Merge tag 'cve-2020-11884' from emailed bundle

Pull s390 fix from Christian Borntraeger:
 "Fix a race between page table upgrade and uaccess on s390.

  This fixes CVE-2020-11884 which allows for a local kernel crash or
  code execution"

* tag 'cve-2020-11884' from emailed bundle:
  s390/mm: fix page table upgrade vs 2ndary address mode accesses

Author: torvalds

arch/s390/lib/uaccess.c

@@ -64,10 +64,13 @@ mm_segment_t enable_sacf_uaccess(void)
 {
 	mm_segment_t old_fs;
 	unsigned long asce, cr;
+	unsigned long flags;
 
 	old_fs = current->thread.mm_segment;
 	if (old_fs & 1)
 		return old_fs;
+	/* protect against a concurrent page table upgrade */
+	local_irq_save(flags);
 	current->thread.mm_segment |= 1;
 	asce = S390_lowcore.kernel_asce;
 	if (likely(old_fs == USER_DS)) {
@@ -83,6 +86,7 @@ mm_segment_t enable_sacf_uaccess(void)
 		__ctl_load(asce, 7, 7);
 		set_cpu_flag(CIF_ASCE_SECONDARY);
 	}
+	local_irq_restore(flags);
 	return old_fs;
 }
 EXPORT_SYMBOL(enable_sacf_uaccess);

arch/s390/mm/pgalloc.c

@@ -70,8 +70,20 @@ static void __crst_table_upgrade(void *arg)
 {
 	struct mm_struct *mm = arg;
 
-	if (current->active_mm == mm)
-		set_user_asce(mm);
+	/* we must change all active ASCEs to avoid the creation of new TLBs */
+	if (current->active_mm == mm) {
+		S390_lowcore.user_asce = mm->context.asce;
+		if (current->thread.mm_segment == USER_DS) {
+			__ctl_load(S390_lowcore.user_asce, 1, 1);
+			/* Mark user-ASCE present in CR1 */
+			clear_cpu_flag(CIF_ASCE_PRIMARY);
+		}
+		if (current->thread.mm_segment == USER_DS_SACF) {
+			__ctl_load(S390_lowcore.user_asce, 7, 7);
+			/* enable_sacf_uaccess does all or nothing */
+			WARN_ON(!test_cpu_flag(CIF_ASCE_SECONDARY));
+		}
+	}
 	__tlb_flush_local();
 }