subject: Re: [PATCH] sg, bsg: mitigate read/write abuse, block uaccess in release

On Thu, Jun 21, 2018 at 2:34 PM Christoph Hellwig <hch@...radead.org> wrote:
>
> On Mon, Jun 18, 2018 at 09:37:01AM -0600, Jens Axboe wrote:
> > It was born with that mode, but I don't think anyone ever really used it.
> > So it might feasible to simply yank it. That said, just doing a prune
> > mode at ->release() time doesn't seem like such a hard task.
>
> Let's try to kill it.  It is a significant amount of code, which does
> fishy things and is probably entirely unused:
>
> ---
> From baec733be1b400d73d0fa2bfc07684598c4172e7 Mon Sep 17 00:00:00 2001
> From: Christoph Hellwig <hch@....de>
> Date: Thu, 21 Jun 2018 14:31:32 +0200
> Subject: bsg: remove read/write support
>
> The code poses a security risk due to user memory access in ->release
> and had an API that can't be used reliably.  As far as we know it was
> never used for real, but if that turns out wrong we'll have to revert
> this commit and come up with a band aid.

FWIW, I just had a look through Debian's codesearch (which AFAIK scans
through the source code of all software that Debian packages) for uses
of struct sg_io_v4: https://codesearch.debian.net/search?q=sg_io_v4

Hits that seem to be using read() or write() with struct sg_io_v4 on
bsg devices:

In the package https://packages.debian.org/stretch/tgt:
  https://sources.debian.org/src/tgt/1:1.0.73-1/usr/bs_sg.c/?hl=131#L131
  https://sources.debian.org/src/tgt/1:1.0.73-1/usr/bs_sg.c/?hl=236#L236
In the package https://packages.debian.org/stretch/sg3-utils:
  https://sources.debian.org/src/sg3-utils/1.42-2/examples/bsg_queue_tst.c/?hl=60#L60