Re: [PATCH] RTIC: selinux: ARM64: Move selinux_state to a separate page

[pnagar@codeaurora.org]

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
>>