- drop unneeded Kconfig "select BUG" (Kamal Mostafa)

- add "hardened_usercopy=off" rare performance needs (Chris von Recklinghausen)
 -----BEGIN PGP SIGNATURE-----
 Comment: Kees Cook <kees@outflux.net>
 
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Merge tag 'hardened-usercopy-v4.19-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux

Pull hardened usercopy updates from Kees Cook:
 "This cleans up a minor Kconfig issue and adds a kernel boot option for
  disabling hardened usercopy for distro users that may have corner-case
  performance issues (e.g. high bandwidth small-packet UDP traffic).

  Summary:

   - drop unneeded Kconfig "select BUG" (Kamal Mostafa)

   - add "hardened_usercopy=off" rare performance needs (Chris von
     Recklinghausen)"

* tag 'hardened-usercopy-v4.19-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux:
  usercopy: Allow boot cmdline disabling of hardening
  usercopy: Do not select BUG with HARDENED_USERCOPY

Documentation/admin-guide/kernel-parameters.txt

@@ -816,6 +816,17 @@
 	disable=	[IPV6]
 			See Documentation/networking/ipv6.txt.
 
+	hardened_usercopy=
+                        [KNL] Under CONFIG_HARDENED_USERCOPY, whether
+                        hardening is enabled for this boot. Hardened
+                        usercopy checking is used to protect the kernel
+                        from reading or writing beyond known memory
+                        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).
+                off     Disable hardened usercopy checks.
+
 	disable_radix	[PPC]
 			Disable RADIX MMU mode on POWER9
 

include/linux/jump_label.h

@@ -299,12 +299,18 @@ struct static_key_false {
 #define DEFINE_STATIC_KEY_TRUE(name)	\
 	struct static_key_true name = STATIC_KEY_TRUE_INIT
 
+#define DEFINE_STATIC_KEY_TRUE_RO(name)	\
+	struct static_key_true name __ro_after_init = STATIC_KEY_TRUE_INIT
+
 #define DECLARE_STATIC_KEY_TRUE(name)	\
 	extern struct static_key_true name
 
 #define DEFINE_STATIC_KEY_FALSE(name)	\
 	struct static_key_false name = STATIC_KEY_FALSE_INIT
 
+#define DEFINE_STATIC_KEY_FALSE_RO(name)	\
+	struct static_key_false name __ro_after_init = STATIC_KEY_FALSE_INIT
+
 #define DECLARE_STATIC_KEY_FALSE(name)	\
 	extern struct static_key_false name
 

mm/usercopy.c

@@ -20,6 +20,8 @@
 #include <linux/sched/task.h>
 #include <linux/sched/task_stack.h>
 #include <linux/thread_info.h>
+#include <linux/atomic.h>
+#include <linux/jump_label.h>
 #include <asm/sections.h>
 
 /*
@@ -240,6 +242,8 @@ static inline void check_heap_object(const void *ptr, unsigned long n,
 	}
 }
 
+static DEFINE_STATIC_KEY_FALSE_RO(bypass_usercopy_checks);
+
 /*
  * Validates that the given object is:
  * - not bogus address
@@ -248,6 +252,9 @@ static inline void check_heap_object(const void *ptr, unsigned long n,
  */
 void __check_object_size(const void *ptr, unsigned long n, bool to_user)
 {
+	if (static_branch_unlikely(&bypass_usercopy_checks))
+		return;
+
 	/* Skip all tests if size is zero. */
 	if (!n)
 		return;
@@ -279,3 +286,21 @@ void __check_object_size(const void *ptr, unsigned long n, bool to_user)
 	check_kernel_text_object((const unsigned long)ptr, n, to_user);
 }
 EXPORT_SYMBOL(__check_object_size);
+
+static bool enable_checks __initdata = true;
+
+static int __init parse_hardened_usercopy(char *str)
+{
+	return strtobool(str, &enable_checks);
+}
+
+__setup("hardened_usercopy=", parse_hardened_usercopy);
+
+static int __init set_hardened_usercopy(void)
+{
+	if (enable_checks == false)
+		static_branch_enable(&bypass_usercopy_checks);
+	return 1;
+}
+
+late_initcall(set_hardened_usercopy);

security/Kconfig

@@ -153,7 +153,6 @@ config HAVE_HARDENED_USERCOPY_ALLOCATOR
 config HARDENED_USERCOPY
 	bool "Harden memory copies between kernel and userspace"
 	depends on HAVE_HARDENED_USERCOPY_ALLOCATOR
-	select BUG
 	imply STRICT_DEVMEM
 	help
 	  This option checks for obviously wrong memory regions when