x86/shadow: fix race condition sampling the dirty vram state

d->arch.hvm_domain.dirty_vram must be read with the domain's paging lock held.

If not, two concurrent hypercalls could both end up attempting to free
dirty_vram (the second of which will free a wild pointer), or both end up
allocating a new dirty_vram structure (the first of which will be leaked).

This is XSA-104.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Tim Deegan <tim@xen.org>
master commit: 46a49b91f1026f64430b84dd83e845a33f06415e
master date: 2014-09-23 14:31:47 +0200

diff --git a/xen/arch/x86/mm/shadow/common.c b/xen/arch/x86/mm/shadow/common.c
index be095f6254cf0eacb001d456625a2f81939943f5..3ed48c41b78489d9d641633f724f96d50c24faff 100644 (file)
--- a/xen/arch/x86/mm/shadow/common.c
+++ b/xen/arch/x86/mm/shadow/common.c
@@ -3486,7 +3486,7 @@ int shadow_track_dirty_vram(struct domain *d,
     int flush_tlb = 0;
     unsigned long i;
     p2m_type_t t;
-    struct sh_dirty_vram *dirty_vram = d->arch.hvm_domain.dirty_vram;
+    struct sh_dirty_vram *dirty_vram;
     struct p2m_domain *p2m = p2m_get_hostp2m(d);
 
     if ( end_pfn < begin_pfn || end_pfn > p2m->max_mapped_pfn + 1 )
@@ -3496,6 +3496,8 @@ int shadow_track_dirty_vram(struct domain *d,
     p2m_lock(p2m_get_hostp2m(d));
     paging_lock(d);
 
+    dirty_vram = d->arch.hvm_domain.dirty_vram;
+
     if ( dirty_vram && (!nr ||
              ( begin_pfn != dirty_vram->begin_pfn
             || end_pfn   != dirty_vram->end_pfn )) )

diff --git a/xen/include/asm-x86/hvm/domain.h b/xen/include/asm-x86/hvm/domain.h
index b1e31871cca805425390f25fa755d0f8cd389ee5..99c5e4486e67a743ada68d6d11de17fb66472b1c 100644 (file)
--- a/xen/include/asm-x86/hvm/domain.h
+++ b/xen/include/asm-x86/hvm/domain.h
@@ -67,7 +67,7 @@ struct hvm_domain {
     /* Memory ranges with pinned cache attributes. */
     struct list_head       pinned_cacheattr_ranges;
 
-    /* VRAM dirty support. */
+    /* VRAM dirty support.  Protect with the domain paging lock. */
     struct sh_dirty_vram *dirty_vram;
 
     /* If one of vcpus of this domain is in no_fill_mode or