xen/x86: p2m: Add preemption in p2m_teardown()

The list p2m->pages contain all the pages used by the P2M. On large
instance this can be quite large and the time spent to call
d->arch.paging.free_page() will take more than 1ms for a 80GB guest
on a Xen running in nested environment on a c5.metal.

By extrapolation, it would take > 100ms for a 8TB guest (what we
current security support). So add some preemption in p2m_teardown()
and propagate to the callers. Note there are 3 places where
the preemption is not enabled:
    - hap_final_teardown()/shadow_final_teardown(): We are
      preventing update the P2M once the domain is dying (so
      no more pages could be allocated) and most of the P2M pages
      will be freed in preemptive manneer when relinquishing the
      resources. So this is fine to disable preemption.
    - shadow_enable(): This is fine because it will undo the allocation
      that may have been made by p2m_alloc_table() (so only the root
      page table).

The preemption is arbitrarily checked every 1024 iterations.

We now need to include <xen/event.h> in p2m-basic in order to
import the definition for local_events_need_delivery() used by
general_preempt_check(). Ideally, the inclusion should happen in
xen/sched.h but it opened a can of worms.

Note that with the current approach, Xen doesn't keep track on whether
the alt/nested P2Ms have been cleared. So there are some redundant work.
However, this is not expected to incurr too much overhead (the P2M lock
shouldn't be contended during teardown). So this is optimization is
left outside of the security event.

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

Signed-off-by: Julien Grall <jgrall@amazon.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
master commit: 8a2111250b424edc49c65c4d41b276766d30635c
master date: 2022-10-11 14:24:48 +0200

diff --git a/xen/arch/x86/mm/hap/hap.c b/xen/arch/x86/mm/hap/hap.c
index 92b201453481a51fda2dbdd3df0b9a31d594c228..34bbe50be06669fe9d76a6e61af6960df5bcdea3 100644 (file)
--- a/xen/arch/x86/mm/hap/hap.c
+++ b/xen/arch/x86/mm/hap/hap.c
@@ -534,17 +534,17 @@ void hap_final_teardown(struct domain *d)
 
     if ( hvm_altp2m_supported() )
         for ( i = 0; i < MAX_ALTP2M; i++ )
-            p2m_teardown(d->arch.altp2m_p2m[i], true);
+            p2m_teardown(d->arch.altp2m_p2m[i], true, NULL);
 
     /* Destroy nestedp2m's first */
     for (i = 0; i < MAX_NESTEDP2M; i++) {
-        p2m_teardown(d->arch.nested_p2m[i], true);
+        p2m_teardown(d->arch.nested_p2m[i], true, NULL);
     }
 
     if ( d->arch.paging.hap.total_pages != 0 )
         hap_teardown(d, NULL);
 
-    p2m_teardown(p2m_get_hostp2m(d), true);
+    p2m_teardown(p2m_get_hostp2m(d), true, NULL);
     /* Free any memory that the p2m teardown released */
     paging_lock(d);
     hap_set_allocation(d, 0, NULL);
@@ -594,14 +594,24 @@ void hap_teardown(struct domain *d, bool *preempted)
         FREE_XENHEAP_PAGE(d->arch.altp2m_eptp);
 
         for ( i = 0; i < MAX_ALTP2M; i++ )
-            p2m_teardown(d->arch.altp2m_p2m[i], false);
+        {
+            p2m_teardown(d->arch.altp2m_p2m[i], false, preempted);
+            if ( preempted && *preempted )
+                return;
+        }
     }
 
     /* Destroy nestedp2m's after altp2m. */
     for ( i = 0; i < MAX_NESTEDP2M; i++ )
-        p2m_teardown(d->arch.nested_p2m[i], false);
+    {
+        p2m_teardown(d->arch.nested_p2m[i], false, preempted);
+        if ( preempted && *preempted )
+            return;
+    }
 
-    p2m_teardown(p2m_get_hostp2m(d), false);
+    p2m_teardown(p2m_get_hostp2m(d), false, preempted);
+    if ( preempted && *preempted )
+        return;
 
     paging_lock(d);
 

diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
index 859edfc95b9f23aed4ea2435b0f4963715628d89..5bc2e483a3e85c2a27b5919a29237b8ce5c526c0 100644 (file)
--- a/xen/arch/x86/mm/p2m.c
+++ b/xen/arch/x86/mm/p2m.c
@@ -737,12 +737,13 @@ int p2m_alloc_table(struct p2m_domain *p2m)
  * hvm fixme: when adding support for pvh non-hardware domains, this path must
  * cleanup any foreign p2m types (release refcnts on them).
  */
-void p2m_teardown(struct p2m_domain *p2m, bool remove_root)
+void p2m_teardown(struct p2m_domain *p2m, bool remove_root, bool *preempted)
 /* Return all the p2m pages to Xen.
  * We know we don't have any extra mappings to these pages */
 {
     struct page_info *pg, *root_pg = NULL;
     struct domain *d;
+    unsigned int i = 0;
 
     if (p2m == NULL)
         return;
@@ -761,8 +762,19 @@ void p2m_teardown(struct p2m_domain *p2m, bool remove_root)
     }
 
     while ( (pg = page_list_remove_head(&p2m->pages)) )
-        if ( pg != root_pg )
-            d->arch.paging.free_page(d, pg);
+    {
+        if ( pg == root_pg )
+            continue;
+
+        d->arch.paging.free_page(d, pg);
+
+        /* Arbitrarily check preemption every 1024 iterations */
+        if ( preempted && !(++i % 1024) && general_preempt_check() )
+        {
+            *preempted = true;
+            break;
+        }
+    }
 
     if ( root_pg )
         page_list_add(root_pg, &p2m->pages);

diff --git a/xen/arch/x86/mm/shadow/common.c b/xen/arch/x86/mm/shadow/common.c
index 8679620f18da14286c21124565556471232a828b..e6af359579b727d5e9c1b6766b08e0b53b47a749 100644 (file)
--- a/xen/arch/x86/mm/shadow/common.c
+++ b/xen/arch/x86/mm/shadow/common.c
@@ -2747,8 +2747,12 @@ int shadow_enable(struct domain *d, u32 mode)
  out_locked:
     paging_unlock(d);
  out_unlocked:
+    /*
+     * This is fine to ignore the preemption here because only the root
+     * will be allocated by p2m_alloc_table().
+     */
     if ( rv != 0 && !pagetable_is_null(p2m_get_pagetable(p2m)) )
-        p2m_teardown(p2m, true);
+        p2m_teardown(p2m, true, NULL);
     if ( rv != 0 && pg != NULL )
     {
         pg->count_info &= ~PGC_count_mask;
@@ -2793,7 +2797,9 @@ void shadow_teardown(struct domain *d, bool *preempted)
 
     paging_unlock(d);
 
-    p2m_teardown(p2m_get_hostp2m(d), false);
+    p2m_teardown(p2m_get_hostp2m(d), false, preempted);
+    if ( preempted && *preempted )
+        return;
 
     paging_lock(d);
 
@@ -2912,7 +2918,7 @@ void shadow_final_teardown(struct domain *d)
         shadow_teardown(d, NULL);
 
     /* It is now safe to pull down the p2m map. */
-    p2m_teardown(p2m_get_hostp2m(d), true);
+    p2m_teardown(p2m_get_hostp2m(d), true, NULL);
     /* Free any shadow memory that the p2m teardown released */
     paging_lock(d);
     shadow_set_allocation(d, 0, NULL);

diff --git a/xen/include/asm-x86/p2m.h b/xen/include/asm-x86/p2m.h
index cab4ca60faae82df7cfa134b6b6fc94cc4e40e16..8ba8cd6a022c1711a6cbf7a1a0f405214fc42c17 100644 (file)
--- a/xen/include/asm-x86/p2m.h
+++ b/xen/include/asm-x86/p2m.h
@@ -599,7 +599,7 @@ int p2m_init(struct domain *d);
 int p2m_alloc_table(struct p2m_domain *p2m);
 
 /* Return all the p2m resources to Xen. */
-void p2m_teardown(struct p2m_domain *p2m, bool remove_root);
+void p2m_teardown(struct p2m_domain *p2m, bool remove_root, bool *preempted);
 void p2m_final_teardown(struct domain *d);
 
 /* Add a page to a domain's p2m table */