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

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Message-ID: <CAG48ez2Fku77S+FM2w5n8J_utOhO0EW_RFbMib=t5Nmwc9pwfQ@mail.gmail.com>
Date:   Fri, 15 Jun 2018 18:44:51 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     axboe@...nel.dk, fujita.tomonori@....ntt.co.jp,
        dgilbert@...erlog.com, 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 Fri, Jun 15, 2018 at 6:40 PM Al Viro <viro@...iv.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@...r.kernel.org>
> > Signed-off-by: Jann Horn <jannh@...gle.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().