diff options
authorChris von Recklinghausen <>2018-07-03 15:43:08 -0400
committerKees Cook <>2018-07-04 08:04:52 -0700
commitb5cb15d9372abc9adc4e844c0c1bf594ca6a7695 (patch)
parent6aa56f44253a6dd802e45d8ab1b48847feaf063a (diff)
usercopy: Allow boot cmdline disabling of hardening
Enabling HARDENED_USERCOPY may cause measurable regressions in networking performance: up to 8% under UDP flood. I ran a small packet UDP flood using pktgen vs. a host b2b connected. On the receiver side the UDP packets are processed by a simple user space process that just reads and drops them: Not very useful from a functional PoV, but it helps to pin-point bottlenecks in the networking stack. When running a kernel with CONFIG_HARDENED_USERCOPY=y, I see a 5-8% regression in the receive tput, compared to the same kernel without this option enabled. With CONFIG_HARDENED_USERCOPY=y, perf shows ~6% of CPU time spent cumulatively in __check_object_size (~4%) and __virt_addr_valid (~2%). The call-chain is: __GI___libc_recvfrom entry_SYSCALL_64_after_hwframe do_syscall_64 __x64_sys_recvfrom __sys_recvfrom inet_recvmsg udp_recvmsg __check_object_size udp_recvmsg() actually calls copy_to_iter() (inlined) and the latters calls check_copy_size() (again, inlined). A generic distro may want to enable HARDENED_USERCOPY in their default kernel config, but at the same time, such distro may want to be able to avoid the performance penalties in with the default configuration and disable the stricter check on a per-boot basis. This change adds a boot parameter that conditionally disables HARDENED_USERCOPY via "hardened_usercopy=off". Signed-off-by: Chris von Recklinghausen <> Signed-off-by: Kees Cook <>
3 files changed, 42 insertions, 0 deletions
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index efc7aa7a0670..560d4dc66f02 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -816,6 +816,17 @@
disable= [IPV6]
See Documentation/networking/ipv6.txt.
+ hardened_usercopy=
+ 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
diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h
index b46b541c67c4..1a0b6f17a5d6 100644
--- a/include/linux/jump_label.h
+++ b/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
extern struct static_key_true name
struct static_key_false name = STATIC_KEY_FALSE_INIT
+ struct static_key_false name __ro_after_init = STATIC_KEY_FALSE_INIT
extern struct static_key_false name
diff --git a/mm/usercopy.c b/mm/usercopy.c
index e9e9325f7638..852eb4e53f06 100644
--- a/mm/usercopy.c
+++ b/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)
@@ -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);
+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;