[Xen-devel] [PATCH v3 1/2] gnttab: don't use possibly unbounded tail calls

["Jan Beulich" <JBeulich@xxxxxxxx>]

There is no guarantee that the compiler would actually translate them
to branches instead of calls, so only ones with a known recursion limit
are okay:
- __release_grant_for_copy() can call itself only once, as
  __acquire_grant_for_copy() won't permit use of multi-level transitive
  grants,
- __acquire_grant_for_copy() is fine to call itself with the last
  argument false, as that prevents further recursion,
- __acquire_grant_for_copy() must not call itself to recover from an
  observed change to the active entry's pin count

This is part of XSA-226.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v2: Zap *page prior to returning ERESTART. Fix i == 0 case in the exit
    path being added to gnttab_copy().

--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -2105,8 +2105,10 @@ __release_grant_for_copy(
 
     if ( td != rd )
     {
-        /* Recursive calls, but they're tail calls, so it's
-           okay. */
+        /*
+         * Recursive calls, but they're bounded (acquire permits only a single
+         * level of transitivity), so it's okay.
+         */
         if ( released_write )
             __release_grant_for_copy(td, trans_gref, 0);
         else if ( released_read )
@@ -2257,10 +2259,11 @@ __acquire_grant_for_copy(
                 return rc;
             }
 
-            /* We dropped the lock, so we have to check that nobody
-               else tried to pin (or, for that matter, unpin) the
-               reference in *this* domain.  If they did, just give up
-               and try again. */
+            /*
+             * We dropped the lock, so we have to check that nobody else tried
+             * to pin (or, for that matter, unpin) the reference in *this*
+             * domain.  If they did, just give up and tell the caller to retry.
+             */
             if ( act->pin != old_pin )
             {
                 __fixup_status_for_copy_pin(act, status);
@@ -2268,9 +2271,8 @@ __acquire_grant_for_copy(
                 active_entry_release(act);
                 grant_read_unlock(rgt);
                 put_page(*page);
-                return __acquire_grant_for_copy(rd, gref, ldom, readonly,
-                                                frame, page, page_off, length,
-                                                allow_transitive);
+                *page = NULL;
+                return ERESTART;
             }
 
             /* The actual remote remote grant may or may not be a
@@ -2576,7 +2578,7 @@ static int gnttab_copy_one(const struct
     {
         gnttab_copy_release_buf(src);
         rc = gnttab_copy_claim_buf(op, &op->source, src, GNTCOPY_source_gref);
-        if ( rc < 0 )
+        if ( rc )
             goto out;
     }
 
@@ -2586,7 +2588,7 @@ static int gnttab_copy_one(const struct
     {
         gnttab_copy_release_buf(dest);
         rc = gnttab_copy_claim_buf(op, &op->dest, dest, GNTCOPY_dest_gref);
-        if ( rc < 0 )
+        if ( rc )
             goto out;
     }
 
@@ -2608,7 +2610,7 @@ static long gnttab_copy(
     {
         if ( i && hypercall_preempt_check() )
         {
-            rc = i;
+            rc = count - i;
             break;
         }
 
@@ -2618,13 +2620,20 @@ static long gnttab_copy(
             break;
         }
 
-        op.status = gnttab_copy_one(&op, &dest, &src);
-        if ( op.status != GNTST_okay )
+        rc = gnttab_copy_one(&op, &dest, &src);
+        if ( rc > 0 )
+        {
+            rc = count - i;
+            break;
+        }
+        if ( rc != GNTST_okay )
         {
             gnttab_copy_release_buf(&src);
             gnttab_copy_release_buf(&dest);
         }
 
+        op.status = rc;
+        rc = 0;
         if ( unlikely(__copy_field_to_guest(uop, &op, status)) )
         {
             rc = -EFAULT;
@@ -3162,6 +3171,7 @@ do_grant_table_op(
         rc = gnttab_copy(copy, count);
         if ( rc > 0 )
         {
+            rc = count - rc;
             guest_handle_add_offset(copy, rc);
             uop = guest_handle_cast(copy, void);
         }



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