Re: [RFC PATCH v2] selinux: security: Move selinux_state to a separate page

From: pnagar
Date: Mon Jan 18 2021 - 01:02:59 EST

On 2021-01-12 22:36, Casey Schaufler wrote:
On 1/12/2021 1:36 AM, pnagar@xxxxxxxxxxxxxx wrote:
On 2021-01-08 22:41, Casey Schaufler wrote:
On 1/8/2021 1:49 AM, Preeti Nagar 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 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.

It would be helpful if you characterized the "various vulnerabilities"
rather than simply asserting their existence. This would allow the reviewer
to determine if the proposed patch addresses the issue.

There might not currently be vulnerabilities, but the system is meant more
specifically to harden valuable assets against future compromises. The key
value add is a third party independent entity keeping a watch on crucial
kernel assets.

Could you characterize the potential vulnerabilities, then?
Seriously, there's a gazillion ways data integrity can be
compromised. Which of those are addressed?

1. Memory Corruption vulnerabilities (example buffer overflows) which can be
exploited to modify the critical kernel assets as a part of a directed attack
or inadvertently are the major type of vulnerabilities which this mechanism
directly addresses.
2. This can be useful in preventing privilege escalation attacks (modifying
some critical kernel structures) which can be caused by exploits targetting
various other types of vulnerabilities such as improper input validation and
integer overflows.

Using this mechanism, 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.

How would this interact with or complement __read_mostly?

Currently, the mechanism we are working on developing is
independent of __read_mostly. This is something we can look more into
while working further on the mechanism.

Please either integrate the two or explain how they differ.
It appears that you haven't considered how you might exploit
or expand the existing mechanism.

On looking up more about __read_mostly and also as David Howells shared, I
understand there two are different.
__read_mostly is seemed to be primarily designed for cache-related performance
improvement [1]. In RTIC design, the idea is of protection and monitoring aspect.
If there are some security-critical kernel assets which are expected to be
updated sometimes or rarely, then, we can monitor the writes to these as well
and check for any unauthorized attempts.

Currently, the change moves selinux_state structure to a separate page. In
future we plan to move more security-related kernel assets to this page to
enhance protection.

What's special about selinux_state? What about the SELinux policy?
How would I, as maintainer of the Smack security module, know if
some Smack data should be treated the same way?

We are investigating more of the SELinux related and other kernel assets
which can be included in the protection. The basis of selinux_state is
because disabling of SELinux is one of the common attack vectors in
Android. We understand any kernel assets, unauthorized changes to which
can give way to security or any other type of attack can be considered to
be a potential asset to be added to the protection.

Yeah, I get that. It looks like this could be a useful mechanism
beyond SELinux. No point in hoarding it.

Thank you! Will try and update the commit header line if possible to convey this

We want to seek your suggestions and comments on the idea and the changes
in the patch.

Signed-off-by: Preeti Nagar <pnagar@xxxxxxxxxxxxxx>
 include/asm-generic/ | 10 ++++++++++
 include/linux/init.h              |  4 ++++
 security/Kconfig                  | 10 ++++++++++
 security/selinux/hooks.c          |  4 ++++
 4 files changed, 28 insertions(+)

diff --git a/include/asm-generic/ b/include/asm-generic/
index b2b3d81..158dbc2 100644
--- a/include/asm-generic/
+++ b/include/asm-generic/
@@ -770,6 +770,15 @@
         *(.scommon)                        \

+#define RTIC_BSS                            \
+    . = ALIGN(PAGE_SIZE);                        \
+    KEEP(*(.bss.rtic))                        \
+    . = ALIGN(PAGE_SIZE);
+#define RTIC_BSS
  * 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 7b53cb3..617adcf 100644
--- a/include/linux/init.h
+++ b/include/linux/init.h
@@ -300,6 +300,10 @@ void __init parse_early_options(char *cmdline);
 /* Data marked not to be saved by software suspend */
 #define __nosavedata __section(".data..nosave")

+#define __rticdata  __section(".bss.rtic")
 #ifdef MODULE
 #define __exit_p(x) x
diff --git a/security/Kconfig b/security/Kconfig
index 7561f6f..66b61b9 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -291,5 +291,15 @@ config LSM

 source "security/Kconfig.hardening"

+        bool "RunTime Integrity Check feature"

Shouldn't this depend on the architecture(s) supporting the

+        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 page aligned special section.
+      This is to enable monitoring and protection of these kernel assets
+      from a higher exception level(EL) against any unauthorized changes.

"if you are unsure ..."

We just thought keeping it generic might be a better idea, thus, moved the
changes to generic files from arch-specific files and thus, kept config also
independent of the arch. Can surely make this config arch dependent if that is
a better approach?

It's kind of silly to enable this if the hardware doesn't
support it, isn't it?

Yes, that makes sense. Will update depends on to the ARM64 arch in the next
version. Thank you.