[Xen-devel] [PATCH] xen/balloon: flush unused mappings before updating P2M table

[Wei Liu <wei.liu2@xxxxxxxxxx>]

Xen balloon driver will update ballooned out pages' P2M entries to point
to scratch page for PV guests. In 24f69373e2 ("xen/balloon: don't alloc
page while non-preemptible", kmap_flush_unused was moved after the
update for P2M table. In that case for 32 bit PV guest we might end up
with

P2M    X -----> scratch_page
M2P    Y -----> X  (Y is mfn in unused kmap entry)

When PVMMU is consulted, it gets confused and returns the wrong value.
Eventually the guest crashes.

Move the flush before __set_phys_to_machine to fix this.

Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
Cc: David Vrabel <david.vrabel@xxxxxxxxxx>
Cc: Konrad Wilk <konrad.wilk@xxxxxxxxxx>
Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
---
 drivers/xen/balloon.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
index 37d06ea..49a809c 100644
--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -392,6 +392,10 @@ static enum bp_state decrease_reservation(unsigned long 
nr_pages, gfp_t gfp)
        if (nr_pages > ARRAY_SIZE(frame_list))
                nr_pages = ARRAY_SIZE(frame_list);
 
+       /* Ensure that ballooned highmem pages don't have kmaps. */
+       kmap_flush_unused();
+       flush_tlb_all();
+
        for (i = 0; i < nr_pages; i++) {
                page = alloc_page(gfp);
                if (page == NULL) {
@@ -432,10 +436,6 @@ static enum bp_state decrease_reservation(unsigned long 
nr_pages, gfp_t gfp)
                balloon_append(pfn_to_page(pfn));
        }
 
-       /* Ensure that ballooned highmem pages don't have kmaps. */
-       kmap_flush_unused();
-       flush_tlb_all();
-
        set_xen_guest_handle(reservation.extent_start, frame_list);
        reservation.nr_extents   = nr_pages;
        ret = HYPERVISOR_memory_op(XENMEM_decrease_reservation, &reservation);
-- 
1.7.10.4


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