On Mon, Dec 16, 2013 at 03:38:05PM +0000, Zoltan Kiss wrote:
[...]

quoted

quoted

+	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

The code snippet above doesn't loop ten times over the whole array if
there's stale pages found, nor does it loop ten times on any stale
pages.

So imagine that we have the very last page in the array staled. This
routine sleeps for 1 second then free all ballooned pages. It's still
not guaranteed at the point we call free_xenballooned_pages all pages
are unmapped, right?

afraid we can't do anything better here, that means a serious
netback bug.
But a malicious guest cannot take advantage of this unless it's find
a way to screw up netback's internal bookkeeping. Then it can block
here indefinitely the teardown of the VIF, and it's associated
resources.

OK.

Wei.