Commit 33bab922 authored by Roger Pau Monne's avatar Roger Pau Monne Committed by Ben Hutchings

xen: fix bio vec merging

commit 462cdace 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: default avatar"Jan H. Schönherr" <jschoenh@amazon.de>
Signed-off-by: default avatarRoger Pau Monné <roger.pau@citrix.com>
Reviewed-by: default avatarJuergen Gross <jgross@suse.com>
Signed-off-by: default avatarKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>
[bwh: Backported to 3.2:
 - s/bfn/mfn/g
 - Adjust context]
Signed-off-by: default avatarBen Hutchings <ben@decadent.org.uk>
parent 31f11713
...@@ -8,6 +8,5 @@ bool xen_biovec_phys_mergeable(const struct bio_vec *vec1, ...@@ -8,6 +8,5 @@ bool xen_biovec_phys_mergeable(const struct bio_vec *vec1,
unsigned long mfn1 = pfn_to_mfn(page_to_pfn(vec1->bv_page)); unsigned long mfn1 = pfn_to_mfn(page_to_pfn(vec1->bv_page));
unsigned long mfn2 = pfn_to_mfn(page_to_pfn(vec2->bv_page)); unsigned long mfn2 = pfn_to_mfn(page_to_pfn(vec2->bv_page));
return __BIOVEC_PHYS_MERGEABLE(vec1, vec2) && return mfn1 + PFN_DOWN(vec1->bv_offset + vec1->bv_len) == mfn2;
((mfn1 == mfn2) || ((mfn1+1) == mfn2));
} }
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