Patch "xen: fix bio vec merging" has been added to the 4.9-stable tree

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

    xen: fix bio vec merging

to the 4.9-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     xen-fix-bio-vec-merging.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 462cdace790ac2ed6aad1b19c9c0af0143b6aab0 Mon Sep 17 00:00:00 2001
From: Roger Pau Monne <roger.pau@citrix.com>
Date: Tue, 18 Jul 2017 15:01:00 +0100
Subject: xen: fix bio vec merging
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

From: Roger Pau Monne <roger.pau@citrix.com>

commit 462cdace790ac2ed6aad1b19c9c0af0143b6aab0 upstream.

The current test for bio vec merging is not fully accurate and can be
tricked into merging bios when certain grant combinations are used.
The result of these malicious bio merges is a bio that extends past
the memory page used by any of the originating bios.

Take into account the following scenario, where a guest creates two
grant references that point to the same mfn, ie: grant 1 -> mfn A,
grant 2 -> mfn A.

These references are then used in a PV block request, and mapped by
the backend domain, thus obtaining two different pfns that point to
the same mfn, pfn B -> mfn A, pfn C -> mfn A.

If those grants happen to be used in two consecutive sectors of a disk
IO operation becoming two different bios in the backend domain, the
checks in xen_biovec_phys_mergeable will succeed, because bfn1 == bfn2
(they both point to the same mfn). However due to the bio merging,
the backend domain will end up with a bio that expands past mfn A into
mfn A + 1.

Fix this by making sure the check in xen_biovec_phys_mergeable takes
into account the offset and the length of the bio, this basically
replicates whats done in __BIOVEC_PHYS_MERGEABLE using mfns (bus
addresses). While there also remove the usage of
__BIOVEC_PHYS_MERGEABLE, since that's already checked by the callers
of xen_biovec_phys_mergeable.

Reported-by: "Jan H. Schönherr" <jschoenh@amazon.de>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 drivers/xen/biomerge.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- a/drivers/xen/biomerge.c
+++ b/drivers/xen/biomerge.c
@@ -10,8 +10,7 @@ bool xen_biovec_phys_mergeable(const str
 	unsigned long bfn1 = pfn_to_bfn(page_to_pfn(vec1->bv_page));
 	unsigned long bfn2 = pfn_to_bfn(page_to_pfn(vec2->bv_page));
 
-	return __BIOVEC_PHYS_MERGEABLE(vec1, vec2) &&
-		((bfn1 == bfn2) || ((bfn1+1) == bfn2));
+	return bfn1 + PFN_DOWN(vec1->bv_offset + vec1->bv_len) == bfn2;
 #else
 	/*
 	 * XXX: Add support for merging bio_vec when using different page


Patches currently in stable-queue which might be from roger.pau@citrix.com are

queue-4.9/xen-blkfront-use-a-right-index-when-checking-requests.patch
queue-4.9/xen-fix-bio-vec-merging.patch