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Date:   Fri, 15 Jun 2018 17:53:10 +0100
From:   Al Viro <viro@ZenIV.linux.org.uk>
To:     Jann Horn <jannh@google.com>
Cc:     axboe@kernel.dk, fujita.tomonori@lab.ntt.co.jp,
        dgilbert@interlog.com, jejb@linux.vnet.ibm.com,
        martin.petersen@oracle.com, linux-block@vger.kernel.org,
        linux-scsi@vger.kernel.org,
        kernel list <linux-kernel@vger.kernel.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        security@kernel.org
Subject: Re: [PATCH] sg, bsg: mitigate read/write abuse, block uaccess in
 release
Message-ID: <20180615165310.GF30522@ZenIV.linux.org.uk>
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 <CAG48ez2Fku77S+FM2w5n8J_utOhO0EW_RFbMib=t5Nmwc9pwfQ@mail.gmail.com>
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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?