xen/arm: p2m: Prevent adding mapping when domain is dying

During the domain destroy process, the domain will still be accessible
until it is fully destroyed. So does the P2M because we don't bail
out early if is_dying is non-zero. If a domain has permission to
modify the other domain's P2M (i.e. dom0, or a stubdomain), then
foreign mapping can be added past relinquish_p2m_mapping().

Therefore, we need to prevent mapping to be added when the domain
is dying. This commit prevents such adding of mapping by adding the
d->is_dying check to p2m_set_entry(). Also this commit enhances the
check in relinquish_p2m_mapping() to make sure that no mappings can
be added in the P2M after the P2M lock is released.

This is part of CVE-2022-33746 / XSA-410.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Signed-off-by: Henry Wang <Henry.Wang@arm.com>
Tested-by: Henry Wang <Henry.Wang@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/arch/arm/p2m.c

@@ -1092,6 +1092,15 @@ int p2m_set_entry(struct p2m_domain *p2m,
 {
     int rc = 0;
 
+    /*
+     * Any reference taken by the P2M mappings (e.g. foreign mapping) will
+     * be dropped in relinquish_p2m_mapping(). As the P2M will still
+     * be accessible after, we need to prevent mapping to be added when the
+     * domain is dying.
+     */
+    if ( unlikely(p2m->domain->is_dying) )
+        return -ENOMEM;
+
     while ( nr )
     {
         unsigned long mask;
@@ -1634,6 +1643,8 @@ int relinquish_p2m_mapping(struct domain *d)
     unsigned int order;
     gfn_t start, end;
 
+    BUG_ON(!d->is_dying);
+    /* No mappings can be added in the P2M after the P2M lock is released. */
     p2m_write_lock(p2m);
 
     start = p2m->lowest_mapped_gfn;