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Subject[RFC PATCH v4 00/12] hardening: statically allocated protected memory
To: Andy Lutomirski <>,
To: Matthew Wilcox <>,
To: Nadav Amit <>
To: Peter Zijlstra <>,
To: Dave Hansen <>,
To: Mimi Zohar <>
To: Thiago Jung Bauermann <>
CC: Kees Cook <>
CC: Ahmed Soliman <>
CC: linux-integrity <>
CC: Kernel Hardening <>
CC: Linux-MM <>
CC: Linux Kernel Mailing List <>

at last I'm able to resume work on the memory protection patchset I've
proposed some time ago. This version should address comments received so
far and introduce support for arm64. Details below.

Patch-set implementing write-rare memory protection for statically
allocated data.
Its purpose is to keep write protected the kernel data which is seldom
modified, especially if altering it can be exploited during an attack.

There is no read overhead, however writing requires special operations that
are probably unsuitable for often-changing data.
The use is opt-in, by applying the modifier __wr_after_init to a variable

As the name implies, the write protection kicks in only after init() is
completed; before that moment, the data is modifiable in the usual way.

Current Limitations:
* supports only data which is allocated statically, at build time.
* supports only x86_64 and arm64;other architectures need to provide own

Some notes:
- in case an architecture doesn't support write rare, the behavior is to
fallback to regular write operations
- before altering any memory, the destination is sanitized
- write rare data is segregated into own set of pages
- only x86_64 and arm64 supported, atm
- the memset_user() assembly functions seems to work, but I'm not too sure
they are really ok
- I've added a simple example: the protection of ima_policy_flags
- the last patch is optional, but it seemed worth to do the refactoring
- the x86_64 user space address range is double the size of the kernel
address space, so it's possible to randomize the beginning of the
mapping of the kernel address space, but on arm64 they have the same
size, so it's not possible to do the same
- I'm not sure if it's correct, since it doesn't seem to be that common in
kernel sources, but instead of using #defines for overriding default
function calls, I'm using "weak" for the default functions.
- unaddressed: Nadav proposed to do:
#define __wr __attribute__((address_space(5)))
but I don't know exactly where to use it atm



* added function for setting memory in user space mapping for arm64
* refactored code, to work with both supported architectures
* reduced dependency on x86_64 specific code, to support by default also
* improved memset_user() for x86_64, but I'm not sure if I understood
correctly what was the best way to enhance it.


* both wr_memset and wr_memcpy are implemented as generic functions
the arch code must provide suitable helpers
* regular initialization for ima_policy_flags: it happens during init
* remove spurious code from the initialization function


* introduce cleaner split between generic and arch code
* add x86_64 specific memset_user()
* replace kernel-space memset() memcopy() with userspace counterpart
* randomize the base address for the alternate map across the entire
available address range from user space (128TB - 64TB)
* convert BUG() to WARN()
* turn verification of written data into debugging option
* wr_rcu_assign_pointer() as special case of wr_assign()
* example with protection of ima_policy_flags
* documentation

Igor Stoppa (12):
__wr_after_init: Core and default arch
__wr_after_init: x86_64: memset_user()
__wr_after_init: x86_64: randomize mapping offset
__wr_after_init: x86_64: enable
__wr_after_init: arm64: memset_user()
__wr_after_init: arm64: enable
__wr_after_init: Documentation: self-protection
__wr_after_init: lkdtm test
__wr_after_init: rodata_test: refactor tests
__wr_after_init: rodata_test: test __wr_after_init
__wr_after_init: test write rare functionality
IMA: turn ima_policy_flags into __wr_after_init

Documentation/security/self-protection.rst | 14 +-
arch/Kconfig | 7 +
arch/arm64/Kconfig | 1 +
arch/arm64/include/asm/uaccess.h | 9 ++
arch/arm64/lib/Makefile | 2 +-
arch/arm64/lib/memset_user.S (new) | 63 ++++++++
arch/x86/Kconfig | 1 +
arch/x86/include/asm/uaccess_64.h | 6 +
arch/x86/lib/usercopy_64.c | 51 ++++++
arch/x86/mm/Makefile | 2 +
arch/x86/mm/prmem.c (new) | 20 +++
drivers/misc/lkdtm/core.c | 3 +
drivers/misc/lkdtm/lkdtm.h | 3 +
drivers/misc/lkdtm/perms.c | 29 ++++
include/linux/prmem.h (new) | 71 ++++++++
mm/Kconfig.debug | 8 +
mm/Makefile | 2 +
mm/prmem.c (new) | 179 +++++++++++++++++++++
mm/rodata_test.c | 69 +++++---
mm/test_write_rare.c (new) | 136 ++++++++++++++++
security/integrity/ima/ima.h | 3 +-
security/integrity/ima/ima_policy.c | 9 +-
22 files changed, 656 insertions(+), 32 deletions(-)
create mode 100644 arch/arm64/lib/memset_user.S
create mode 100644 arch/x86/mm/prmem.c
create mode 100644 include/linux/prmem.h
create mode 100644 mm/prmem.c
create mode 100644 mm/test_write_rare.c


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