linux-kernel - Re: [RFC 5/7] x86/asm: Rearrange struct cpu_tss to enlarge SYSENTER

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Date:   Mon, 13 Nov 2017 11:19:25 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>
Cc:     Borislav Petkov <bpetkov@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC 5/7] x86/asm: Rearrange struct cpu_tss to enlarge
 SYSENTER_stack and fix alignment

On 11/10/2017 08:05 PM, Andy Lutomirski wrote:
>  struct tss_struct {
>  	/*
> +	 * Space for the temporary SYSENTER stack.  Used for the entry
> +	 * trampoline as well.  Size it such that tss_struct ends up
> +	 * as a multiple of PAGE_SIZE.  This calculation assumes that
> +	 * io_bitmap is a multiple of PAGE_SIZE (8192 bytes) plus one
> +	 * long.
> +	 */
> +	unsigned long		SYSENTER_stack_canary;
> +	unsigned long		SYSENTER_stack[(PAGE_SIZE - sizeof(struct x86_hw_tss)) / sizeof(unsigned long) - 2];
> +
> +	/*
>  	 * The hardware state:
>  	 */
>  	struct x86_hw_tss	x86_tss;
> @@ -337,15 +347,9 @@ struct tss_struct {
>  	 * be within the limit.
>  	 */
>  	unsigned long		io_bitmap[IO_BITMAP_LONGS + 1];
> -
> -	/*
> -	 * Space for the temporary SYSENTER stack.
> -	 */
> -	unsigned long		SYSENTER_stack_canary;
> -	unsigned long		SYSENTER_stack[64];
>  } ____cacheline_aligned;


If io_bitmap[] is already page-size-aligned, how does it help us to move
SYSENTER_stack[]?

It seems like it would be easier to just leave SYSENTER_stack[] where it
is, make it SYSENTER_stack[0], and just find somewhere else to choose
how much to bloat the tss_struct *allocation* instead of trying to make
sure that sizeof(tss_struct) matches the allocation.

That SYSENTER_stack[] size calculation is pretty hideous. :)