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SubjectRe: [PATCH] RTIC: selinux: ARM64: Move selinux_state to a separate page
On 2021-02-16 23:39, Nick Desaulniers wrote:
> On Tue, Feb 16, 2021 at 2:19 AM Preeti Nagar <pnagar@codeaurora.org>
> wrote:
>>
>> The changes introduce a new security feature, RunTime Integrity Check
>> (RTIC), designed to protect Linux Kernel at runtime. The motivation
>> behind these changes is:
>> 1. The system protection offered by Security Enhancements(SE) for
>> Android relies on the assumption of kernel integrity. If the kernel
>> itself is compromised (by a perhaps as yet unknown future
>> vulnerability),
>> SE for Android security mechanisms could potentially be disabled and
>> rendered ineffective.
>> 2. Qualcomm Snapdragon devices use Secure Boot, which adds
>> cryptographic
>> checks to each stage of the boot-up process, to assert the
>> authenticity
>> of all secure software images that the device executes. However, due
>> to
>> various vulnerabilities in SW modules, the integrity of the system can
>> be
>> compromised at any time after device boot-up, leading to un-authorized
>> SW executing.
>>
>> The feature's idea is to move some sensitive kernel structures to a
>> separate page and monitor further any unauthorized changes to these,
>> from higher Exception Levels using stage 2 MMU. Moving these to a
>> different page will help avoid getting page faults from un-related
>> data.
>> The mechanism we have been working on removes the write permissions
>> for
>> HLOS in the stage 2 page tables for the regions to be monitored, such
>> that any modification attempts to these will lead to faults being
>> generated and handled by handlers. If the protected assets are moved
>> to
>> a separate page, faults will be generated corresponding to change
>> attempts
>> to these assets only. If not moved to a separate page, write attempts
>> to
>> un-related data present on the monitored pages will also be generated.
>>
>> Using this feature, some sensitive variables of the kernel which are
>> initialized after init or are updated rarely can also be protected
>> from
>> simple overwrites and attacks trying to modify these.
>>
>> Currently, the change moves selinux_state structure to a separate
>> page.
>> The page is 2MB aligned not 4K to avoid TLB related performance impact
>> as,
>> for some CPU core designs, the TLB does not cache 4K stage 2 (IPA to
>> PA)
>> mappings if the IPA comes from a stage 1 mapping. In future, we plan
>> to
>> move more security-related kernel assets to this page to enhance
>> protection.
>>
>> Signed-off-by: Preeti Nagar <pnagar@codeaurora.org>
>
> This addresses my feedback from the RFC regarding the section symbols.
> No comment on whether there is a better approach, or the 2MB vs page
> alignment, but perhaps other folks cc'ed can please take a look.
>
> Acked-by: Nick Desaulniers <ndesaulniers@google.com>
>
Thank you! I look forward to reviews and suggestions from the added
folks.

>> ---
>> The RFC patch reviewed available at:
>> https://lore.kernel.org/linux-security-module/1610099389-28329-1-git-send-email-pnagar@codeaurora.org/
>> ---
>> include/asm-generic/vmlinux.lds.h | 10 ++++++++++
>> include/linux/init.h | 6 ++++++
>> security/Kconfig | 11 +++++++++++
>> security/selinux/hooks.c | 2 +-
>> 4 files changed, 28 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/asm-generic/vmlinux.lds.h
>> b/include/asm-generic/vmlinux.lds.h
>> index b97c628..d1a5434 100644
>> --- a/include/asm-generic/vmlinux.lds.h
>> +++ b/include/asm-generic/vmlinux.lds.h
>> @@ -770,6 +770,15 @@
>> *(.scommon)
>> \
>> }
>>
>> +#ifdef CONFIG_SECURITY_RTIC
>> +#define RTIC_BSS
>> \
>> + . = ALIGN(SZ_2M);
>> \
>> + KEEP(*(.bss.rtic))
>> \
>> + . = ALIGN(SZ_2M);
>> +#else
>> +#define RTIC_BSS
>> +#endif
>> +
>> /*
>> * Allow archectures to redefine BSS_FIRST_SECTIONS to add extra
>> * sections to the front of bss.
>> @@ -782,6 +791,7 @@
>> . = ALIGN(bss_align);
>> \
>> .bss : AT(ADDR(.bss) - LOAD_OFFSET) {
>> \
>> BSS_FIRST_SECTIONS
>> \
>> + RTIC_BSS
>> \
>> . = ALIGN(PAGE_SIZE);
>> \
>> *(.bss..page_aligned)
>> \
>> . = ALIGN(PAGE_SIZE);
>> \
>> diff --git a/include/linux/init.h b/include/linux/init.h
>> index e668832..e6d452a 100644
>> --- a/include/linux/init.h
>> +++ b/include/linux/init.h
>> @@ -300,6 +300,12 @@ void __init parse_early_options(char *cmdline);
>> /* Data marked not to be saved by software suspend */
>> #define __nosavedata __section(".data..nosave")
>>
>> +#ifdef CONFIG_SECURITY_RTIC
>> +#define __rticdata __section(".bss.rtic")
>> +#else
>> +#define __rticdata
>> +#endif
>> +
>> #ifdef MODULE
>> #define __exit_p(x) x
>> #else
>> diff --git a/security/Kconfig b/security/Kconfig
>> index 7561f6f..1af913a 100644
>> --- a/security/Kconfig
>> +++ b/security/Kconfig
>> @@ -291,5 +291,16 @@ config LSM
>>
>> source "security/Kconfig.hardening"
>>
>> +config SECURITY_RTIC
>> + bool "RunTime Integrity Check feature"
>> + depends on ARM64
>> + help
>> + RTIC(RunTime Integrity Check) feature is to protect Linux
>> kernel
>> + at runtime. This relocates some of the security sensitive
>> kernel
>> + structures to a separate RTIC specific page.
>> +
>> + This is to enable monitoring and protection of these kernel
>> assets
>> + from a higher exception level(EL) against any unauthorized
>> changes.
>> +
>> endmenu
>>
>> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
>> index 644b17e..59d7eee 100644
>> --- a/security/selinux/hooks.c
>> +++ b/security/selinux/hooks.c
>> @@ -104,7 +104,7 @@
>> #include "audit.h"
>> #include "avc_ss.h"
>>
>> -struct selinux_state selinux_state;
>> +struct selinux_state selinux_state __rticdata;
>>
>> /* SECMARK reference count */
>> static atomic_t selinux_secmark_refcount = ATOMIC_INIT(0);
>> --
>> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
>> member
>> of Code Aurora Forum, hosted by The Linux Foundation
>>

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