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

Vlastimil Babka
2021-05-05T11:08:41Z

On 5/5/21 2:30 AM, Rick Edgecombe wrote:
> 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.

Yeah, it's a good idea. I've once used a similar approach locally during
debugging a problem that appeared to be stray writes hitting page tables, and
without PKS I indeed made the whole pages read-only when not touched by the
designated code.

> 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.

I can see in patch 8/9 that you are flipping the MSR around individual
operations on page table entries. In my patch I hooked making the page table
writable to obtaining the page table lock (IIRC I had only the PTE level fully
handled though). Wonder if that would be better tradeoff even for your MSR approach?

Vlastimil

> 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(-)
>