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From: Ingo Molnar <mingo@kernel.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org,
	Borislav Petkov <bp@alien8.de>, Oleg Nesterov <oleg@redhat.com>,
	Denys Vlasenko <dvlasenk@redhat.com>
Subject: Re: [PATCH 1/3] x86: Create and use a TOP_OF_KERNEL_STACK_PADDING macro
Date: Mon, 16 Mar 2015 09:56:33 +0100	[thread overview]
Message-ID: <20150316085632.GA19903@gmail.com> (raw)
In-Reply-To: <02bf2f54b8dcb76a62a142b6dfe07d4ef7fc582e.1426009661.git.luto@amacapital.net>


* Andy Lutomirski <luto@amacapital.net> wrote:

> x86_32, unlike x86_64, pads the top of the kernel stack.  Document
> this padding and give it a name.
> 
> This should make no change whatsoever to the compiled kernel image.
> It also doesn't fix any of the current bugs in this area.
> 
> Signed-off-by: Andy Lutomirski <luto@amacapital.net>
> ---
>  arch/x86/include/asm/processor.h   |  3 ++-
>  arch/x86/include/asm/thread_info.h | 30 ++++++++++++++++++++++++++++++
>  2 files changed, 32 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index 48a61c1c626e..88d9aa745898 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -849,7 +849,8 @@ extern unsigned long thread_saved_pc(struct task_struct *tsk);
>  #define task_pt_regs(task)                                             \
>  ({                                                                     \
>         struct pt_regs *__regs__;                                       \
> -       __regs__ = (struct pt_regs *)(KSTK_TOP(task_stack_page(task))-8); \
> +       __regs__ = (struct pt_regs *)(KSTK_TOP(task_stack_page(task)) - \
> +				     TOP_OF_KERNEL_STACK_PADDING);     \
>         __regs__ - 1;                                                   \
>  })
>  
> diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
> index 7740edd56fed..74fd74ca50d3 100644
> --- a/arch/x86/include/asm/thread_info.h
> +++ b/arch/x86/include/asm/thread_info.h
> @@ -49,6 +49,36 @@ struct thread_info {
>  #define init_thread_info	(init_thread_union.thread_info)
>  #define init_stack		(init_thread_union.stack)
>  
> +#ifdef CONFIG_X86_32
> +
> +/*
> + * TOP_OF_KERNEL_STACK_PADDING is a number of unused bytes that we
> + * reserve at the top of the kernel stack.  We do it because of a nasty
> + * 32-bit corner case.  On x86_32, the hardware stack frame is
> + * variable-length.  Except for vm86 mode, struct pt_regs assumes a
> + * maximum-length frame.  If we enter from CPL 0, the top 8 bytes of
> + * pt_regs don't actually exist.  Ordinarily this doesn't matter, but it
> + * does in at least one case:
> + *
> + * If we take an NMI early enough in sysenter, the we can end up with

s/the/then

I fixed this up in the commit.

> + * pt_regs that extends above sp0.  On the way out, in the espfix code,
> + * we can read the saved SS value, but that value will be above sp0.
> + * Without this offset, that can result in a page fault.  (We are
> + * careful that, in this case, the value we read doesn't matter.)
> + *
> + * In vm86 mode, the hardware frame is much longer still, but we neither
> + * access the extra members from NMI context, nor do we write such a
> + * frame at sp0 at all.
> + */

Thanks,

	Ingo

  parent reply	other threads:[~2015-03-16  8:56 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-03-10 18:05 [PATCH 0/3] sp0, ss1, and sp1 docs and minor fixes Andy Lutomirski
2015-03-10 18:05 ` [PATCH 1/3] x86: Create and use a TOP_OF_KERNEL_STACK_PADDING macro Andy Lutomirski
2015-03-10 19:22   ` Denys Vlasenko
2015-03-10 19:47     ` Andy Lutomirski
2015-03-13 14:08     ` Denys Vlasenko
2015-03-16  8:56   ` Ingo Molnar [this message]
2015-03-16 12:08   ` [tip:x86/asm] x86/asm/entry: Create and use a ' TOP_OF_KERNEL_STACK_PADDING' macro tip-bot for Andy Lutomirski
2015-03-17  8:45   ` tip-bot for Andy Lutomirski
2015-03-10 18:05 ` [PATCH 2/3] x86: Unify and fix init sp0 Andy Lutomirski
2015-03-11 11:21   ` Borislav Petkov
2015-03-16 12:09   ` [tip:x86/asm] x86/asm/entry: Unify and fix initial thread_struct: :sp0 values tip-bot for Andy Lutomirski
2015-03-17  8:45   ` tip-bot for Andy Lutomirski
2015-03-10 18:06 ` [PATCH 3/3] x86_32: Document our abuse of ss1 and sp1 Andy Lutomirski
2015-03-10 19:13   ` Denys Vlasenko
2015-03-10 20:06     ` Andy Lutomirski
2015-03-10 20:52       ` Denys Vlasenko
2015-03-16 12:09   ` [tip:x86/asm] x86/asm/entry/32: Document our abuse of x86_hw_tss: :ss1 and x86_hw_tss::sp1 tip-bot for Andy Lutomirski
2015-03-16 15:36     ` Andy Lutomirski
2015-03-17  8:45   ` tip-bot for Andy Lutomirski

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