[xen staging] x86/shadow: use __put_user() instead of __copy_to_user()

[patchbot@xxxxxxx]

commit a8cd9b8aff93b5d55f126910dde77f90d973ac76
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Jan 26 14:13:18 2021 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Jan 26 14:13:18 2021 +0100

    x86/shadow: use __put_user() instead of __copy_to_user()
    
    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 <jbeulich@xxxxxxxx>
    Reviewed-by: Tim Deegan <tim@xxxxxxx>
---
 xen/arch/x86/mm/shadow/multi.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/mm/shadow/multi.c b/xen/arch/x86/mm/shadow/multi.c
index d24ccde035..da46eae835 100644
--- 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, int entries, mfn_t 
mfn)
     /* 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);
--
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