[PATCH 2/3] mm: security: Allow default HARDENED_USERCOPY to be set at compile time

[Mel Gorman]

HARDENED_USERCOPY defaults to on if enabled at compile time. Allow
hardened_usercopy= default to be set at compile time similar to
init_on_alloc= and init_on_free=. The intent is that hardening
options that can be disabled at runtime can set their default at
build time.

Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
---
 Documentation/admin-guide/kernel-parameters.txt | 4 +++-
 mm/usercopy.c                                   | 3 ++-
 security/Kconfig.hardening                      | 8 ++++++++
 3 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 3872bc6ec49d..5d759b20540a 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1773,7 +1773,9 @@
 			allocation boundaries as a proactive defense
 			against bounds-checking flaws in the kernel's
 			copy_to_user()/copy_from_user() interface.
-		on	Perform hardened usercopy checks (default).
+			The default is determined by
+			CONFIG_HARDENED_USERCOPY_DEFAULT_ON.
+		on	Perform hardened usercopy checks.
 		off	Disable hardened usercopy checks.
 
 	hardlockup_all_cpu_backtrace=
diff --git a/mm/usercopy.c b/mm/usercopy.c
index 83c164aba6e0..4cf33305347a 100644
--- a/mm/usercopy.c
+++ b/mm/usercopy.c
@@ -255,7 +255,8 @@ void __check_object_size(const void *ptr, unsigned long n, bool to_user)
 }
 EXPORT_SYMBOL(__check_object_size);
 
-static bool enable_checks __initdata = true;
+static bool enable_checks __initdata =
+		IS_ENABLED(CONFIG_HARDENED_USERCOPY_DEFAULT_ON);
 
 static int __init parse_hardened_usercopy(char *str)
 {
diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
index 00e6e2ed0c43..537a6431892e 100644
--- a/security/Kconfig.hardening
+++ b/security/Kconfig.hardening
@@ -293,6 +293,14 @@ config HARDENED_USERCOPY
 	  or are part of the kernel text. This prevents entire classes
 	  of heap overflow exploits and similar kernel memory exposures.
 
+config HARDENED_USERCOPY_DEFAULT_ON
+	bool "Harden memory copies by default"
+	depends on HARDENED_USERCOPY
+	default n
+	help
+	  This has the effect of setting "hardened_usercopy=on" on the kernel
+	  command line. This can be disabled with "hardened_usercopy=off".
+
 endmenu
 
 menu "Hardening of kernel data structures"
-- 
2.43.0