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SubjectRe: [PATCH 02/16] x86/entry/32: Enter the kernel via trampoline stack
On Tue, Jan 16, 2018 at 8:36 AM, Joerg Roedel <joro@8bytes.org> wrote:
> From: Joerg Roedel <jroedel@suse.de>
>
> Use the sysenter stack as a trampoline stack to enter the
> kernel. The sysenter stack is already in the cpu_entry_area
> and will be mapped to userspace when PTI is enabled.
>
> Signed-off-by: Joerg Roedel <jroedel@suse.de>
> ---
> arch/x86/entry/entry_32.S | 89 +++++++++++++++++++++++++++++++++++-----
> arch/x86/include/asm/switch_to.h | 6 +--
> arch/x86/kernel/asm-offsets_32.c | 4 +-
> arch/x86/kernel/cpu/common.c | 5 ++-
> arch/x86/kernel/process.c | 2 -
> arch/x86/kernel/process_32.c | 6 +++
> 6 files changed, 91 insertions(+), 21 deletions(-)
>
> diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
> index eb8c5615777b..5a7bdb73be9f 100644
> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -222,6 +222,47 @@
> .endm
>
> /*
> + * Switch from the entry-trampline stack to the kernel stack of the
> + * running task.
> + *
> + * nr_regs is the number of dwords to push from the entry stack to the
> + * task stack. If it is > 0 it expects an irq frame at the bottom of the
> + * stack.
> + *
> + * check_user != 0 it will add a check to only switch stacks if the
> + * kernel entry was from user-space.
> + */
> +.macro SWITCH_TO_KERNEL_STACK nr_regs=0 check_user=0

How about marking nr_regs with :req to force everyone to be explicit?

> +
> + .if \check_user > 0 && \nr_regs > 0
> + testb $3, (\nr_regs - 4)*4(%esp) /* CS */
> + jz .Lend_\@
> + .endif
> +
> + pushl %edi
> + movl %esp, %edi
> +
> + /*
> + * TSS_sysenter_stack is the offset from the bottom of the
> + * entry-stack
> + */
> + movl TSS_sysenter_stack + ((\nr_regs + 1) * 4)(%esp), %esp

This is incomprehensible. You're adding what appears to be the offset
of sysenter_stack within the TSS to something based on esp and
dereferencing that to get the new esp. That't not actually what
you're doing, but please change asm_offsets.c (as in my previous
email) to avoid putting serious arithmetic in it and then do the
arithmetic right here so that it's possible to follow what's going on.

> +
> + /* Copy the registers over */
> + .if \nr_regs > 0
> + i = 0
> + .rept \nr_regs
> + pushl (\nr_regs - i) * 4(%edi)
> + i = i + 1
> + .endr
> + .endif
> +
> + mov (%edi), %edi
> +
> +.Lend_\@:
> +.endm
> +
> +/*
> * %eax: prev task
> * %edx: next task
> */
> @@ -401,7 +442,9 @@ ENTRY(xen_sysenter_target)
> * 0(%ebp) arg6
> */
> ENTRY(entry_SYSENTER_32)
> - movl TSS_sysenter_stack(%esp), %esp
> + /* Kernel stack is empty */
> + SWITCH_TO_KERNEL_STACK

This would be more readable if you put nr_regs in here.

> +
> .Lsysenter_past_esp:
> pushl $__USER_DS /* pt_regs->ss */
> pushl %ebp /* pt_regs->sp (stashed in bp) */
> @@ -521,6 +564,10 @@ ENDPROC(entry_SYSENTER_32)
> ENTRY(entry_INT80_32)
> ASM_CLAC
> pushl %eax /* pt_regs->orig_ax */
> +
> + /* Stack layout: ss, esp, eflags, cs, eip, orig_eax */
> + SWITCH_TO_KERNEL_STACK nr_regs=6 check_user=1
> +

Why check_user?

> @@ -655,6 +702,10 @@ END(irq_entries_start)
> common_interrupt:
> ASM_CLAC
> addl $-0x80, (%esp) /* Adjust vector into the [-256, -1] range */
> +
> + /* Stack layout: ss, esp, eflags, cs, eip, vector */
> + SWITCH_TO_KERNEL_STACK nr_regs=6 check_user=1

LGTM.

> ENTRY(nmi)
> ASM_CLAC
> +
> + /* Stack layout: ss, esp, eflags, cs, eip */
> + SWITCH_TO_KERNEL_STACK nr_regs=5 check_user=1

This is wrong, I think. If you get an nmi in kernel mode but while
still on the sysenter stack, you blow up. IIRC we have some crazy
code already to handle this (for nmi and #DB), and maybe that's
already adequate or can be made adequate, but at the very least this
needs a big comment explaining why it's okay.

> diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
> index eb5f7999a893..20e5f7ab8260 100644
> --- a/arch/x86/include/asm/switch_to.h
> +++ b/arch/x86/include/asm/switch_to.h
> @@ -89,13 +89,9 @@ static inline void refresh_sysenter_cs(struct thread_struct *thread)
> /* This is used when switching tasks or entering/exiting vm86 mode. */
> static inline void update_sp0(struct task_struct *task)
> {
> - /* On x86_64, sp0 always points to the entry trampoline stack, which is constant: */
> -#ifdef CONFIG_X86_32
> - load_sp0(task->thread.sp0);
> -#else
> + /* sp0 always points to the entry trampoline stack, which is constant: */
> if (static_cpu_has(X86_FEATURE_XENPV))
> load_sp0(task_top_of_stack(task));
> -#endif
> }
>
> #endif /* _ASM_X86_SWITCH_TO_H */
> diff --git a/arch/x86/kernel/asm-offsets_32.c b/arch/x86/kernel/asm-offsets_32.c
> index 654229bac2fc..7270dd834f4b 100644
> --- a/arch/x86/kernel/asm-offsets_32.c
> +++ b/arch/x86/kernel/asm-offsets_32.c
> @@ -47,9 +47,11 @@ void foo(void)
> BLANK();
>
> /* Offset from the sysenter stack to tss.sp0 */
> - DEFINE(TSS_sysenter_stack, offsetof(struct cpu_entry_area, tss.x86_tss.sp0) -
> + DEFINE(TSS_sysenter_stack, offsetof(struct cpu_entry_area, tss.x86_tss.sp1) -
> offsetofend(struct cpu_entry_area, entry_stack_page.stack));



>
> + OFFSET(TSS_sp1, tss_struct, x86_tss.sp1);
> +
> #ifdef CONFIG_CC_STACKPROTECTOR
> BLANK();
> OFFSET(stack_canary_offset, stack_canary, canary);
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index ef29ad001991..20a71c914e59 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -1649,11 +1649,12 @@ void cpu_init(void)
> enter_lazy_tlb(&init_mm, curr);
>
> /*
> - * Initialize the TSS. Don't bother initializing sp0, as the initial
> - * task never enters user mode.
> + * Initialize the TSS. sp0 points to the entry trampoline stack
> + * regardless of what task is running.
> */
> set_tss_desc(cpu, &get_cpu_entry_area(cpu)->tss.x86_tss);
> load_TR_desc();
> + load_sp0((unsigned long)(cpu_entry_stack(cpu) + 1));

It's high time we unified the 32-bit and 64-bit versions of the code.
This isn't necessarily needed for your series, though.

> diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
> index 5224c6099184..452eeac00b80 100644
> --- a/arch/x86/kernel/process_32.c
> +++ b/arch/x86/kernel/process_32.c
> @@ -292,6 +292,12 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
> this_cpu_write(cpu_current_top_of_stack,
> (unsigned long)task_stack_page(next_p) +
> THREAD_SIZE);
> + /*
> + * TODO: Find a way to let cpu_current_top_of_stack point to
> + * cpu_tss_rw.x86_tss.sp1. Doing so now results in stack corruption with
> + * iret exceptions.
> + */
> + this_cpu_write(cpu_tss_rw.x86_tss.sp1, next_p->thread.sp0);

Do you know what the issue is?

As a general comment, the interaction between this patch and vm86 is a
bit scary. In vm86 mode, the kernel gets entered with extra stuff on
the stack, which may screw up all your offsets.

--Andy

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