Messages in this thread | | | Subject | Re: [PATCH v10 00/18] Enable FSGSBASE instructions | From | Vegard Nossum <> | Date | Sun, 10 May 2020 10:09:14 +0200 |
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On 4/24/20 1:21 AM, Sasha Levin wrote: > 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 v9: > > - Rebase on top of v5.7-rc1 and re-test. > - Work around changes in 2fff071d28b5 ("x86/process: Unify > copy_thread_tls()"). > - Work around changes in c7ca0b614513 ("Revert "x86/ptrace: Prevent > ptrace from clearing the FS/GS selector" and fix the test"). > > > > 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 | 10 +- > arch/x86/kernel/process.h | 69 ++++++ > arch/x86/kernel/process_64.c | 142 +++++++------ > arch/x86/kernel/ptrace.c | 17 +- > tools/testing/selftests/x86/fsgsbase.c | 24 ++- > 16 files changed, 606 insertions(+), 129 deletions(-) > create mode 100644 Documentation/x86/x86_64/fsgs.rst
So FWIW I've done some overnight fuzz testing of this patch set and haven't seen any problems. Will try a couple of other kernel configs too.
Vegard
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