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From:   Brian Gerst <brgerst@gmail.com>
Date:   Tue, 26 Jan 2021 01:04:48 -0500
Message-ID: <CAMzpN2j6gYGPj07RK3AD9Y6PkZH7ozVLtFvqHiWgW5=7uF4tyg@mail.gmail.com>
Subject: Re: [PATCH V2 3/6] x86_32/sysenter: switch to the task stack without
 emptying the entry stack
To:     Lai Jiangshan <jiangshanlai@gmail.com>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Lai Jiangshan <laijs@linux.alibaba.com>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Joerg Roedel <jroedel@suse.de>,
        Ricardo Neri <ricardo.neri-calderon@linux.intel.com>,
        Reinette Chatre <reinette.chatre@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Gabriel Krisman Bertazi <krisman@collabora.com>,
        Kees Cook <keescook@chromium.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Jens Axboe <axboe@kernel.dk>,
        Arvind Sankar <nivedita@alum.mit.edu>,
        Ard Biesheuvel <ardb@kernel.org>,
        Andi Kleen <ak@linux.intel.com>,
        Mike Rapoport <rppt@kernel.org>, Mike Hommey <mh@glandium.org>,
        Mark Gross <mgross@linux.intel.com>,
        Fenghua Yu <fenghua.yu@intel.com>,
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        Jay Lang <jaytlang@mit.edu>,
        "Chang S. Bae" <chang.seok.bae@intel.com>
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On Mon, Jan 25, 2021 at 11:35 AM Lai Jiangshan <jiangshanlai@gmail.com> wrote:
>
> From: Lai Jiangshan <laijs@linux.alibaba.com>
>
> 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@linux.alibaba.com>
> ---
>  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