These patches are based on work from a team at Graz University of Technology posted here: https://github.com/IAIK/KAISER
The KAISER approach keeps two copies of the page tables: one for running in the kernel and one for running userspace. But, there are a few structures that are needed for switching in and out of the kernel and a good subset of *those* are per-cpu data. Here's a short summary of the things we map to userspace: * The gdt_page's virtual address is pointed to by the LGDT instruction. It is needed to define the segments. Deeply required by CPU to run. * cpu_tss tells the CPU, among other things, where the new stacks are after user<->kernel transitions. Needed by the CPU to make ring transitions. * exception_stacks are needed at interrupt and exception entry so that we have a place to store data to, among other things, get a free register to load the kernel CR3. Signed-off-by: Dave Hansen <[email protected]> Cc: Moritz Lipp <[email protected]> Cc: Daniel Gruss <[email protected]> Cc: Michael Schwarz <[email protected]> Cc: Richard Fellner <[email protected]> Cc: Andy Lutomirski <[email protected]> Cc: Linus Torvalds <[email protected]> Cc: Kees Cook <[email protected]> Cc: Hugh Dickins <[email protected]> Cc: [email protected] --- b/arch/x86/include/asm/desc.h | 2 +- b/arch/x86/include/asm/processor.h | 2 +- b/arch/x86/kernel/cpu/common.c | 4 ++-- b/arch/x86/kernel/process.c | 2 +- 4 files changed, 5 insertions(+), 5 deletions(-) diff -puN arch/x86/include/asm/desc.h~kaiser-prep-x86-percpu-user-mapped arch/x86/include/asm/desc.h --- a/arch/x86/include/asm/desc.h~kaiser-prep-x86-percpu-user-mapped 2017-11-08 10:45:29.252681395 -0800 +++ b/arch/x86/include/asm/desc.h 2017-11-08 10:45:29.261681395 -0800 @@ -45,7 +45,7 @@ struct gdt_page { struct desc_struct gdt[GDT_ENTRIES]; } __attribute__((aligned(PAGE_SIZE))); -DECLARE_PER_CPU_PAGE_ALIGNED(struct gdt_page, gdt_page); +DECLARE_PER_CPU_PAGE_ALIGNED_USER_MAPPED(struct gdt_page, gdt_page); /* Provide the original GDT */ static inline struct desc_struct *get_cpu_gdt_rw(unsigned int cpu) diff -puN arch/x86/include/asm/processor.h~kaiser-prep-x86-percpu-user-mapped arch/x86/include/asm/processor.h --- a/arch/x86/include/asm/processor.h~kaiser-prep-x86-percpu-user-mapped 2017-11-08 10:45:29.254681395 -0800 +++ b/arch/x86/include/asm/processor.h 2017-11-08 10:45:29.261681395 -0800 @@ -346,7 +346,7 @@ struct tss_struct { unsigned long SYSENTER_stack[64]; } ____cacheline_aligned; -DECLARE_PER_CPU_SHARED_ALIGNED(struct tss_struct, cpu_tss); +DECLARE_PER_CPU_SHARED_ALIGNED_USER_MAPPED(struct tss_struct, cpu_tss); /* * sizeof(unsigned long) coming from an extra "long" at the end diff -puN arch/x86/kernel/cpu/common.c~kaiser-prep-x86-percpu-user-mapped arch/x86/kernel/cpu/common.c --- a/arch/x86/kernel/cpu/common.c~kaiser-prep-x86-percpu-user-mapped 2017-11-08 10:45:29.256681395 -0800 +++ b/arch/x86/kernel/cpu/common.c 2017-11-08 10:45:29.262681395 -0800 @@ -98,7 +98,7 @@ static const struct cpu_dev default_cpu static const struct cpu_dev *this_cpu = &default_cpu; -DEFINE_PER_CPU_PAGE_ALIGNED(struct gdt_page, gdt_page) = { .gdt = { +DEFINE_PER_CPU_PAGE_ALIGNED_USER_MAPPED(struct gdt_page, gdt_page) = { .gdt = { #ifdef CONFIG_X86_64 /* * We need valid kernel segments for data and code in long mode too @@ -1343,7 +1343,7 @@ static const unsigned int exception_stac [DEBUG_STACK - 1] = DEBUG_STKSZ }; -static DEFINE_PER_CPU_PAGE_ALIGNED(char, exception_stacks +DEFINE_PER_CPU_PAGE_ALIGNED_USER_MAPPED(char, exception_stacks [(N_EXCEPTION_STACKS - 1) * EXCEPTION_STKSZ + DEBUG_STKSZ]); /* May not be marked __init: used by software suspend */ diff -puN arch/x86/kernel/process.c~kaiser-prep-x86-percpu-user-mapped arch/x86/kernel/process.c --- a/arch/x86/kernel/process.c~kaiser-prep-x86-percpu-user-mapped 2017-11-08 10:45:29.257681395 -0800 +++ b/arch/x86/kernel/process.c 2017-11-08 10:45:29.262681395 -0800 @@ -46,7 +46,7 @@ * section. Since TSS's are completely CPU-local, we want them * on exact cacheline boundaries, to eliminate cacheline ping-pong. */ -__visible DEFINE_PER_CPU_SHARED_ALIGNED(struct tss_struct, cpu_tss) = { +__visible DEFINE_PER_CPU_SHARED_ALIGNED_USER_MAPPED(struct tss_struct, cpu_tss) = { .x86_tss = { /* * .sp0 is only used when entering ring 0 from a lower _
