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

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Message-ID: <20180615171033.GG30522@ZenIV.linux.org.uk>
Date:   Fri, 15 Jun 2018 18:10:33 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Jann Horn <jannh@...gle.com>
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 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@...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().
> 
> 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...