FSGSBASE is a 64-bit instruction set to allow read/write FS/GSBASE from any privileges. Since introduced in Ivybridge, enabling efforts has been revolving in a quite long period of time, for various reasons [2,3,4]. After the extended discussions [1], the new ABIs are finally introduced to customize FS/GSBASE separately from the selector.
Benefits: Some performance benefit in context switch is expected by skipping MSR write for GSBASE. User-level programs (such as JAVA-based) may benefit from avoiding system calls to edit FS/GSBASE. Major changes in the kernel: * In a context switch, a thread's FS/GSBASE will be secured regardless of its selector, base on the discussion [1]. * (Subsequently) ptracer should expect a divergence of FS/GS index and base values. There was a controversial debate on the concerns for a backward compatibility (mostly for GDB. [7,8]). We finally concluded it is insignificant in real usages. * On the paranoid_entry, GSBASE is updated to point the per_CPU base and the original GSBASE is restored at the exit. Virtualization: A FSGSBASE-enabled VM can be located on a host either with HW virtualization or with SW emulation. KVM advertises FSGSBASE when physical CPU has. The emulation is supported in QEMU/TCG [5]. In a pool of the mixed systems, VMM may disable FSGSBASE for seamless VM migrations [6]. Update from v2 [10]: * Separate out the preparatory patches [11] (now merged to the tip) * Bisect the paranoid_entry update patch * Edit minor nits Updates from v1 [9]: * Update the GSBASE update mechanism on the paranoid entry/exit. * Exclude ptracer backward compatibility patches. * Include FSGSBASE documentation and enumerating capability for user space * Add the TAINT_INSECURE flag. [1] Recent discussion on LKML: https://marc.info/?t=150147053700001&r=1&w=2 [2] Andy Lutomirski’s patchwork work : https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/log/?h=x86/fsgsbase [3] Patchset shown in 2016: https://lore.kernel.org/patchwork/patch/660520 [4] Patchset shown in 2014: https://lore.kernel.org/patchwork/patch/460288 [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] Version 1: https://lore.kernel.org/patchwork/cover/934843 [10] Version 2: https://lore.kernel.org/patchwork/cover/912063 [11] x86: infra to enable FSGSBASE https://lore.kernel.org/patchwork/cover/988180 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: Add 'unsafe_fsgsbase' to enable CR4.FSGSBASE x86/fsgsbase/64: Preserve FS/GS state in __switch_to() if FSGSBASE is on selftests/x86/fsgsbase: Test WRGSBASE x86/fsgsbase/64: Enable FSGSBASE by default and add a chicken bit Chang S. Bae (5): taint: Introduce a new taint flag (insecure) x86/fsgsbase/64: Enable FSGSBASE instructions in the helper functions x86/fsgsbase/64: When copying a thread, use the FSGSBASE instructions if available x86/fsgsbase/64: Introduce the new FIND_PERCPU_BASE macro x86/fsgsbase/64: Use the per-CPU base as GSBASE at the paranoid_entry .../admin-guide/kernel-parameters.txt | 2 + Documentation/sysctl/kernel.txt | 1 + Documentation/x86/fsgs.txt | 104 +++++++++++++ arch/x86/entry/entry_64.S | 73 +++++++-- arch/x86/include/asm/fsgsbase.h | 140 +++++++++++++++++- arch/x86/include/asm/inst.h | 15 ++ arch/x86/include/uapi/asm/hwcap2.h | 3 + arch/x86/kernel/cpu/common.c | 22 +++ arch/x86/kernel/process_64.c | 128 +++++++++++++--- include/linux/kernel.h | 3 +- kernel/panic.c | 1 + tools/testing/selftests/x86/fsgsbase.c | 110 +++++++++++++- 12 files changed, 556 insertions(+), 46 deletions(-) create mode 100644 Documentation/x86/fsgs.txt -- 2.19.1