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From: <security@xenproject.org>
Subject: x86/ept: atomically modify entries in ept_next_level
ept_next_level was passing a live PTE pointer to ept_set_middle_entry,
which was then modified without taking into account that the PTE could
be part of a live EPT table. This wasn't a security issue because the
pages returned by p2m_alloc_ptp are zeroed, so adding such an entry
before actually initializing it didn't allow a guest to access
physical memory addresses it wasn't supposed to access.
This is part of XSA-328.
Reviewed-by: Jan Beulich <jbeulich@suse.com>
--- a/xen/arch/x86/mm/p2m-ept.c
+++ b/xen/arch/x86/mm/p2m-ept.c
@@ -307,6 +307,8 @@ static int ept_next_level(struct p2m_dom
ept_entry_t *ept_entry, *next = NULL, e;
u32 shift, index;
+ ASSERT(next_level);
+
shift = next_level * EPT_TABLE_ORDER;
index = *gfn_remainder >> shift;
@@ -323,16 +325,20 @@ static int ept_next_level(struct p2m_dom
if ( !is_epte_present(&e) )
{
+ int rc;
+
if ( e.sa_p2mt == p2m_populate_on_demand )
return GUEST_TABLE_POD_PAGE;
if ( read_only )
return GUEST_TABLE_MAP_FAILED;
- next = ept_set_middle_entry(p2m, ept_entry);
+ next = ept_set_middle_entry(p2m, &e);
if ( !next )
return GUEST_TABLE_MAP_FAILED;
- /* e is now stale and hence may not be used anymore below. */
+
+ rc = atomic_write_ept_entry(p2m, ept_entry, e, next_level);
+ ASSERT(rc == 0);
}
/* The only time sp would be set here is if we had hit a superpage */
else if ( is_epte_superpage(&e) )
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