I am continuing to build on the work Juerg, Tycho and Julian have done on XPFO. After the last round of updates, we were seeing very significant performance penalties when stale TLB entries were flushed actively after an XPFO TLB update. Benchmark for measuring performance is kernel build using parallel make. To get full protection from ret2dir attackes, we must flush stale TLB entries. Performance penalty from flushing stale TLB entries goes up as the number of cores goes up. On a desktop class machine with only 4 cores, enabling TLB flush for stale entries causes system time for "make -j4" to go up by a factor of 2.61x but on a larger machine with 96 cores, system time with "make -j60" goes up by a factor of 26.37x! I have been working on reducing this performance penalty.
I implemented two solutions to reduce performance penalty and that has had large impact. XPFO code flushes TLB every time a page is allocated to userspace. It does so by sending IPIs to all processors to flush TLB. Back to back allocations of pages to userspace on multiple processors results in a storm of IPIs. Each one of these incoming IPIs is handled by a processor by flushing its TLB. To reduce this IPI storm, I have added a per CPU flag that can be set to tell a processor to flush its TLB. A processor checks this flag on every context switch. If the flag is set, it flushes its TLB and clears the flag. This allows for multiple TLB flush requests to a single CPU to be combined into a single request. A kernel TLB entry for a page that has been allocated to userspace is flushed on all processors unlike the previous version of this patch. A processor could hold a stale kernel TLB entry that was removed on another processor until the next context switch. A local userspace page allocation by the currently running process could force the TLB flush earlier for such entries. The other solution reduces the number of TLB flushes required, by performing TLB flush for multiple pages at one time when pages are refilled on the per-cpu freelist. If the pages being addedd to per-cpu freelist are marked for userspace allocation, TLB entries for these pages can be flushed upfront and pages tagged as currently unmapped. When any such page is allocated to userspace, there is no need to performa a TLB flush at that time any more. This batching of TLB flushes reduces performance imapct further. I measured system time for parallel make with unmodified 4.20 kernel, 4.20 with XPFO patches before these patches and then again after applying each of these patches. Here are the results: Hardware: 96-core Intel Xeon Platinum 8160 CPU @ 2.10GHz, 768 GB RAM make -j60 all 4.20 950.966s 4.20+XPFO 25073.169s 26.37x 4.20+XPFO+Deferred flush 1372.874s 1.44x 4.20+XPFO+Deferred flush+Batch update 1255.021s 1.32x Hardware: 4-core Intel Core i5-3550 CPU @ 3.30GHz, 8G RAM make -j4 all 4.20 607.671s 4.20+XPFO 1588.646s 2.61x 4.20+XPFO+Deferred flush 803.989s 1.32x 4.20+XPFO+Deferred flush+Batch update 795.728s 1.31x 30+% overhead is still very high and there is room for improvement. Performance with this patch set is good enough to use these as starting point for further refinement before we merge it into main kernel, hence RFC. I have dropped the patch "mm, x86: omit TLB flushing by default for XPFO page table modifications" since not flushing TLB leaves kernel wide open to attack and there is no point in enabling XPFO without flushing TLB every time kernel TLB entries for pages are removed. I also dropped the patch "EXPERIMENTAL: xpfo, mm: optimize spin lock usage in xpfo_kmap". There was not a measurable improvement in performance with this patch and it introduced a possibility for deadlock that Laura found. What remains to be done beyond this patch series: 1. Performance improvements: Ideas to explore - (1) Add a freshly freed page to per cpu freelist and not make a kernel TLB entry for it, (2) kernel mappings private to an mm, (3) Any others?? 2. Re-evaluate the patch "arm64/mm: Add support for XPFO to swiotlb" from Juerg. I dropped it for now since swiotlb code for ARM has changed a lot in 4.20. 3. Extend the patch "xpfo, mm: Defer TLB flushes for non-current CPUs" to other architectures besides x86. --------------------------------------------------------- Juerg Haefliger (5): mm, x86: Add support for eXclusive Page Frame Ownership (XPFO) swiotlb: Map the buffer if it was unmapped by XPFO arm64/mm: Add support for XPFO arm64/mm, xpfo: temporarily map dcache regions lkdtm: Add test for XPFO Julian Stecklina (2): xpfo, mm: remove dependency on CONFIG_PAGE_EXTENSION xpfo, mm: optimize spinlock usage in xpfo_kunmap Khalid Aziz (2): xpfo, mm: Defer TLB flushes for non-current CPUs (x86 only) xpfo, mm: Optimize XPFO TLB flushes by batching them together Tycho Andersen (5): mm: add MAP_HUGETLB support to vm_mmap x86: always set IF before oopsing from page fault xpfo: add primitives for mapping underlying memory arm64/mm: disable section/contiguous mappings if XPFO is enabled mm: add a user_virt_to_phys symbol .../admin-guide/kernel-parameters.txt | 2 + arch/arm64/Kconfig | 1 + arch/arm64/mm/Makefile | 2 + arch/arm64/mm/flush.c | 7 + arch/arm64/mm/mmu.c | 2 +- arch/arm64/mm/xpfo.c | 64 +++++ arch/x86/Kconfig | 1 + arch/x86/include/asm/pgtable.h | 26 ++ arch/x86/include/asm/tlbflush.h | 1 + arch/x86/mm/Makefile | 2 + arch/x86/mm/fault.c | 6 + arch/x86/mm/pageattr.c | 23 +- arch/x86/mm/tlb.c | 38 +++ arch/x86/mm/xpfo.c | 181 ++++++++++++++ drivers/misc/lkdtm/Makefile | 1 + drivers/misc/lkdtm/core.c | 3 + drivers/misc/lkdtm/lkdtm.h | 5 + drivers/misc/lkdtm/xpfo.c | 194 +++++++++++++++ include/linux/highmem.h | 15 +- include/linux/mm.h | 2 + include/linux/mm_types.h | 8 + include/linux/page-flags.h | 18 +- include/linux/xpfo.h | 95 ++++++++ include/trace/events/mmflags.h | 10 +- kernel/dma/swiotlb.c | 3 +- mm/Makefile | 1 + mm/mmap.c | 19 +- mm/page_alloc.c | 7 + mm/util.c | 32 +++ mm/xpfo.c | 223 ++++++++++++++++++ security/Kconfig | 29 +++ 31 files changed, 977 insertions(+), 44 deletions(-) create mode 100644 arch/arm64/mm/xpfo.c create mode 100644 arch/x86/mm/xpfo.c create mode 100644 drivers/misc/lkdtm/xpfo.c create mode 100644 include/linux/xpfo.h create mode 100644 mm/xpfo.c -- 2.17.1
