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[Xen-changelog] [xen stable-4.9] gnttab: correct logic to get page references during map requests



commit 0984d0dc1243a266391c8caafc0fa5be2a71f4a6
Author:     George Dunlap <george.dunlap@xxxxxxxxxx>
AuthorDate: Tue Jun 20 15:47:46 2017 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Jun 20 15:47:46 2017 +0200

    gnttab: correct logic to get page references during map requests
    
    The rules for reference counting are somewhat complicated:
    
    * Each of GNTTAB_host_map and GNTTAB_device_map need their own
    reference count
    
    * If the mapping is writeable:
     - GNTTAB_host_map needs a type count under only some conditions
     - GNTTAB_device_map always needs a type count
    
    If the mapping succeeds, we need to keep all of these; if the mapping
    fails, we need to release whatever references we have acquired so far.
    
    Additionally, the code that does a lot of this calculation "inherits"
    a reference as part of the process of finding out who the owner is.
    
    Finally, if the grant is mapped as writeable (without the
    GNTMAP_readonly flag), but the hypervisor cannot grab a
    PGT_writeable_page type, the entire operation should fail.
    
    Unfortunately, the current code has several logic holes:
    
    * If a grant is mapped only GNTTAB_device_map, and with a writeable
      mapping, but in conditions where a *host* type count is not
      necessary, the code will fail to grab the necessary type count.
    
    * If a grant is mapped both GNTTAB_device_map and GNTTAB_host_map,
      with a writeable mapping, in conditions where the host type count is
      not necessary, *and* where the page cannot be changed to type
      PGT_writeable, the condition will not be detected.
    
    In both cases, this means that on success, the type count will be
    erroneously reduced when the grant is unmapped.  In the second case,
    the type count will be erroneously reduced on the failure path as
    well.  (In the first case the failure path logic has the same hole
    as the reference grabbing logic.)
    
    Additionally, the return value of get_page() is not checked; but this
    may fail even if the first get_page() succeeded due to a reference
    counting overflow.
    
    First of all, simplify the restoration logic by explicitly counting
    the reference and type references acquired.
    
    Consider each mapping type separately, explicitly marking the
    'incoming' reference as used so we know when we need to grab a second
    one.
    
    Finally, always check the return value of get_page[_type]() and go to
    the failure path if appropriate.
    
    This is part of XSA-224.
    
    Reported-by: Jan Beulich <jbeulich@xxxxxxxx>
    Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx>
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: 75b384ece635adc55c2bafbdc2d8959c10542c31
    master date: 2017-06-20 14:46:21 +0200
---
 xen/common/grant_table.c | 58 +++++++++++++++++++++++++++---------------------
 1 file changed, 33 insertions(+), 25 deletions(-)

diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index 672582f..34086f9 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -758,12 +758,12 @@ __gnttab_map_grant_ref(
     struct grant_table *lgt, *rgt;
     struct vcpu   *led;
     int            handle;
-    unsigned long  frame = 0, nr_gets = 0;
+    unsigned long  frame = 0;
     struct page_info *pg = NULL;
     int            rc = GNTST_okay;
     u32            old_pin;
     u32            act_pin;
-    unsigned int   cache_flags;
+    unsigned int   cache_flags, refcnt = 0, typecnt = 0;
     struct active_grant_entry *act = NULL;
     struct grant_mapping *mt;
     grant_entry_header_t *shah;
@@ -889,11 +889,17 @@ __gnttab_map_grant_ref(
     else
         owner = page_get_owner(pg);
 
+    if ( owner )
+        refcnt++;
+
     if ( !pg || (owner == dom_io) )
     {
         /* Only needed the reference to confirm dom_io ownership. */
         if ( pg )
+        {
             put_page(pg);
+            refcnt--;
+        }
 
         if ( paging_mode_external(ld) )
         {
@@ -921,27 +927,38 @@ __gnttab_map_grant_ref(
     }
     else if ( owner == rd || owner == dom_cow )
     {
-        if ( gnttab_host_mapping_get_page_type(op, ld, rd) )
+        if ( (op->flags & GNTMAP_device_map) && !(op->flags & GNTMAP_readonly) 
)
         {
             if ( (owner == dom_cow) ||
                  !get_page_type(pg, PGT_writable_page) )
                 goto could_not_pin;
+            typecnt++;
         }
 
-        nr_gets++;
         if ( op->flags & GNTMAP_host_map )
         {
-            rc = create_grant_host_mapping(op->host_addr, frame, op->flags, 0);
-            if ( rc != GNTST_okay )
-                goto undo_out;
-
+            /*
+             * Only need to grab another reference if device_map claimed
+             * the other one.
+             */
             if ( op->flags & GNTMAP_device_map )
             {
-                nr_gets++;
-                (void)get_page(pg, rd);
-                if ( !(op->flags & GNTMAP_readonly) )
-                    get_page_type(pg, PGT_writable_page);
+                if ( !get_page(pg, rd) )
+                    goto could_not_pin;
+                refcnt++;
+            }
+
+            if ( gnttab_host_mapping_get_page_type(op, ld, rd) )
+            {
+                if ( (owner == dom_cow) ||
+                     !get_page_type(pg, PGT_writable_page) )
+                    goto could_not_pin;
+                typecnt++;
             }
+
+            rc = create_grant_host_mapping(op->host_addr, frame, op->flags, 0);
+            if ( rc != GNTST_okay )
+                goto undo_out;
         }
     }
     else
@@ -950,8 +967,6 @@ __gnttab_map_grant_ref(
         if ( !rd->is_dying )
             gdprintk(XENLOG_WARNING, "Could not pin grant frame %lx\n",
                      frame);
-        if ( owner != NULL )
-            put_page(pg);
         rc = GNTST_general_error;
         goto undo_out;
     }
@@ -1014,18 +1029,11 @@ __gnttab_map_grant_ref(
     return;
 
  undo_out:
-    if ( nr_gets > 1 )
-    {
-        if ( !(op->flags & GNTMAP_readonly) )
-            put_page_type(pg);
-        put_page(pg);
-    }
-    if ( nr_gets > 0 )
-    {
-        if ( gnttab_host_mapping_get_page_type(op, ld, rd) )
-            put_page_type(pg);
+    while ( typecnt-- )
+        put_page_type(pg);
+
+    while ( refcnt-- )
         put_page(pg);
-    }
 
     grant_read_lock(rgt);
 
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.9

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