[01/17] x86/shadow: use __put_user() instead of __copy_to_user()

Commit Message

Jan Beulich Jan. 14, 2021, 3:03 p.m. UTC

In a subsequent patch I would almost have broken the logic here, if I
hadn't happened to read through the comment at the top of
safe_write_entry(): __copy_from_user() does not provide a guarantee
shadow_write_entries() requires - it's only an optimization that it
makes use of __put_user_size() for certain sizes. Use __put_user()
directly, which does expand to a single (memory accessing) insn.

Signed-off-by: Jan Beulich <[e-mail]>
---
In a future patch I guess I'll make this write store the intended data
instead of doing this "no-op" write, making the subsequent loop start
from 1 in the success case. In fact I also think safe_write_entry()
would better go away, in favor of direct use of write_atomic().

Patch

--- a/xen/arch/x86/mm/shadow/multi.c
+++ b/xen/arch/x86/mm/shadow/multi.c
@@ -776,9 +776,9 @@  shadow_write_entries(void *d, void *s, i
     /* Because we mirror access rights at all levels in the shadow, an
      * l2 (or higher) entry with the RW bit cleared will leave us with
      * no write access through the linear map.
-     * We detect that by writing to the shadow with copy_to_user() and
+     * We detect that by writing to the shadow with __put_user() and
      * using map_domain_page() to get a writeable mapping if we need to. */
-    if ( __copy_to_user(d, d, sizeof (unsigned long)) != 0 )
+    if ( __put_user(*dst, dst) )
     {
         perfc_incr(shadow_linear_map_failed);
         map = map_domain_page(mfn);