[Xen-changelog] [xen stable-4.9] x86/mm: fix race condition in modify_xen_mappings()

[patchbot@xxxxxxx]

commit ddfca4005697afd6169153a817bcb527c1520078
Author:     Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
AuthorDate: Thu Nov 16 11:46:38 2017 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Nov 16 11:46:38 2017 +0100

    x86/mm: fix race condition in modify_xen_mappings()
    
    In modify_xen_mappings(), a L1/L2 page table shall be freed,
    if all entries of this page table are empty. Corresponding
    L2/L3 PTE will need be cleared in such scenario.
    
    However, concurrent paging structure modifications on different
    CPUs may cause the L2/L3 PTEs to be already be cleared or set
    to reference a superpage.
    
    Therefore the logic to enumerate the L1/L2 page table and to
    reset the corresponding L2/L3 PTE need to be protected with
    spinlock. And the _PAGE_PRESENT and _PAGE_PSE flags need be
    checked after the lock is obtained.
    
    Suggested-by: Jan Beulich <jbeulich@xxxxxxxx>
    Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: b9ee1fd7b98064cf27d0f8f1adf1f5359b72c97f
    master date: 2017-11-14 17:11:26 +0100
---
 xen/arch/x86/mm.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 3641f0d..c9c9a29 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -6552,6 +6552,27 @@ int modify_xen_mappings(unsigned long s, unsigned long 
e, unsigned int nf)
              */
             if ( (nf & _PAGE_PRESENT) || ((v != e) && (l1_table_offset(v) != 
0)) )
                 continue;
+            if ( locking )
+                spin_lock(&map_pgdir_lock);
+
+            /*
+             * L2E may be already cleared, or set to a superpage, by
+             * concurrent paging structure modifications on other CPUs.
+             */
+            if ( !(l2e_get_flags(*pl2e) & _PAGE_PRESENT) )
+            {
+                if ( locking )
+                    spin_unlock(&map_pgdir_lock);
+                goto check_l3;
+            }
+
+            if ( l2e_get_flags(*pl2e) & _PAGE_PSE )
+            {
+                if ( locking )
+                    spin_unlock(&map_pgdir_lock);
+                continue;
+            }
+
             pl1e = l2e_to_l1e(*pl2e);
             for ( i = 0; i < L1_PAGETABLE_ENTRIES; i++ )
                 if ( l1e_get_intpte(pl1e[i]) != 0 )
@@ -6560,11 +6581,16 @@ int modify_xen_mappings(unsigned long s, unsigned long 
e, unsigned int nf)
             {
                 /* Empty: zap the L2E and free the L1 page. */
                 l2e_write_atomic(pl2e, l2e_empty());
+                if ( locking )
+                    spin_unlock(&map_pgdir_lock);
                 flush_area(NULL, FLUSH_TLB_GLOBAL); /* flush before free */
                 free_xen_pagetable(pl1e);
             }
+            else if ( locking )
+                spin_unlock(&map_pgdir_lock);
         }
 
+ check_l3:
         /*
          * If we are not destroying mappings, or not done with the L3E,
          * skip the empty&free check.
@@ -6572,6 +6598,21 @@ int modify_xen_mappings(unsigned long s, unsigned long 
e, unsigned int nf)
         if ( (nf & _PAGE_PRESENT) ||
              ((v != e) && (l2_table_offset(v) + l1_table_offset(v) != 0)) )
             continue;
+        if ( locking )
+            spin_lock(&map_pgdir_lock);
+
+        /*
+         * L3E may be already cleared, or set to a superpage, by
+         * concurrent paging structure modifications on other CPUs.
+         */
+        if ( !(l3e_get_flags(*pl3e) & _PAGE_PRESENT) ||
+              (l3e_get_flags(*pl3e) & _PAGE_PSE) )
+        {
+            if ( locking )
+                spin_unlock(&map_pgdir_lock);
+            continue;
+        }
+
         pl2e = l3e_to_l2e(*pl3e);
         for ( i = 0; i < L2_PAGETABLE_ENTRIES; i++ )
             if ( l2e_get_intpte(pl2e[i]) != 0 )
@@ -6580,9 +6621,13 @@ int modify_xen_mappings(unsigned long s, unsigned long 
e, unsigned int nf)
         {
             /* Empty: zap the L3E and free the L2 page. */
             l3e_write_atomic(pl3e, l3e_empty());
+            if ( locking )
+                spin_unlock(&map_pgdir_lock);
             flush_area(NULL, FLUSH_TLB_GLOBAL); /* flush before free */
             free_xen_pagetable(pl2e);
         }
+        else if ( locking )
+            spin_unlock(&map_pgdir_lock);
     }
 
     flush_area(NULL, FLUSH_TLB_GLOBAL);
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.9

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