[PATCH 1/2] gnttab: remove guest_physmap_remove_page() call from gnttab_map_frame()

[Jan Beulich <jbeulich@xxxxxxxx>]

Without holding appropriate locks, attempting to remove a prior mapping
of the underlying page is pointless, as the same (or another) mapping
could be re-established by a parallel request on another vCPU. Move the
code to Arm's gnttab_set_frame_gfn(). Of course this new placement
doesn't improve things in any way as far as the security of grant status
frame mappings goes (see XSA-379). Proper locking would be needed here
to allow status frames to be mapped securely.

In turn this then requires replacing the other use in
gnttab_unpopulate_status_frames(), which yet in turn requires adjusting
x86's gnttab_set_frame_gfn(). Note that with proper locking inside
guest_physmap_remove_page() combined with checking the GFN's mapping
there against the passed in MFN, there then is no issue with the
involved multiple gnttab_set_frame_gfn()-s potentially returning varying
values (due to a racing XENMAPSPACE_grant_table request).

This, as a side effect, does away with gnttab_map_frame() having a local
variable "gfn" which shadows a function parameter of the same name.

Together with XSA-379 this points out that XSA-255's addition to
gnttab_map_frame() was really useless.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -92,7 +92,7 @@ struct grant_table {
     struct radix_tree_root maptrack_tree;
 
     /* Domain to which this struct grant_table belongs. */
-    const struct domain *domain;
+    struct domain *domain;
 
     struct grant_table_arch arch;
 };
@@ -1795,8 +1795,8 @@ gnttab_unpopulate_status_frames(struct d
         {
             int rc = gfn_eq(gfn, INVALID_GFN)
                      ? 0
-                     : guest_physmap_remove_page(d, gfn,
-                                                 page_to_mfn(pg), 0);
+                     : gnttab_set_frame_gfn(gt, true, i, INVALID_GFN,
+                                            page_to_mfn(pg));
 
             if ( rc )
             {
@@ -1806,7 +1806,6 @@ gnttab_unpopulate_status_frames(struct d
                 domain_crash(d);
                 return rc;
             }
-            gnttab_set_frame_gfn(gt, true, i, INVALID_GFN);
         }
 
         BUG_ON(page_get_owner(pg) != d);
@@ -4132,6 +4131,9 @@ int gnttab_map_frame(struct domain *d, u
     struct grant_table *gt = d->grant_table;
     bool status = false;
 
+    if ( gfn_eq(gfn, INVALID_GFN) )
+        return -EINVAL;
+
     grant_write_lock(gt);
 
     if ( evaluate_nospec(gt->gt_version == 2) && (idx & 
XENMAPIDX_grant_table_status) )
@@ -4144,24 +4146,18 @@ int gnttab_map_frame(struct domain *d, u
     else
         rc = gnttab_get_shared_frame_mfn(d, idx, mfn);
 
-    if ( !rc && paging_mode_translate(d) )
-    {
-        gfn_t gfn = gnttab_get_frame_gfn(gt, status, idx);
-
-        if ( !gfn_eq(gfn, INVALID_GFN) )
-            rc = guest_physmap_remove_page(d, gfn, *mfn, 0);
-    }
-
     if ( !rc )
     {
+        struct page_info *pg = mfn_to_page(*mfn);
+
         /*
          * Make sure gnttab_unpopulate_status_frames() won't (successfully)
          * free the page until our caller has completed its operation.
          */
-        if ( get_page(mfn_to_page(*mfn), d) )
-            gnttab_set_frame_gfn(gt, status, idx, gfn);
-        else
+        if ( !get_page(pg, d) )
             rc = -EBUSY;
+        else if ( (rc = gnttab_set_frame_gfn(gt, status, idx, gfn, *mfn)) )
+            put_page(pg);
     }
 
     grant_write_unlock(gt);
--- a/xen/include/asm-arm/grant_table.h
+++ b/xen/include/asm-arm/grant_table.h
@@ -71,11 +71,17 @@ int replace_grant_host_mapping(unsigned
         XFREE((gt)->arch.status_gfn);                                    \
     } while ( 0 )
 
-#define gnttab_set_frame_gfn(gt, st, idx, gfn)                           \
-    do {                                                                 \
-        ((st) ? (gt)->arch.status_gfn : (gt)->arch.shared_gfn)[idx] =    \
-            (gfn);                                                       \
-    } while ( 0 )
+#define gnttab_set_frame_gfn(gt, st, idx, gfn, mfn)                      \
+    ({                                                                   \
+        int rc_ = 0;                                                     \
+        gfn_t ogfn = gnttab_get_frame_gfn(gt, st, idx);                  \
+        if ( gfn_eq(ogfn, INVALID_GFN) || gfn_eq(ogfn, gfn) ||           \
+             (rc_ = guest_physmap_remove_page((gt)->domain, ogfn, mfn,   \
+                                              0)) == 0 )                 \
+            ((st) ? (gt)->arch.status_gfn                                \
+                  : (gt)->arch.shared_gfn)[idx] = (gfn);                 \
+        rc_;                                                             \
+    })
 
 #define gnttab_get_frame_gfn(gt, st, idx) ({                             \
    (st) ? gnttab_status_gfn(NULL, gt, idx)                               \
--- a/xen/include/asm-x86/grant_table.h
+++ b/xen/include/asm-x86/grant_table.h
@@ -37,7 +37,12 @@ static inline int replace_grant_host_map
 
 #define gnttab_init_arch(gt) 0
 #define gnttab_destroy_arch(gt) do {} while ( 0 )
-#define gnttab_set_frame_gfn(gt, st, idx, gfn) do {} while ( 0 )
+#define gnttab_set_frame_gfn(gt, st, idx, gfn, mfn)                      \
+    (gfn_eq(gfn, INVALID_GFN)                                            \
+     ? guest_physmap_remove_page((gt)->domain,                           \
+                                 gnttab_get_frame_gfn(gt, st, idx),      \
+                                 mfn, 0)                                 \
+     : 0 /* Handled in add_to_physmap_one(). */)
 #define gnttab_get_frame_gfn(gt, st, idx) ({                             \
     mfn_t mfn_ = (st) ? gnttab_status_mfn(gt, idx)                       \
                       : gnttab_shared_mfn(gt, idx);                      \