When entering the kernel from userland, use the per-task PTI stack instead of the per-cpu trampoline stack. Like the trampoline stack, the PTI stack is mapped both in the kernel and in the user page-table. Using a per-task stack which is mapped into the kernel and the user page-table instead of a per-cpu stack will allow executing more code before switching to the kernel stack and to the kernel page-table.
Additional changes will be made to later to switch to the kernel stack (which is only mapped in the kernel page-table). Signed-off-by: Alexandre Chartre <alexandre.char...@oracle.com> --- arch/x86/entry/entry_64.S | 42 +++++++++----------------------- arch/x86/include/asm/pti.h | 8 ++++++ arch/x86/include/asm/switch_to.h | 7 +++++- 3 files changed, 26 insertions(+), 31 deletions(-) diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index 458af12ed9a1..29beab46bedd 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -194,19 +194,9 @@ syscall_return_via_sysret: /* rcx and r11 are already restored (see code above) */ POP_REGS pop_rdi=0 skip_r11rcx=1 - /* - * Now all regs are restored except RSP and RDI. - * Save old stack pointer and switch to trampoline stack. - */ - movq %rsp, %rdi - movq PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %rsp - UNWIND_HINT_EMPTY - - pushq RSP-RDI(%rdi) /* RSP */ - pushq (%rdi) /* RDI */ - /* * We are on the trampoline stack. All regs except RDI are live. + * We are on the trampoline stack. All regs except RSP are live. * We can do future final exit work right here. */ STACKLEAK_ERASE_NOCLOBBER @@ -214,7 +204,7 @@ syscall_return_via_sysret: SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi popq %rdi - popq %rsp + movq RSP-ORIG_RAX(%rsp), %rsp USERGS_SYSRET64 SYM_CODE_END(entry_SYSCALL_64) @@ -606,24 +596,6 @@ SYM_INNER_LABEL(swapgs_restore_regs_and_return_to_usermode, SYM_L_GLOBAL) #endif POP_REGS pop_rdi=0 - /* - * The stack is now user RDI, orig_ax, RIP, CS, EFLAGS, RSP, SS. - * Save old stack pointer and switch to trampoline stack. - */ - movq %rsp, %rdi - movq PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %rsp - UNWIND_HINT_EMPTY - - /* Copy the IRET frame to the trampoline stack. */ - pushq 6*8(%rdi) /* SS */ - pushq 5*8(%rdi) /* RSP */ - pushq 4*8(%rdi) /* EFLAGS */ - pushq 3*8(%rdi) /* CS */ - pushq 2*8(%rdi) /* RIP */ - - /* Push user RDI on the trampoline stack. */ - pushq (%rdi) - /* * We are on the trampoline stack. All regs except RDI are live. * We can do future final exit work right here. @@ -634,6 +606,7 @@ SYM_INNER_LABEL(swapgs_restore_regs_and_return_to_usermode, SYM_L_GLOBAL) /* Restore RDI. */ popq %rdi + addq $8, %rsp /* skip regs->orig_ax */ SWAPGS INTERRUPT_RETURN @@ -1062,6 +1035,15 @@ SYM_CODE_START_LOCAL(error_entry) SWITCH_TO_KERNEL_CR3 scratch_reg=%rax .Lerror_entry_from_usermode_after_swapgs: + /* + * We are on the trampoline stack. With PTI, the trampoline + * stack is a per-thread stack so we are all set and we can + * return. + * + * Without PTI, the trampoline stack is a per-cpu stack and + * we need to switch to the normal thread stack. + */ + ALTERNATIVE "", "ret", X86_FEATURE_PTI /* Put us onto the real thread stack. */ popq %r12 /* save return addr in %12 */ movq %rsp, %rdi /* arg0 = pt_regs pointer */ diff --git a/arch/x86/include/asm/pti.h b/arch/x86/include/asm/pti.h index 5484e69ff8d3..ed211fcc3a50 100644 --- a/arch/x86/include/asm/pti.h +++ b/arch/x86/include/asm/pti.h @@ -17,8 +17,16 @@ extern void pti_check_boottime_disable(void); extern void pti_finalize(void); extern void pti_clone_pgtable(struct mm_struct *mm, unsigned long start, unsigned long end, enum pti_clone_level level); +static inline bool pti_enabled(void) +{ + return static_cpu_has(X86_FEATURE_PTI); +} #else static inline void pti_check_boottime_disable(void) { } +static inline bool pti_enabled(void) +{ + return false; +} #endif #endif /* __ASSEMBLY__ */ diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h index 9f69cc497f4b..457458228462 100644 --- a/arch/x86/include/asm/switch_to.h +++ b/arch/x86/include/asm/switch_to.h @@ -3,6 +3,7 @@ #define _ASM_X86_SWITCH_TO_H #include <linux/sched/task_stack.h> +#include <asm/pti.h> struct task_struct; /* one of the stranger aspects of C forward declarations */ @@ -76,8 +77,12 @@ static inline void update_task_stack(struct task_struct *task) * doesn't work on x86-32 because sp1 and * cpu_current_top_of_stack have different values (because of * the non-zero stack-padding on 32bit). + * + * If PTI is enabled, sp0 points to the PTI stack (mapped in + * the kernel and user page-table) which is used when entering + * the kernel. */ - if (static_cpu_has(X86_FEATURE_XENPV)) + if (static_cpu_has(X86_FEATURE_XENPV) || pti_enabled()) load_sp0(task_top_of_stack(task)); #endif } -- 2.18.4