From: =?UTF-8?q?Roger=20Pau=20Monn=C3=A9?= <roger.pau@citrix.com>
Subject: x86: protect conditional lock taking from speculative execution

Conditionally taken locks that use the pattern:

if ( lock )
    spin_lock(...);

Need an else branch in order to issue an speculation barrier in the else case,
just like it's done in case the lock needs to be acquired.

eval_nospec() could be used on the condition itself, but that would result in a
double barrier on the branch where the lock is taken.

Introduce a new pair of helpers, {gfn,spin}_lock_if() that can be used to
conditionally take a lock in a speculation safe way.

This is part of XSA-453 / CVE-2024-2193

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 03cf7ca23e0e876075954c558485b267b7d02406)

--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -5249,8 +5249,7 @@ static l3_pgentry_t *virt_to_xen_l3e(uns
         if ( !pl3e )
             return NULL;
         clear_page(pl3e);
-        if ( locking )
-            spin_lock(&map_pgdir_lock);
+        spin_lock_if(locking, &map_pgdir_lock);
         if ( !(l4e_get_flags(*pl4e) & _PAGE_PRESENT) )
         {
             l4_pgentry_t l4e = l4e_from_paddr(__pa(pl3e), __PAGE_HYPERVISOR);
@@ -5284,8 +5283,7 @@ static l2_pgentry_t *virt_to_xen_l2e(uns
         if ( !pl2e )
             return NULL;
         clear_page(pl2e);
-        if ( locking )
-            spin_lock(&map_pgdir_lock);
+        spin_lock_if(locking, &map_pgdir_lock);
         if ( !(l3e_get_flags(*pl3e) & _PAGE_PRESENT) )
         {
             l3e_write(pl3e, l3e_from_paddr(__pa(pl2e), __PAGE_HYPERVISOR));
@@ -5317,8 +5315,7 @@ l1_pgentry_t *virt_to_xen_l1e(unsigned l
         if ( !pl1e )
             return NULL;
         clear_page(pl1e);
-        if ( locking )
-            spin_lock(&map_pgdir_lock);
+        spin_lock_if(locking, &map_pgdir_lock);
         if ( !(l2e_get_flags(*pl2e) & _PAGE_PRESENT) )
         {
             l2e_write(pl2e, l2e_from_paddr(__pa(pl1e), __PAGE_HYPERVISOR));
@@ -5353,6 +5350,8 @@ l1_pgentry_t *virt_to_xen_l1e(unsigned l
     do {                      \
         if ( locking )        \
             l3t_lock(page);   \
+        else                            \
+            block_lock_speculation();   \
     } while ( false )
 
 #define L3T_UNLOCK(page)                           \
@@ -5503,8 +5502,7 @@ int map_pages_to_xen(
             if ( l3e_get_flags(ol3e) & _PAGE_GLOBAL )
                 flush_flags |= FLUSH_TLB_GLOBAL;
 
-            if ( locking )
-                spin_lock(&map_pgdir_lock);
+            spin_lock_if(locking, &map_pgdir_lock);
             if ( (l3e_get_flags(*pl3e) & _PAGE_PRESENT) &&
                  (l3e_get_flags(*pl3e) & _PAGE_PSE) )
             {
@@ -5601,8 +5599,7 @@ int map_pages_to_xen(
                 if ( l2e_get_flags(*pl2e) & _PAGE_GLOBAL )
                     flush_flags |= FLUSH_TLB_GLOBAL;
 
-                if ( locking )
-                    spin_lock(&map_pgdir_lock);
+                spin_lock_if(locking, &map_pgdir_lock);
                 if ( (l2e_get_flags(*pl2e) & _PAGE_PRESENT) &&
                      (l2e_get_flags(*pl2e) & _PAGE_PSE) )
                 {
@@ -5640,8 +5637,7 @@ int map_pages_to_xen(
             {
                 unsigned long base_mfn;
 
-                if ( locking )
-                    spin_lock(&map_pgdir_lock);
+                spin_lock_if(locking, &map_pgdir_lock);
 
                 ol2e = *pl2e;
                 /*
@@ -5693,8 +5689,7 @@ int map_pages_to_xen(
         {
             unsigned long base_mfn;
 
-            if ( locking )
-                spin_lock(&map_pgdir_lock);
+            spin_lock_if(locking, &map_pgdir_lock);
 
             ol3e = *pl3e;
             /*
@@ -5827,8 +5822,8 @@ int modify_xen_mappings(unsigned long s,
                           l2e_from_pfn(l3e_get_pfn(*pl3e) +
                                        (i << PAGETABLE_ORDER),
                                        l3e_get_flags(*pl3e)));
-            if ( locking )
-                spin_lock(&map_pgdir_lock);
+
+            spin_lock_if(locking, &map_pgdir_lock);
             if ( (l3e_get_flags(*pl3e) & _PAGE_PRESENT) &&
                  (l3e_get_flags(*pl3e) & _PAGE_PSE) )
             {
@@ -5882,8 +5877,8 @@ int modify_xen_mappings(unsigned long s,
                     l1e_write(&pl1e[i],
                               l1e_from_pfn(l2e_get_pfn(*pl2e) + i,
                                            l2e_get_flags(*pl2e) & ~_PAGE_PSE));
-                if ( locking )
-                    spin_lock(&map_pgdir_lock);
+
+                spin_lock_if(locking, &map_pgdir_lock);
                 if ( (l2e_get_flags(*pl2e) & _PAGE_PRESENT) &&
                      (l2e_get_flags(*pl2e) & _PAGE_PSE) )
                 {
@@ -5925,8 +5920,7 @@ int modify_xen_mappings(unsigned long s,
              */
             if ( (nf & _PAGE_PRESENT) || ((v != e) && (l1_table_offset(v) != 0)) )
                 continue;
-            if ( locking )
-                spin_lock(&map_pgdir_lock);
+            spin_lock_if(locking, &map_pgdir_lock);
 
             /*
              * L2E may be already cleared, or set to a superpage, by
@@ -5971,8 +5965,7 @@ int modify_xen_mappings(unsigned long s,
         if ( (nf & _PAGE_PRESENT) ||
              ((v != e) && (l2_table_offset(v) + l1_table_offset(v) != 0)) )
             continue;
-        if ( locking )
-            spin_lock(&map_pgdir_lock);
+        spin_lock_if(locking, &map_pgdir_lock);
 
         /*
          * L3E may be already cleared, or set to a superpage, by
--- a/xen/arch/x86/mm/mm-locks.h
+++ b/xen/arch/x86/mm/mm-locks.h
@@ -347,6 +347,15 @@ static inline void p2m_unlock(struct p2m
 #define p2m_locked_by_me(p)   mm_write_locked_by_me(&(p)->lock)
 #define gfn_locked_by_me(p,g) p2m_locked_by_me(p)
 
+static always_inline void gfn_lock_if(bool condition, struct p2m_domain *p2m,
+                                      gfn_t gfn, unsigned int order)
+{
+    if ( condition )
+        gfn_lock(p2m, gfn, order);
+    else
+        block_lock_speculation();
+}
+
 /* PoD lock (per-p2m-table)
  *
  * Protects private PoD data structs: entry and cache
--- a/xen/arch/x86/mm/p2m.c
+++ b/xen/arch/x86/mm/p2m.c
@@ -502,9 +502,8 @@ mfn_t __get_gfn_type_access(struct p2m_d
         return _mfn(gfn_l);
     }
 
-    if ( locked )
-        /* Grab the lock here, don't release until put_gfn */
-        gfn_lock(p2m, gfn, 0);
+    /* Grab the lock here, don't release until put_gfn */
+    gfn_lock_if(locked, p2m, gfn, 0);
 
     mfn = p2m->get_entry(p2m, gfn, t, a, q, page_order, NULL);
 
--- a/xen/include/xen/spinlock.h
+++ b/xen/include/xen/spinlock.h
@@ -208,6 +208,14 @@ static always_inline void spin_lock_irq(
         block_lock_speculation();                               \
     })
 
+/* Conditionally take a spinlock in a speculation safe way. */
+static always_inline void spin_lock_if(bool condition, spinlock_t *l)
+{
+    if ( condition )
+        _spin_lock(l);
+    block_lock_speculation();
+}
+
 #define spin_unlock(l)                _spin_unlock(l)
 #define spin_unlock_irq(l)            _spin_unlock_irq(l)
 #define spin_unlock_irqrestore(l, f)  _spin_unlock_irqrestore(l, f)
