Re: [PATCH V2 3/6] x86_32/sysenter: switch to the task stack without emptying the entry stack

[Brian Gerst]

On Mon, Jan 25, 2021 at 11:35 AM Lai Jiangshan <jiangshanlai@xxxxxxxxx> wrote:
>
> From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
>
> Like the way x86_64 uses the "old" stack, we can save the entry stack
> pointer to a register and switch to the task stack.  So that we have
> space on the "old" stack to save more things or scratch registers.
>
> Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> ---
>  arch/x86/entry/entry_32.S | 11 +++++------
>  1 file changed, 5 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
> index 3b4d1a63d1f0..4513702ba45d 100644
> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -905,19 +905,18 @@ SYM_FUNC_START(entry_SYSENTER_32)
>         pushl   %eax
>         BUG_IF_WRONG_CR3 no_user_check=1
>         SWITCH_TO_KERNEL_CR3 scratch_reg=%eax
> -       popl    %eax
> -       popfl
>
> -       /* Stack empty again, switch to task stack */
> -       movl    TSS_entry2task_stack(%esp), %esp
> +       /* Switch to task stack */
> +       movl    %esp, %eax
> +       movl    (2*4+TSS_entry2task_stack)(%esp), %esp
>
>  .Lsysenter_past_esp:
>         pushl   $__USER_DS              /* pt_regs->ss */
>         pushl   $0                      /* pt_regs->sp (placeholder) */
> -       pushfl                          /* pt_regs->flags (except IF = 0) */
> +       pushl   4(%eax)                 /* pt_regs->flags (except IF = 0) */

__KERNEL_DS isn't loaded at this point, so this needs an explicit %ss:
override.  You probably didn't catch this because the default
__USER_DS was still loaded.

>         pushl   $__USER_CS              /* pt_regs->cs */
>         pushl   $0                      /* pt_regs->ip = 0 (placeholder) */
> -       pushl   %eax                    /* pt_regs->orig_ax */
> +       pushl   (%eax)                  /* pt_regs->orig_ax */

Add an %ss: override here too.

>         SAVE_ALL pt_regs_ax=$-ENOSYS    /* save rest, stack already switched */
>
>         /*
> --
> 2.19.1.6.gb485710b
>

--
Brian Gerst