x86/shadow: shrink struct page_info's shadow_flags to 16 bits

This is to avoid it overlapping the linear_pt_count field needed for PV
domains. Introduce a separate, HVM-only pagetable_dying field to replace
the sole one left in the upper 16 bits.

Note that the accesses to ->shadow_flags in shadow_{pro,de}mote() get
switched to non-atomic, non-bitops operations, as {test,set,clear}_bit()
are not allowed on uint16_t fields and hence their use would have
required ugly casts. This is fine because all updates of the field ought
to occur with the paging lock held, and other updates of it use |= and
&= as well (i.e. using atomic operations here didn't really guard
against potentially racing updates elsewhere).

This is part of XSA-280.

Reported-by: Prgmr.com Security <security@prgmr.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>

diff --git a/xen/arch/x86/mm/shadow/common.c b/xen/arch/x86/mm/shadow/common.c
index 958a4b7fbe11a2c906a36f134060117f6c457a00..f76e5e7564be1600bbf50fe88c450e51ff8f4517 100644 (file)
--- a/xen/arch/x86/mm/shadow/common.c
+++ b/xen/arch/x86/mm/shadow/common.c
@@ -787,10 +787,14 @@ void shadow_promote(struct domain *d, mfn_t gmfn, unsigned int type)
 
     /* Is the page already shadowed? */
     if ( !test_and_set_bit(_PGC_page_table, &page->count_info) )
+    {
         page->shadow_flags = 0;
+        if ( is_hvm_domain(d) )
+            page->pagetable_dying = false;
+    }
 
-    ASSERT(!test_bit(type, &page->shadow_flags));
-    set_bit(type, &page->shadow_flags);
+    ASSERT(!(page->shadow_flags & (1u << type)));
+    page->shadow_flags |= 1u << type;
     TRACE_SHADOW_PATH_FLAG(TRCE_SFLAG_PROMOTE);
 }
 
@@ -799,9 +803,9 @@ void shadow_demote(struct domain *d, mfn_t gmfn, u32 type)
     struct page_info *page = mfn_to_page(gmfn);
 
     ASSERT(test_bit(_PGC_page_table, &page->count_info));
-    ASSERT(test_bit(type, &page->shadow_flags));
+    ASSERT(page->shadow_flags & (1u << type));
 
-    clear_bit(type, &page->shadow_flags);
+    page->shadow_flags &= ~(1u << type);
 
     if ( (page->shadow_flags & SHF_page_type_mask) == 0 )
     {
@@ -2405,7 +2409,7 @@ void sh_remove_shadows(struct domain *d, mfn_t gmfn, int fast, int all)
     if ( !fast && all && (pg->count_info & PGC_page_table) )
     {
         printk(XENLOG_G_ERR "can't find all shadows of mfn %"PRI_mfn
-               " (shadow_flags=%08x)\n", mfn_x(gmfn), pg->shadow_flags);
+               " (shadow_flags=%04x)\n", mfn_x(gmfn), pg->shadow_flags);
         domain_crash(d);
     }
 

diff --git a/xen/arch/x86/mm/shadow/multi.c b/xen/arch/x86/mm/shadow/multi.c
index 669c3f2c5e5bc291b55b052e99be9f0758e4e3ab..7e9cbc69be97e505a869d723ddc4944cb8fc1837 100644 (file)
--- a/xen/arch/x86/mm/shadow/multi.c
+++ b/xen/arch/x86/mm/shadow/multi.c
@@ -3298,8 +3298,8 @@ static int sh_page_fault(struct vcpu *v,
 
     /* Unshadow if we are writing to a toplevel pagetable that is
      * flagged as a dying process, and that is not currently used. */
-    if ( sh_mfn_is_a_page_table(gmfn)
-         && (mfn_to_page(gmfn)->shadow_flags & SHF_pagetable_dying) )
+    if ( sh_mfn_is_a_page_table(gmfn) && is_hvm_domain(d) &&
+         mfn_to_page(gmfn)->pagetable_dying )
     {
         int used = 0;
         struct vcpu *tmp;
@@ -4261,9 +4261,9 @@ int sh_rm_write_access_from_sl1p(struct domain *d, mfn_t gmfn,
     ASSERT(mfn_valid(smfn));
 
     /* Remember if we've been told that this process is being torn down */
-    if ( curr->domain == d )
+    if ( curr->domain == d && is_hvm_domain(d) )
         curr->arch.paging.shadow.pagetable_dying
-            = !!(mfn_to_page(gmfn)->shadow_flags & SHF_pagetable_dying);
+            = mfn_to_page(gmfn)->pagetable_dying;
 
     sp = mfn_to_page(smfn);
 
@@ -4580,10 +4580,10 @@ static void sh_pagetable_dying(paddr_t gpa)
                    : shadow_hash_lookup(d, mfn_x(gmfn), SH_type_l2_pae_shadow);
         }
 
-        if ( mfn_valid(smfn) )
+        if ( mfn_valid(smfn) && is_hvm_domain(d) )
         {
             gmfn = _mfn(mfn_to_page(smfn)->v.sh.back);
-            mfn_to_page(gmfn)->shadow_flags |= SHF_pagetable_dying;
+            mfn_to_page(gmfn)->pagetable_dying = true;
             shadow_unhook_mappings(d, smfn, 1/* user pages only */);
             flush = 1;
         }
@@ -4621,9 +4621,9 @@ static void sh_pagetable_dying(paddr_t gpa)
     smfn = shadow_hash_lookup(d, mfn_x(gmfn), SH_type_l4_64_shadow);
 #endif
 
-    if ( mfn_valid(smfn) )
+    if ( mfn_valid(smfn) && is_hvm_domain(d) )
     {
-        mfn_to_page(gmfn)->shadow_flags |= SHF_pagetable_dying;
+        mfn_to_page(gmfn)->pagetable_dying = true;
         shadow_unhook_mappings(d, smfn, 1/* user pages only */);
         /* Now flush the TLB: we removed toplevel mappings. */
         flush_tlb_mask(d->dirty_cpumask);

diff --git a/xen/arch/x86/mm/shadow/private.h b/xen/arch/x86/mm/shadow/private.h
index 701ab494df130f838ff7e3d50525e86a0d1e8b78..f7424650cec246ef34e2a6b96af8f7cfcd1e96c6 100644 (file)
--- a/xen/arch/x86/mm/shadow/private.h
+++ b/xen/arch/x86/mm/shadow/private.h
@@ -290,8 +290,6 @@ static inline void sh_terminate_list(struct page_list_head *tmp_list)
 
 #endif /* (SHADOW_OPTIMIZATIONS & SHOPT_OUT_OF_SYNC) */
 
-#define SHF_pagetable_dying (1u<<31)
-
 static inline int sh_page_has_multiple_shadows(struct page_info *pg)
 {
     u32 shadows;

diff --git a/xen/include/asm-x86/mm.h b/xen/include/asm-x86/mm.h
index 7565f28f0f3048e4bbe111325376c32e113effc0..1ea173c555c4c1a42036e41bdc776c7206870ecc 100644 (file)
--- a/xen/include/asm-x86/mm.h
+++ b/xen/include/asm-x86/mm.h
@@ -259,8 +259,15 @@ struct page_info
          * Guest pages with a shadow.  This does not conflict with
          * tlbflush_timestamp since page table pages are explicitly not
          * tracked for TLB-flush avoidance when a guest runs in shadow mode.
+         *
+         * pagetable_dying is used for HVM domains only. The layout here has
+         * to avoid re-use of the space used by linear_pt_count, which (only)
+         * PV guests use.
          */
-        u32 shadow_flags;
+        struct {
+            uint16_t shadow_flags;
+            bool pagetable_dying;
+        };
 
         /* When in use as a shadow, next shadow in this hash chain. */
         __pdx_t next_shadow;