[PATCH RFC 0/9] PKS write protected page tables

Rick Edgecombe
2021-05-05T00:32:27Z

This is a POC for write protecting page tables with PKS (Protection Keys for
Supervisor) [1]. The basic idea is to make the page tables read only, except
temporarily on a per-cpu basis when they need to be modified. I’m looking for
opinions on whether people like the general direction of this in terms of
value and implementation.

Why would people want this?
===========================
Page tables are the basis for many types of protections and as such, are a
juicy target for attackers. Mapping them read-only will make them harder to
use in attacks.

This protects against an attacker that has acquired the ability to write to
the page tables. It's not foolproof because an attacker who can execute
arbitrary code can either disable PKS directly, or simply call the same
functions that the kernel uses for legitimate page table writes.

Why use PKS for this?
=====================
PKS is an upcoming CPU feature that allows supervisor virtual memory
permissions to be changed without flushing the TLB, like PKU does for user
memory. Protecting page tables would normally be really expensive because you
would have to do it with paging itself. PKS helps by providing a way to toggle
the writability of the page tables with just a per-cpu MSR.

Performance impacts
===================
Setting direct map permissions on whatever random page gets allocated for a
page table would result in a lot of kernel range shootdowns and direct map
large page shattering. So the way the PKS page table memory is created is
similar to this module page clustering series[2], where a cache of pages is
replenished from 2MB pages such that the direct map permissions and associated
breakage is localized on the direct map. In the PKS page tables case, a PKS
key is pre-applied to the direct map for pages in the cache.

There would be some costs of memory overhead in order to protect the direct
map page tables. There would also be some extra kernel range shootdowns to
replenish the cache on occasion, from setting the PKS key on the direct map of
the new pages. I don’t have any actual performance data yet.

This is based on V6 [1] of the core PKS infrastructure patches. PKS
infrastructure follow-on’s are planned to enable keys to be set to the same
permissions globally. Since this usage needs a key to be set globally
read-only by default, a small temporary solution is hacked up in patch 8. Long
term, PKS protected page tables would use a better and more generic solution
to achieve this.

[1]
https://lore.kernel.org/lkml/20210401225833.566238-1-ira.weiny@intel.com/
[2]
https://lore.kernel.org/lkml/20210405203711.1095940-1-rick.p.edgecombe@intel.com
/

Thanks,

Rick

Rick Edgecombe (9):
list: Support getting most recent element in list_lru
list: Support list head not in object for list_lru
x86/mm/cpa: Add grouped page allocations
mm: Explicitly zero page table lock ptr
x86, mm: Use cache of page tables
x86/mm/cpa: Add set_memory_pks()
x86/mm/cpa: Add perm callbacks to grouped pages
x86, mm: Protect page tables with PKS
x86, cpa: PKS protect direct map page tables

arch/x86/boot/compressed/ident_map_64.c | 5 +
arch/x86/include/asm/pgalloc.h | 6 +
arch/x86/include/asm/pgtable.h | 26 +-
arch/x86/include/asm/pgtable_64.h | 33 ++-
arch/x86/include/asm/pkeys_common.h | 8 +-
arch/x86/include/asm/set_memory.h | 23 ++
arch/x86/mm/init.c | 40 +++
arch/x86/mm/pat/set_memory.c | 312 +++++++++++++++++++++++-
arch/x86/mm/pgtable.c | 144 ++++++++++-
include/asm-generic/pgalloc.h | 42 +++-
include/linux/list_lru.h | 26 ++
include/linux/mm.h | 7 +
mm/Kconfig | 6 +-
mm/list_lru.c | 38 ++-
mm/memory.c | 1 +
mm/swap.c | 7 +
mm/swap_state.c | 6 +
17 files changed, 705 insertions(+), 25 deletions(-)

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2.30.2