[Xen-changelog] [xen stable-4.9] x86/shadow: shrink struct page_info's shadow_flags to 16 bits

[patchbot@xxxxxxx]

commit ca5ede63978f79db910f638472ab51d35d703f27
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Nov 20 15:50:57 2018 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Nov 20 15:50:57 2018 +0100

    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@xxxxxxxxx>
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Tim Deegan <tim@xxxxxxx>
    master commit: 789589968ed90e82a832dbc60e958c76b787be7e
    master date: 2018-11-20 14:59:54 +0100
---
 xen/arch/x86/mm/shadow/common.c  | 14 +++++++++-----
 xen/arch/x86/mm/shadow/multi.c   | 16 ++++++++--------
 xen/arch/x86/mm/shadow/private.h |  2 --
 xen/include/asm-x86/mm.h         |  9 ++++++++-
 4 files changed, 25 insertions(+), 16 deletions(-)

diff --git a/xen/arch/x86/mm/shadow/common.c b/xen/arch/x86/mm/shadow/common.c
index 1189e754b9..5c02a98468 100644
--- a/xen/arch/x86/mm/shadow/common.c
+++ b/xen/arch/x86/mm/shadow/common.c
@@ -955,10 +955,14 @@ 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);
 }
 
@@ -967,9 +971,9 @@ 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 )
     {
@@ -2799,7 +2803,7 @@ void sh_remove_shadows(struct domain *d, mfn_t gmfn, int 
fast, int all)
     if ( !fast && all && (pg->count_info & PGC_page_table) )
     {
         SHADOW_ERROR("can't find all shadows of mfn %"PRI_mfn" "
-                     "(shadow_flags=%08x)\n",
+                     "(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 a53d3db56c..7dc39d7565 100644
--- a/xen/arch/x86/mm/shadow/multi.c
+++ b/xen/arch/x86/mm/shadow/multi.c
@@ -3394,8 +3394,8 @@ 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;
@@ -4366,9 +4366,9 @@ 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);
 
@@ -4684,10 +4684,10 @@ static void sh_pagetable_dying(struct vcpu *v, 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;
         }
@@ -4724,9 +4724,9 @@ static void sh_pagetable_dying(struct vcpu *v, 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->domain_dirty_cpumask);
diff --git a/xen/arch/x86/mm/shadow/private.h b/xen/arch/x86/mm/shadow/private.h
index 990afece4c..d8177209e0 100644
--- a/xen/arch/x86/mm/shadow/private.h
+++ b/xen/arch/x86/mm/shadow/private.h
@@ -292,8 +292,6 @@ 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 56604e209e..7e4ffeb160 100644
--- a/xen/include/asm-x86/mm.h
+++ b/xen/include/asm-x86/mm.h
@@ -171,8 +171,15 @@ 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;
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.9

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