Patch "xen: fix bio vec merging" has been added to the 4.12-stable tree

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

xen: fix bio vec merging

to the 4.12-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-4.12 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Roger Pau Monne <[email protected]>
Date: Tue, 18 Jul 2017 15:01:00 +0100
Subject: xen: fix bio vec merging
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From: Roger Pau Monne <[email protected]>

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.

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

@@ -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 [email protected] are

queue-4.12/xen-blkfront-use-a-right-index-when-checking-requests.patch queue-4.12/xen-fix-bio-vec-merging.patch