x86/mm: add speculation barriers to open coded locks

Add a speculation barrier to the clearly identified open-coded lock taking
functions.

Note that the memory sharing page_lock() replacement (_page_lock()) is left
as-is, as the code is experimental and not security supported.

This is part of XSA-453 / CVE-2024-2193

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 42a572a38e)

xen/arch/x86/mm.c

@@ -2004,7 +2004,7 @@ static inline bool current_locked_page_ne_check(struct page_info *page) {
 #define current_locked_page_ne_check(x) true
 #endif
 
-int page_lock(struct page_info *page)
+int page_lock_unsafe(struct page_info *page)
 {
     unsigned long x, nx;
 
@@ -2065,7 +2065,7 @@ void page_unlock(struct page_info *page)
  * l3t_lock(), so to avoid deadlock we must avoid grabbing them in
  * reverse order.
  */
-static void l3t_lock(struct page_info *page)
+static always_inline void l3t_lock(struct page_info *page)
 {
     unsigned long x, nx;
 
@@ -2074,6 +2074,8 @@ static void l3t_lock(struct page_info *page)
             cpu_relax();
         nx = x | PGT_locked;
     } while ( cmpxchg(&page->u.inuse.type_info, x, nx) != x );
+
+    block_lock_speculation();
 }
 
 static void l3t_unlock(struct page_info *page)

xen/include/asm-x86/mm.h

@@ -393,7 +393,9 @@ const struct platform_bad_page *get_platform_badpages(unsigned int *array_size);
  * The use of PGT_locked in mem_sharing does not collide, since mem_sharing is
  * only supported for hvm guests, which do not have PV PTEs updated.
  */
-int page_lock(struct page_info *page);
+int page_lock_unsafe(struct page_info *page);
+#define page_lock(pg)   lock_evaluate_nospec(page_lock_unsafe(pg))
+
 void page_unlock(struct page_info *page);
 
 void put_page_type(struct page_info *page);