FSGSBASE is 64-bit instruction set to allow read/write FS/GS base from any privilege. As introduced from Ivybridge, enabling effort has been revolving quite long [2,3,4] for various reasons. After extended discussions [1], this patchset is proposed to introduce new ABIs of customizing FS/GS base (separate from its selector).
FSGSBASE-enabled VM can be located on hosts with either HW virtualization or SW emulation. KVM advertises FSGSBASE when physical CPU has and emulation is supported in QEMU/TCG [5]. In a pool of mixed systems, VMM may disable FSGSBASE for seamless VM migrations [6]. A couple of major benefits are expected. Kernel will have performance improvement in context switch by skipping MSR write for GS base. User-level programs (such as JAVA-based) benefit from avoiding system calls to edit FS/GS base. Changes when FSGSBASE enabled: (1) In context switch, a thread's FS/GS base is secured regardless of its selector base on the discussion [1]. (2) (Subsequently) ptracer should expect divergence of FS/GS index and base values. There was controveral debate on the concerns with backward incompatibility with that. (Cases for GDB than other toolchains [7,8]) Current patchset as baseline version does not contain support for the backward compatibility (3) On paranoid entry, GS base is updated to per_CPU base and the original base is restored at the exit Updates from V1 [9]: * (3), instead of comparing current and kernel GS bases * (2); does not include ptracer backward compatibility patches. * With (3), CPU number is stored as early as possible (before hitting IST stack) than at vDSO initialization. * Include FSGSBASE documentation and enumerating capability for user space * Add TAINT_INSECURE flag and vDSO cleanup for the (CPU number) initialization [1] Recent discussion on LKML: https://marc.info/?t=150147053700001&r=1&w=2 [2] Andy Lutomirski’s rebase work : https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/log/?h=x86/fsgsbase [3] Patch set shown in year 2016: https://marc.info/?t=145857711900001&r=1&w=2 [4] First patch set: https://lkml.org/lkml/2015/4/10/573 [5] QEMU with FSGSBASE emulation: https://github.com/qemu/qemu/blob/026aaf47c02b79036feb830206cfebb2a726510d/target/i386/translate.c#L8186 [6] 5-level EPT: http://lkml.kernel.org/r/9ddf602b-6c8b-8c1e-ab46-07ed12366...@redhat.com [7] RR/FSGSBASE: https://mail.mozilla.org/pipermail/rr-dev/2018-March/000616.html [8] CRIU/FSGSBASE: https://lists.openvz.org/pipermail/criu/2018-March/040654.html [9] V1: https://lkml.org/lkml/2018/3/19/1699 Andi Kleen (3): x86/fsgsbase/64: Add intrinsics/macros for FSGSBASE instructions x86/elf: Enumerate kernel FSGSBASE capability in AT_HWCAP2 x86/fsgsbase/64: Add documentation for FSGSBASE Andy Lutomirski (4): x86/fsgsbase/64: Make ptrace read FS/GS base accurately x86/fsgsbase/64: Add 'unsafe_fsgsbase' to enable CR4.FSGSBASE x86/fsgsbase/64: Preserve FS/GS state in __switch_to if FSGSBASE is on x86/fsgsbase/64: Enable FSGSBASE by default and add a chicken bit Chang S. Bae (8): x86/fsgsbase/64: Introduce FS/GS base helper functions x86/fsgsbase/64: Use FS/GS base helpers in core dump x86/fsgsbase/64: Factor out load FS/GS segments from __switch_to x86/vdso: Move out the CPU number store taint: Add taint for insecure x86/fsgsbase/64: Enable FSGSBASE instructions in helper functions x86/fsgsbase/64: When copying a thread, use FSGSBASE if enabled x86/fsgsbase/64: Use per-CPU base as GS base on paranoid_entry Documentation/admin-guide/kernel-parameters.txt | 2 + Documentation/sysctl/kernel.txt | 1 + Documentation/x86/entry_64.txt | 9 + Documentation/x86/fsgs.txt | 104 +++++++++ arch/x86/entry/entry_64.S | 74 +++++-- arch/x86/entry/vdso/vgetcpu.c | 2 +- arch/x86/entry/vdso/vma.c | 38 +--- arch/x86/include/asm/elf.h | 6 +- arch/x86/include/asm/fsgsbase.h | 171 +++++++++++++++ arch/x86/include/asm/inst.h | 15 ++ arch/x86/include/asm/segment.h | 4 + arch/x86/include/asm/vgtod.h | 2 - arch/x86/include/uapi/asm/hwcap2.h | 3 + arch/x86/kernel/cpu/common.c | 39 ++++ arch/x86/kernel/process_64.c | 274 ++++++++++++++++++++---- arch/x86/kernel/ptrace.c | 28 +-- arch/x86/kernel/setup_percpu.c | 30 ++- include/linux/kernel.h | 3 +- kernel/panic.c | 1 + 19 files changed, 681 insertions(+), 125 deletions(-) create mode 100644 Documentation/x86/fsgs.txt create mode 100644 arch/x86/include/asm/fsgsbase.h -- 2.7.4
