gnttab: correct GNTTABOP_cache_flush empty batch handling

Jann validly points out that with a caller bogusly requesting a zero-
element batch with non-zero high command bits (the ones used for
continuation encoding), the assertion right before the call to
hypercall_create_continuation() would trigger. A similar situation would
arise afaict for non-empty batches with op and/or length zero in every
element.

While we want the former to succeed (as we do elsewhere for similar
no-op requests), the latter can clearly be converted to an error, as
this is a state that can't be the result of a prior operation.

Take the opportunity and also correct the order of argument checks:
We shouldn't accept zero-length elements with unknown bits set in "op".
Also constify cache_flush()'s first parameter.

Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andre Przywara <andre.przywara@linaro.org>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>

diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index c5950f2b3f11a3ac6c88c7f712d8fa305eec6666..bce224be6e6948cc4c97ff237cb1b75ac10d4291 100644 (file)
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -3208,7 +3208,7 @@ gnttab_swap_grant_ref(XEN_GUEST_HANDLE_PARAM(gnttab_swap_grant_ref_t) uop,
     return 0;
 }
 
-static int cache_flush(gnttab_cache_flush_t *cflush, grant_ref_t *cur_ref)
+static int cache_flush(const gnttab_cache_flush_t *cflush, grant_ref_t *cur_ref)
 {
     struct domain *d, *owner;
     struct page_info *page;
@@ -3218,19 +3218,17 @@ static int cache_flush(gnttab_cache_flush_t *cflush, grant_ref_t *cur_ref)
 
     if ( (cflush->offset >= PAGE_SIZE) ||
          (cflush->length > PAGE_SIZE) ||
-         (cflush->offset + cflush->length > PAGE_SIZE) )
+         (cflush->offset + cflush->length > PAGE_SIZE) ||
+         (cflush->op & ~(GNTTAB_CACHE_INVAL | GNTTAB_CACHE_CLEAN)) )
         return -EINVAL;
 
     if ( cflush->length == 0 || cflush->op == 0 )
-        return 0;
+        return !*cur_ref ? 0 : -EILSEQ;
 
     /* currently unimplemented */
     if ( cflush->op & GNTTAB_CACHE_SOURCE_GREF )
         return -EOPNOTSUPP;
 
-    if ( cflush->op & ~(GNTTAB_CACHE_INVAL|GNTTAB_CACHE_CLEAN) )
-        return -EINVAL;
-
     d = rcu_lock_current_domain();
     mfn = cflush->a.dev_bus_addr >> PAGE_SHIFT;
 
@@ -3310,6 +3308,9 @@ gnttab_cache_flush(XEN_GUEST_HANDLE_PARAM(gnttab_cache_flush_t) uop,
         *cur_ref = 0;
         guest_handle_add_offset(uop, 1);
     }
+
+    *cur_ref = 0;
+
     return 0;
 }