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Date:   Tue, 15 Jan 2019 17:28:43 -0800
From:   Laura Abbott <labbott@...hat.com>
To:     Khalid Aziz <khalid.aziz@...cle.com>, juergh@...il.com,
        tycho@...ho.ws, jsteckli@...zon.de, ak@...ux.intel.com,
        torvalds@...ux-foundation.org, liran.alon@...cle.com,
        keescook@...gle.com, konrad.wilk@...cle.com
Cc:     deepa.srinivasan@...cle.com, chris.hyser@...cle.com,
        tyhicks@...onical.com, dwmw@...zon.co.uk,
        andrew.cooper3@...rix.com, jcm@...hat.com,
        boris.ostrovsky@...cle.com, kanth.ghatraju@...cle.com,
        joao.m.martins@...cle.com, jmattson@...gle.com,
        pradeep.vincent@...cle.com, john.haxby@...cle.com,
        tglx@...utronix.de, kirill.shutemov@...ux.intel.com, hch@....de,
        steven.sistare@...cle.com, kernel-hardening@...ts.openwall.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v7 00/16] Add support for eXclusive Page Frame
 Ownership

On 1/10/19 1:09 PM, Khalid Aziz wrote:
> 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 2.614x but on a larger machine with 96
> cores, system time with "make -j60" goes up by a factor of 26.366x!
> I have been working on reducing this performance penalty.
> 
> I implemented a solution to reduce performance penalty and
> that has had large impact. When XPFO code flushes stale TLB entries,
> it does so for all CPUs on the system which may include CPUs that
> may not have any matching TLB entries or may never be scheduled to
> run the userspace task causing TLB flush. Problem is made worse by
> the fact that if number of entries being flushed exceeds
> tlb_single_page_flush_ceiling, it results in a full TLB flush on
> every CPU. A rogue process can launch a ret2dir attack only from a
> CPU that has dual mapping for its pages in physmap in its TLB. We
> can hence defer TLB flush on a CPU until a process that would have
> caused a TLB flush is scheduled on that CPU. I have added a cpumask
> to task_struct which is then used to post pending TLB flush on CPUs
> other than the one a process is running on. This cpumask is checked
> when a process migrates to a new CPU and TLB is flushed at that
> time. I measured system time for parallel make with unmodified 4.20
> kernel, 4.20 with XPFO patches before this optimization and then
> again after applying this optimization. Here are the results:
> 
> Hardware: 96-core Intel Xeon Platinum 8160 CPU @ 2.10GHz, 768 GB RAM
> make -j60 all
> 
> 4.20				915.183s
> 4.20+XPFO			24129.354s	26.366x
> 4.20+XPFO+Deferred flush	1216.987s	 1.330xx
> 
> 
> Hardware: 4-core Intel Core i5-3550 CPU @ 3.30GHz, 8G RAM
> make -j4 all
> 
> 4.20				607.671s
> 4.20+XPFO			1588.646s	2.614x
> 4.20+XPFO+Deferred flush	794.473s	1.307xx
> 
> 30+% overhead is still very high and there is room for improvement.
> Dave Hansen had suggested batch updating TLB entries and Tycho had
> created an initial implementation but I have not been able to get
> that to work correctly. I am still working on it and I suspect we
> will see a noticeable improvement in performance with that. In the
> code I added, I post a pending full TLB flush to all other CPUs even
> when number of TLB entries being flushed on current CPU does not
> exceed tlb_single_page_flush_ceiling. There has to be a better way
> to do this. I just haven't found an efficient way to implemented
> delayed limited TLB flush on other CPUs.
> 
> I am not entirely sure if switch_mm_irqs_off() is indeed the right
> place to perform the pending TLB flush for a CPU. Any feedback on
> that will be very helpful. Delaying full TLB flushes on other CPUs
> seems to help tremendously, so if there is a better way to implement
> the same thing than what I have done in patch 16, I am open to
> ideas.
> 
> 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.
> 
> Since not flushing stale TLB entries creates a false sense of
> security, I would recommend making TLB flush mandatory and eliminate
> the "xpfotlbflush" kernel parameter (patch "mm, x86: omit TLB
> flushing by default for XPFO page table modifications").
> 
> What remains to be done beyond this patch series:
> 
> 1. Performance improvements
> 2. Remove xpfotlbflush parameter
> 3. 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.
> 4. 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 (4):
>    mm, x86: omit TLB flushing by default for XPFO page table
>      modifications
>    xpfo, mm: remove dependency on CONFIG_PAGE_EXTENSION
>    xpfo, mm: optimize spinlock usage in xpfo_kunmap
>    EXPERIMENTAL: xpfo, mm: optimize spin lock usage in xpfo_kmap
> 
> Khalid Aziz (2):
>    xpfo, mm: Fix hang when booting with "xpfotlbflush"
>    xpfo, mm: Defer TLB flushes for non-current CPUs (x86 only)
> 
> 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                          |  58 ++++
>   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                           |  10 +
>   arch/x86/mm/pageattr.c                        |  23 +-
>   arch/x86/mm/tlb.c                             |  27 ++
>   arch/x86/mm/xpfo.c                            | 171 ++++++++++++
>   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                    |  13 +
>   include/linux/sched.h                         |   9 +
>   include/linux/xpfo.h                          |  90 +++++++
>   include/trace/events/mmflags.h                |  10 +-
>   kernel/dma/swiotlb.c                          |   3 +-
>   mm/Makefile                                   |   1 +
>   mm/mmap.c                                     |  19 +-
>   mm/page_alloc.c                               |   3 +
>   mm/util.c                                     |  32 +++
>   mm/xpfo.c                                     | 247 ++++++++++++++++++
>   security/Kconfig                              |  29 ++
>   32 files changed, 974 insertions(+), 43 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
> 

So this seems to blow up immediately on my arm64 box with a config
based on Fedora:

[   11.008243] Unable to handle kernel paging request at virtual address ffff8003f8602f9b
[   11.016133] Mem abort info:
[   11.018926]   ESR = 0x96000007
[   11.021967]   Exception class = DABT (current EL), IL = 32 bits
[   11.027858]   SET = 0, FnV = 0
[   11.030904]   EA = 0, S1PTW = 0
[   11.034030] Data abort info:
[   11.036896]   ISV = 0, ISS = 0x00000007
[   11.040718]   CM = 0, WnR = 0
[   11.043672] swapper pgtable: 4k pages, 48-bit VAs, pgdp = (____ptrval____)
[   11.050523] [ffff8003f8602f9b] pgd=00000043ffff7803, pud=00000043fe113803, pmd=00000043fc376803, pte=00e80043f8602f13
[   11.061094] Internal error: Oops: 96000007 [#3] SMP
[   11.065948] Modules linked in: xfs libcrc32c sdhci_of_arasan sdhci_pltfm sdhci i2c_xgene_slimpro cqhci gpio_dwapb xhci_plat_hcd gpio_xgene_sb gpio_keys
[   11.079454] CPU: 3 PID: 577 Comm: systemd-getty-g Tainted: G      D           4.20.0-xpfo+ #9
[   11.087936] Hardware name: www.apm.com American Megatrends/American Megatrends, BIOS 3.07.06 20/03/2015
[   11.097285] pstate: 00400005 (nzcv daif +PAN -UAO)
[   11.102057] pc : __memcpy+0x20/0x180
[   11.105616] lr : __access_remote_vm+0x7c/0x1f0
[   11.110036] sp : ffff000011cb3c20
[   11.113333] x29: ffff000011cb3c20 x28: ffff8003f8602000
[   11.118619] x27: 0000000000000f9b x26: 0000000000001000
[   11.123904] x25: 000083ffffffffff x24: cccccccccccccccd
[   11.129189] x23: ffff8003d7c53000 x22: 0000000000000044
[   11.134474] x21: 0000fffff0591f9b x20: 0000000000000044
[   11.139759] x19: 0000000000000044 x18: 0000000000000000
[   11.145044] x17: 0000000000000002 x16: 0000000000000000
[   11.150329] x15: 0000000000000000 x14: 0000000000000000
[   11.155614] x13: 0000000000000000 x12: 0000000000000000
[   11.160899] x11: 0000000000000000 x10: 0000000000000000
[   11.166184] x9 : 0000000000000000 x8 : 0000000000000000
[   11.171469] x7 : 0000000000000000 x6 : ffff8003d7c53000
[   11.176754] x5 : 00e00043f8602fd3 x4 : 0000000000000005
[   11.182038] x3 : 00000003f8602000 x2 : 000000000000003f
[   11.187323] x1 : ffff8003f8602f9b x0 : ffff8003d7c53000
[   11.192609] Process systemd-getty-g (pid: 577, stack limit = 0x(____ptrval____))
[   11.199967] Call trace:
[   11.202400]  __memcpy+0x20/0x180
[   11.205611]  access_remote_vm+0x4c/0x60
[   11.209428]  environ_read+0x12c/0x260
[   11.213071]  __vfs_read+0x48/0x158
[   11.216454]  vfs_read+0x94/0x150
[   11.219665]  ksys_read+0x54/0xb0
[   11.222875]  __arm64_sys_read+0x24/0x30
[   11.226691]  el0_svc_handler+0x94/0x110
[   11.230508]  el0_svc+0x8/0xc
[   11.233375] Code: f2400c84 540001c0 cb040042 36000064 (38401423)
[   11.239439] ---[ end trace 4132d3416fb70591 ]---

I'll see if I get some time tomorrow to dig into this unless
someone spots a problem sooner.

Thanks,
Laura

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