VT-d: correct ordering of operations in cleanup_domid_map()

The function may be called without any locks held (leaving aside the
domctl one, which we surely don't want to depend on here), so needs to
play safe wrt other accesses to domid_map[] and domid_bitmap[]. This is
to avoid context_set_domain_id()'s writing of domid_map[] to be reset to
zero right away in the case of it racing the freeing of a DID.

For the interaction with context_set_domain_id() and did_to_domain_id()
see the code comment.

{check_,}cleanup_domid_map() are called with pcidevs_lock held or during
domain cleanup only (and pcidevs_lock is also held around
context_set_domain_id()), i.e. racing calls with the same (dom, iommu)
tuple cannot occur.

domain_iommu_domid(), besides its use by cleanup_domid_map(), has its
result used only to control flushing, and hence a stale result would
only lead to a stray extra flush.

This is CVE-2022-26357 / XSA-399.

Fixes: b9c20c78789f ("VT-d: per-iommu domain-id")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
master commit: d9eca7bb6c6636eb87bb17b08ba7de270f47ecd0
master date: 2022-04-05 14:12:27 +0200

diff --git a/xen/drivers/passthrough/vtd/iommu.c b/xen/drivers/passthrough/vtd/iommu.c
index d8393d31d8eb3a6db333a6bdcae07ef96f44d961..e7ba56e8099dcf3b896c1e0186e789c58cdd7f3e 100644 (file)
--- a/xen/drivers/passthrough/vtd/iommu.c
+++ b/xen/drivers/passthrough/vtd/iommu.c
@@ -1770,8 +1770,14 @@ static int domain_context_unmap(struct domain *domain, u8 devfn,
             goto out;
         }
 
+        /*
+         * Update domid_map[] /before/ domid_bitmap[] to avoid a race with
+         * context_set_domain_id(), setting the slot to DOMID_INVALID for
+         * ->domid_map[] reads to produce a suitable value while the bit is
+         * still set.
+         */
+        iommu->domid_map[iommu_domid] = DOMID_INVALID;
         clear_bit(iommu_domid, iommu->domid_bitmap);
-        iommu->domid_map[iommu_domid] = 0;
     }
 
 out: