[Trusty SRU: CVE-2017-12134] xen: fix bio vec merging

Stefan Bader stefan.bader at canonical.com
Tue Apr 17 09:46:52 UTC 2018

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

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.

CC: stable at vger.kernel.org
Reported-by: "Jan H. Schönherr" <jschoenh at amazon.de>
Signed-off-by: Roger Pau Monné <roger.pau at citrix.com>
Reviewed-by: Juergen Gross <jgross at suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk at oracle.com>


(backported from commit 462cdace790ac2ed6aad1b19c9c0af0143b6aab0)
[smb: pfn_to_gfn in newer code returns the same as pfn_to_mfn]
Signed-off-by: Stefan Bader <stefan.bader at canonical.com>
Already applied in Xenial, Trusty still needs it but needed a bit
of tweaking. Basically the newer code uses pfn_to_gfn which either
returns a pfn or a mfn. Which is the same behavior as pfn_to_mfn.


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

diff --git a/drivers/xen/biomerge.c b/drivers/xen/biomerge.c
index 0edb91c..4a77fe8 100644
--- a/drivers/xen/biomerge.c
+++ b/drivers/xen/biomerge.c
@@ -9,7 +9,6 @@ bool xen_biovec_phys_mergeable(const struct bio_vec *vec1,
 	unsigned long mfn1 = pfn_to_mfn(page_to_pfn(vec1->bv_page));
 	unsigned long mfn2 = pfn_to_mfn(page_to_pfn(vec2->bv_page));
-	return __BIOVEC_PHYS_MERGEABLE(vec1, vec2) &&
-		((mfn1 == mfn2) || ((mfn1+1) == mfn2));
+	return mfn1 + PFN_DOWN(vec1->bv_offset + vec1->bv_len) == mfn2;

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