Re: [PATCH 12/43] x86/espfix/64: Stop assuming that pt_regs is on the entry stack

[Borislav Petkov]

On Fri, Nov 24, 2017 at 06:23:40PM +0100, Ingo Molnar wrote:
> From: Andy Lutomirski <luto@xxxxxxxxxx>
> 
> When we start using an entry trampoline, a #GP from userspace will
> be delivered on the entry stack, not on the task stack.  Fix the
> espfix64 #DF fixup to set up #GP according to TSS.SP0, rather than
> assuming that pt_regs + 1 == SP0.  This won't change anything
> without an entry stack, but it will make the code continue to work
> when an entry stack is added.
> 
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Borislav Petkov <bpetkov@xxxxxxx>
> Cc: Brian Gerst <brgerst@xxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
> Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Link: https://lkml.kernel.org/r/b1ef4136616c6bd2a75d1fd2736d1d54437d65a8.1511497875.git.luto@xxxxxxxxxx
> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> ---
>  arch/x86/kernel/traps.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 2008dd0f8ccb..1bd43f044c62 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -359,7 +359,8 @@ dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
>  		regs->cs == __KERNEL_CS &&
>  		regs->ip == (unsigned long)native_irq_return_iret)
>  	{
> -		struct pt_regs *normal_regs = task_pt_regs(current);
> +		struct pt_regs *normal_regs =
> +			(struct pt_regs *)this_cpu_read(cpu_tss.x86_tss.sp0) - 1;

Just let that line stick out. Also, you can shorten it by renaming
normal_regs to something much shorter - it is a local variable and the
comment already explains everything you you can just as well have:

	 struct pt_regs *r = (struct pt_regs *)this_cpu_read(cpu_tss.x86_tss.sp0) - 1;

-- 
Regards/Gruss,
    Boris.

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