Re: scsi: use-after-free in bio_copy_from_iter

[Johannes Thumshirn]

On Mon, Dec 05, 2016 at 07:03:39PM +0000, Al Viro wrote:
> 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.

Yes I realized that as well when I had enough distance between me and the
code...

> 
> 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?

I did and it didn't trigger. What triggers is (as expected) a
	WARN_ON((size_t)mxsize > count);
We have count at 80 and mxsize (which ends in hp->dxfer_len) at 65499. But the
65499 bytes are the len of the data we're suppost to be copying in via the
iov. I'm still rather confused what's happening here, sorry.

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