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    Subject[PATCH v12 00/18] Enable FSGSBASE instructions
    Date
    Benefits:
    Currently a user process that wishes to read or write the FS/GS base must
    make a system call. But recent X86 processors have added new instructions
    for use in 64-bit mode that allow direct access to the FS and GS segment
    base addresses. The operating system controls whether applications can
    use these instructions with a %cr4 control bit.

    In addition to benefits to applications, performance improvements to the
    OS context switch code are possible by making use of these instructions. 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.

    Changes from v11:

    - Rebase to v5.7-rc5, fix 32bit compilation error.


    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/entry/64: Clean up paranoid exit
    x86/fsgsbase/64: Use FSGSBASE in switch_to() if available
    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/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
    x86/fsgsbase/64: Enable FSGSBASE instructions in helper functions
    x86/fsgsbase/64: Use FSGSBASE instructions on thread copy and ptrace
    selftests/x86/fsgsbase: Test ptracer-induced GS base write with
    FSGSBASE

    Sasha Levin (1):
    x86/fsgsbase/64: move save_fsgs to header file

    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

    .../admin-guide/kernel-parameters.txt | 2 +
    Documentation/x86/entry_64.rst | 9 +
    Documentation/x86/x86_64/fsgs.rst | 199 ++++++++++++++++++
    Documentation/x86/x86_64/index.rst | 1 +
    arch/x86/entry/calling.h | 40 ++++
    arch/x86/entry/entry_64.S | 131 +++++++++---
    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.c | 9 +-
    arch/x86/kernel/process.h | 72 +++++++
    arch/x86/kernel/process_64.c | 142 +++++++------
    arch/x86/kernel/ptrace.c | 17 +-
    tools/testing/selftests/x86/fsgsbase.c | 24 ++-
    16 files changed, 608 insertions(+), 129 deletions(-)
    create mode 100644 Documentation/x86/x86_64/fsgs.rst

    --
    2.20.1

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