Messages in this thread | | | From | "Chang S. Bae" <> | Subject | [PATCH v8 00/17] Enable FSGSBASE instructions | Date | Thu, 12 Sep 2019 13:06:41 -0700 |
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Introduction: FSGSBASE is a 64-bit instruction set to allow read/write FS/GS base from user and kernel spaces. Currently a user process reads/writes to FS/GS base needs a system call. Read/write to FS/GS base allows performance improvement specifically during context switch. A third party reported out promising performance numbers out of their initial benchmarking of the previous version of this patch series [9].
Enablement check: The kernel provides information about the enabled state of FSGSBASE to applications using the ELF_AUX vector. If the HWCAP2_FSGSBASE bit is set in the AUX vector, the kernel has FSGSBASE instructions enabled and applications can use them.
Kernel changes: Major changes made in the kernel are in context switch, paranoid path, and ptrace. In a context switch, a task's FS/GS base will be secured regardless of its selector. In the paranoid path, GS base is unconditionally overwritten to the kernel GS base on entry and the original GS base is restored on exit. Ptrace includes divergence of FS/GS index and base values.
Security: For mitigating the Spectre v1 SWAPGS issue, LFENCE instructions were added on most kernel entries. Those patches are dependent on previous behaviors that users couldn't load a kernel address into the GS base. These patches change that assumption since the user can load any address into GS base. The changes to the kernel entry path in this patch series take account of the SWAPGS issue.
Updates from v7 [7]: (1) Consider FSGSBASE when determining which Spectre SWAPGS mitigations are required. (2) Fixed save_fsgs() to be aware of interrupt conditions (3) Made selftest changes based on Andy's previous fixes and cleanups (4) Included Andy's paranoid exit cleanup (5) Included documentation rewritten by Thomas (6) Carried on Thomas' edits on multiple changelogs and comments (7) Used '[FS|GS] base' consistently, except for selftest where GSBASE has been already used in its test messages (8) Dropped the READ_MSR_GSBASE macro
Updates from the last merged version [8] (that was reverted) are (1), (2) and (7) items. Thomas' SOB has been removed, except for the documentation patch.
Previous versions: [1-6]
[1] version 1: https://lkml.kernel.org/r/1521481767-22113-1-git-send-email-chang.seok.bae@intel.com/ [2] version 2: https://lkml.kernel.org/r/1527789525-8857-1-git-send-email-chang.seok.bae@intel.com/ [3] version 3: https://lkml.kernel.org/r/20181023184234.14025-1-chang.seok.bae@intel.com/ [4] version 4: https://lkml.kernel.org/r/20190116224849.8617-1-chang.seok.bae@intel.com/ [5] version 5: https://lkml.kernel.org/r/20190201205319.15995-1-chang.seok.bae@intel.com/ [6] version 6: https://lkml.kernel.org/r/1552680405-5265-1-git-send-email-chang.seok.bae@intel.com/ [7] version 7: https://lkml.kernel.org/r/1557309753-24073-1-git-send-email-chang.seok.bae@intel.com/ [8] previously merged point (right before reverted): https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?h=x86-cpu-for-linus&id=697096b14444f458fb81212d1c82d7846e932455 [9] initial benchmark: https://www.phoronix.com/scan.php?page=article&item=linux-wip-fsgsbase&num=1
Andi Kleen (2): x86/fsgsbase/64: Add intrinsics for FSGSBASE instructions x86/elf: Enumerate kernel FSGSBASE capability in AT_HWCAP2
Andy Lutomirski (4): x86/cpu: Add 'unsafe_fsgsbase' to enable CR4.FSGSBASE x86/fsgsbase/64: Use FSGSBASE in switch_to() if available x86/entry/64: Clean up paranoid exit x86/fsgsbase/64: Enable FSGSBASE on 64bit by default and add a chicken bit
Chang S. Bae (9): x86/ptrace: Prevent ptrace from clearing the FS/GS selector selftests/x86/fsgsbase: Test GS selector on ptracer-induced GS base write x86/fsgsbase/64: Enable FSGSBASE instructions in helper functions x86/fsgsbase/64: Use FSGSBASE instructions on thread copy and ptrace x86/entry/64: Switch CR3 before SWAPGS in paranoid entry x86/entry/64: Introduce the FIND_PERCPU_BASE macro x86/entry/64: Handle FSGSBASE enabled paranoid entry/exit x86/entry/64: Document GSBASE handling in the paranoid path selftests/x86/fsgsbase: Test ptracer-induced GS base write with FSGSBASE
Thomas Gleixner (1): Documentation/x86/64: Add documentation for GS/FS addressing mode
Tony Luck (1): x86/speculation/swapgs: Check FSGSBASE in enabling SWAPGS mitigation
Documentation/admin-guide/kernel-parameters.txt | 2 + Documentation/x86/entry_64.rst | 9 ++ Documentation/x86/x86_64/fsgs.rst | 200 ++++++++++++++++++++++++ Documentation/x86/x86_64/index.rst | 1 + arch/x86/entry/calling.h | 40 +++++ arch/x86/entry/entry_64.S | 134 ++++++++++++---- arch/x86/include/asm/fsgsbase.h | 45 ++++-- arch/x86/include/asm/inst.h | 15 ++ arch/x86/include/uapi/asm/hwcap2.h | 3 + arch/x86/kernel/cpu/bugs.c | 6 +- arch/x86/kernel/cpu/common.c | 22 +++ arch/x86/kernel/process_64.c | 121 ++++++++++++-- arch/x86/kernel/ptrace.c | 14 +- tools/testing/selftests/x86/fsgsbase.c | 24 ++- 14 files changed, 564 insertions(+), 72 deletions(-) create mode 100644 Documentation/x86/x86_64/fsgs.rst
-- 2.7.4
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