iov_iter: Add missing speculation barrier to copy_from_user_iter()

The results of "access_ok()" can be mis-speculated.  The result is that
the CPU can end speculatively:

	if (access_ok(from, size))
		// Right here

For the same reason as done in copy_from_user() in commit 74e19ef
("uaccess: Add speculation barrier to copy_from_user()"), add a speculation
barrier to copy_from_user_iter().

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://patch.msgid.link/6b73e69cc7168c89df4eab0a216e3ed4cca36b0a.1763396724.git.christophe.leroy@csgroup.eu

Author: chleroy

lib/iov_iter.c

@@ -49,11 +49,19 @@ size_t copy_from_user_iter(void __user *iter_from, size_t progress,
 
 	if (should_fail_usercopy())
 		return len;
-	if (can_do_masked_user_access())
+	if (can_do_masked_user_access()) {
 		iter_from = mask_user_address(iter_from);
-	else if (!access_ok(iter_from, len))
-		return res;
+	} else {
+		if (!access_ok(iter_from, len))
+			return res;
 
+		/*
+		 * Ensure that bad access_ok() speculation will not
+		 * lead to nasty side effects *after* the copy is
+		 * finished:
+		 */
+		barrier_nospec();
+	}
 	to += progress;
 	instrument_copy_from_user_before(to, iter_from, len);
 	res = raw_copy_from_user(to, iter_from, len);