[xen staging-4.14] gnttab: never permit mapping transitive grants

[patchbot@xxxxxxx]

commit 4a505ed5c4b7b333e22fb485ddfa5a19029045ad
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Fri Mar 5 15:34:07 2021 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Mar 5 15:34:07 2021 +0100

    gnttab: never permit mapping transitive grants
    
    Transitive grants allow an intermediate domain I to grant a target
    domain T access to a page which origin domain O did grant I access to.
    As an implementation restriction, T is not allowed to map such a grant.
    This restriction is currently tried to be enforced by marking active
    entries resulting from transitive grants as is-sub-page; sub-page grants
    for obvious reasons don't allow mapping. However, marking (and checking)
    only active entries is insufficient, as a map attempt may also occur on
    a grant not otherwise in use. When not presently in use (pin count zero)
    the grant type itself needs checking. Otherwise T may be able to map an
    unrelated page owned by I. This is because the "transitive" sub-
    structure of the v2 union would end up being interpreted as "full_page"
    sub-structure instead. The low 32 bits of the GFN used would match the
    grant reference specified in I's transitive grant entry, while the upper
    32 bits could be random (depending on how exactly I sets up its grant
    table entries).
    
    Note that if one mapping already exists and the granting domain _then_
    changes the grant to GTF_transitive (which the domain is not supposed to
    do), the changed type will only be honored after the pin count has gone
    back to zero. This is no different from e.g. GTF_readonly or
    GTF_sub_page becoming set when a grant is already in use.
    
    While adjusting the implementation, also adjust commentary in the public
    header to better reflect reality.
    
    Fixes: 3672ce675c93 ("Transitive grant support")
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Julien Grall <jgrall@xxxxxxxxxx>
    master commit: b339e3a976b1680f57051adabcb98281198f7eac
    master date: 2021-02-18 13:16:12 +0100
---
 xen/common/grant_table.c         | 9 +++++----
 xen/include/public/grant_table.h | 6 ++++--
 2 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index 9f0cae52c0..4a6ae52ae3 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -836,9 +836,10 @@ static int _set_status_v2(const grant_entry_header_t *shah,
         mask |= GTF_sub_page;
 
     /* If not already pinned, check the grant domid and type. */
-    if ( !act->pin && ((((scombo.flags & mask) != GTF_permit_access) &&
-                        ((scombo.flags & mask) != GTF_transitive)) ||
-                       (scombo.domid != ldomid)) )
+    if ( !act->pin &&
+         ((((scombo.flags & mask) != GTF_permit_access) &&
+           (mapflag || ((scombo.flags & mask) != GTF_transitive))) ||
+          (scombo.domid != ldomid)) )
         PIN_FAIL(done, GNTST_general_error,
                  "Bad flags (%x) or dom (%d); expected d%d, flags %x\n",
                  scombo.flags, scombo.domid, ldomid, mask);
@@ -864,7 +865,7 @@ static int _set_status_v2(const grant_entry_header_t *shah,
     if ( !act->pin )
     {
         if ( (((scombo.flags & mask) != GTF_permit_access) &&
-              ((scombo.flags & mask) != GTF_transitive)) ||
+              (mapflag || ((scombo.flags & mask) != GTF_transitive))) ||
              (scombo.domid != ldomid) ||
              (!readonly && (scombo.flags & GTF_readonly)) )
         {
diff --git a/xen/include/public/grant_table.h b/xen/include/public/grant_table.h
index 3b7bf93d74..84b1d26b36 100644
--- a/xen/include/public/grant_table.h
+++ b/xen/include/public/grant_table.h
@@ -166,11 +166,13 @@ typedef struct grant_entry_v1 grant_entry_v1_t;
 #define GTF_type_mask       (3U<<0)
 
 /*
- * Subflags for GTF_permit_access.
+ * Subflags for GTF_permit_access and GTF_transitive.
  *  GTF_readonly: Restrict @domid to read-only mappings and accesses. [GST]
  *  GTF_reading: Grant entry is currently mapped for reading by @domid. [XEN]
  *  GTF_writing: Grant entry is currently mapped for writing by @domid. [XEN]
- *  GTF_PAT, GTF_PWT, GTF_PCD: (x86) cache attribute flags for the grant [GST]
+ * Further subflags for GTF_permit_access only.
+ *  GTF_PAT, GTF_PWT, GTF_PCD: (x86) cache attribute flags to be used for
+ *                             mappings of the grant [GST]
  *  GTF_sub_page: Grant access to only a subrange of the page.  @domid
  *                will only be allowed to copy from the grant, and not
  *                map it. [GST]
--
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