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@xxxxxxxxx/ > [2] > https://lore.kernel.org/lkml/20210405203711.1095940-1-rick.p.edgecombe@xxxxxxxxx > / > > 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(-) >
