linux-kernel - Re: [PATCH net-next v2 2/9] xen-netback: Change TX path from grant copy to mapping

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Date:	Mon, 16 Dec 2013 19:06:54 +0000
From:	Wei Liu <wei.liu2@...rix.com>
To:	Zoltan Kiss <zoltan.kiss@...rix.com>
CC:	Wei Liu <wei.liu2@...rix.com>, <ian.campbell@...rix.com>,
	<xen-devel@...ts.xenproject.org>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <jonathan.davies@...rix.com>
Subject: Re: [PATCH net-next v2 2/9] xen-netback: Change TX path from grant
 copy to mapping

On Mon, Dec 16, 2013 at 06:57:44PM +0000, Zoltan Kiss wrote:
> On 16/12/13 18:21, Wei Liu wrote:
> >On Mon, Dec 16, 2013 at 03:38:05PM +0000, Zoltan Kiss wrote:
> >[...]
> >>>>+	for (i = 0; i < MAX_PENDING_REQS; ++i) {
> >>>>+		if (vif->grant_tx_handle[i] != NETBACK_INVALID_HANDLE) {
> >>>>+			i = 0;
> >>>>+			unmap_timeout++;
> >>>>+			msleep(1000);
> >>>>+			if (unmap_timeout > 9 &&
> >>>>+				net_ratelimit())
> >>>>+				netdev_err(vif->dev,
> >>>>+					"Page still granted! Index: %x\n", i);
> >>>>+		}
> >>>>+	}
> >>>>+
> >>>>+	free_xenballooned_pages(MAX_PENDING_REQS, vif->mmap_pages);
> >>>>+
> >>>
> >>>If some pages are stuck and you just free them will it cause Dom0 to
> >>>crash? I mean, if those pages are recycled by other balloon page users.
> >>>
> >>>Even if it will not cause Dom0 to crash, will it leak any resource in
> >>>Dom0? At plain sight it looks like at least grant table entry is leaked,
> >>>isn't it? We need to be careful about this because a malicious might be
> >>>able to DoS Dom0 with resource leakage.
> >>Yes, if we call free_xenballooned_pages while something is still
> >>mapped, Xen kills Dom0 because balloon driver tries to touch the PTE
> >>of a grant mapped page. That's why we make sure before that
> >>everything is unmapped, and repeat an error message if it's not. I'm
> 
> There is an "i = 0" if we find a valid handle. So we start again

Oops, missed that.

> checking the whole array from the second element (incorrectly, it
> should be "i = -1"!), and we print an incorrect error message, but
> essentially we are not leaving the loop, unless the first element
> was the problematic. We can modify that to "i--" or "i = -1" if we
> want to recheck the whole array. It shouldn't happen at this point
> that we transmit new packets, starting from the beginning is just an
> extra safety check.
> Also, we should modify i after the printing of the error message.
> 

So I did help find a bug though. :-)

Wei.

> Zoli
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