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SubjectRe: [PATCH v3] add param that allows bootline control of hardened usercopy
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On 06/30/2018 04:11 PM, Kees Cook wrote:
> On Wed, Jun 27, 2018 at 5:07 AM, Chris von Recklinghausen
> <crecklin@redhat.com> wrote:
>> Enabling HARDENED_USER_COPY causes measurable regressions in
> nit: HARDENED_USERCOPY
>
>> networking performance, up to 8% under UDP flood.
>>
>> I'm running an 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:
>>
>> https://github.com/netoptimizer/network-testing/blob/master/src/udp_sink.c
>>
>> 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).
> Thanks for including these details!
>
>> A generic distro may want to enable HARDENED_USER_COPY in their default
> same nit :)

Sorry, I'll fix those.

>
>> 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 at boot time.
>>
>> v2->v3:
>> add benchmark details to commit comments
>> Don't add new item to Documentation/admin-guide/kernel-parameters.rst
>> rename boot param to "hardened_usercopy="
>> update description in Documentation/admin-guide/kernel-parameters.txt
>> static_branch_likely -> static_branch_unlikely
>> add __ro_after_init versions of DEFINE_STATIC_KEY_FALSE,
>> DEFINE_STATIC_KEY_TRUE
>> disable_huc_atboot -> enable_checks (strtobool "on" == true)
>>
>> v1->v2:
>> remove CONFIG_HUC_DEFAULT_OFF
>> default is now enabled, boot param disables
>> move check to __check_object_size so as to not break optimization of
>> __builtin_constant_p()
>> include linux/atomic.h before linux/jump_label.h
>>
>> Signed-off-by: Chris von Recklinghausen <crecklin@redhat.com>
>> ---
>> .../admin-guide/kernel-parameters.txt | 11 ++++++++
>> include/linux/jump_label.h | 6 +++++
>> include/linux/thread_info.h | 5 ++++
>> mm/usercopy.c | 26 +++++++++++++++++++
>> 4 files changed, 48 insertions(+)
>>
>> 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=
>> + [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
>>
>> 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
>> +
>> #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
>>
>> diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
>> index 8d8821b3689a..ab24fe2d3f87 100644
>> --- a/include/linux/thread_info.h
>> +++ b/include/linux/thread_info.h
>> @@ -109,6 +109,11 @@ static inline int arch_within_stack_frames(const void * const stack,
>> #endif
>>
>> #ifdef CONFIG_HARDENED_USERCOPY
>> +#include <linux/atomic.h>
>> +#include <linux/jump_label.h>
>> +
>> +DECLARE_STATIC_KEY_FALSE(bypass_usercopy_checks);
>> +
> This isn't needed any more since bypass_usercopy_checks is internal to
> __check_object_size() now.

I'll remove that.

>
>> extern void __check_object_size(const void *ptr, unsigned long n,
>> bool to_user);
>>
>> diff --git a/mm/usercopy.c b/mm/usercopy.c
>> index e9e9325f7638..39f8b1409618 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>
>>
>> /*
>> @@ -248,6 +250,9 @@ 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 +284,24 @@ 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);
>> +
>> +DEFINE_STATIC_KEY_FALSE_RO(bypass_usercopy_checks);
> This can be static.
>
>> +EXPORT_SYMBOL(bypass_usercopy_checks);
> No longer needs to be exported.
>
>> +
>> +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);
> Otherwise, yeah, this looks good if the copy_from_iter() path can't be improved.

The last issue I'm chasing is build failures on ARCH=m68k. The error is
atomic_read and friends needed by the jump label code not being found.
The config has CONFIG_BROKEN_ON_SMP=y, so the jump label calls I added
will only be made #ifndef CONFIG_BROKEN_ON_SMP. Do you think that's
worth a mention in the blurb that's added to
Documentation/admin-guide/kernel-parameters.txt?

Thanks,

Chris

>
> -Kees
>

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