[PATCH 3/3] mm/usercopy: return 1 from hardened_usercopy __setup() handler

[Randy Dunlap <rdunlap@infradead.org>]

__setup() handlers should return 1 if the command line option is handled
and 0 if not (or maybe never return 0; it just pollutes init's
environment). This prevents:

  Unknown kernel command line parameters \
  "BOOT_IMAGE=/boot/bzImage-517rc5 hardened_usercopy=off", will be \
  passed to user space.

  Run /sbin/init as init process
   with arguments:
     /sbin/init
   with environment:
     HOME=/
     TERM=linux
     BOOT_IMAGE=/boot/bzImage-517rc5
     hardened_usercopy=off
or
     hardened_usercopy=on
but when "hardened_usercopy=foo" is used, there is no Unknown kernel
command line parameter.

Return 1 to indicate that the boot option has been handled.
Print a warning if strtobool() returns an error on the option string,
but do not mark this as in unknown command line option and do not cause
init's environment to be polluted with this string.

Fixes: b5cb15d9372ab ("usercopy: Allow boot cmdline disabling of hardening")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: Igor Zhbanov <i.zhbanov@omprussia.ru>
Link: lore.kernel.org/r/64644a2f-4a20-bab3-1e15-3b2cdd0defe3@omprussia.ru
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Chris von Recklinghausen <crecklin@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
---
Sorry, I missed this third mm/ instance. Hence the patch 3/3.

 mm/usercopy.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- linux-next-20220217.orig/mm/usercopy.c
+++ linux-next-20220217/mm/usercopy.c
@@ -284,7 +284,10 @@ static bool enable_checks __initdata = t
 
 static int __init parse_hardened_usercopy(char *str)
 {
-	return strtobool(str, &enable_checks);
+	if (strtobool(str, &enable_checks))
+		pr_warn("Invalid option string for hardened_usercopy: '%s'\n",
+			str);
+	return 1;
 }
 
 __setup("hardened_usercopy=", parse_hardened_usercopy);