linux-kernel - Re: scsi: use-after-free in bio_copy_from

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Date:   Mon, 5 Dec 2016 19:03:39 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Johannes Thumshirn <jthumshirn@...e.de>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>,
        Doug Gilbert <dgilbert@...erlog.com>, jejb@...ux.vnet.ibm.com,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        linux-scsi <linux-scsi@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, axboe@...nel.dk,
        linux-block@...r.kernel.org, David Rientjes <rientjes@...gle.com>,
        Hannes Reinecke <hare@...e.de>, mhocko@...e.cz
Subject: Re: scsi: use-after-free in bio_copy_from_iter

On Mon, Dec 05, 2016 at 04:17:53PM +0100, Johannes Thumshirn wrote:
> 633         hp = &srp->header;
> [...]
> 646                 hp->dxferp = (char __user *)buf + cmd_size;

> So the memory for hp->dxferp comes from:
> 633         hp = &srp->header;

????

> >From my debug instrumentation I see that the dxferp ends up in the
> iovec_iter's kvec->iov_base and the faulting address is always dxferp + n *
> 4k with n in [1, 16] (and we're copying 16 4k pages from the iovec into the
> bio).

_Address_ of hp->dxferp comes from that assignment; the value is 'buf'
argument of sg_write() + small offset.  In this case, it should point
inside a pipe buffer, which is, indeed, at a kernel address.  Who'd
allocated srp is irrelevant.

And if you end up dereferencing more than one page worth there, you do have
a problem - pipe buffers are not going to be that large.  Could you slap
	WARN_ON((size_t)input_size > count);
right after the calculation of input_size in sg_write() and see if it triggers
on your reproducer?