linux-kernel - Re: [RFC 11/14] x86: add support for Dynamic Kernel Stacks

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Date: Tue, 12 Mar 2024 14:58:00 -0700
From: Andi Kleen <ak@...ux.intel.com>
To: Pasha Tatashin <pasha.tatashin@...een.com>
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Subject: Re: [RFC 11/14] x86: add support for Dynamic Kernel Stacks

Pasha Tatashin <pasha.tatashin@...een.com> writes:

> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index d6375b3c633b..651c558b10eb 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1198,6 +1198,9 @@ do_kern_addr_fault(struct pt_regs *regs, unsigned long hw_error_code,
>  	if (is_f00f_bug(regs, hw_error_code, address))
>  		return;
>  
> +	if (dynamic_stack_fault(current, address))
> +		return;

Probably I'm missing something here, but since you are on the same stack as
you are trying to fix up, how can this possibly work?

Fault on missing stack
#PF
<will push more stuff onto the missing stack causing a double fault>

You would need a separate stack just to handle this. But the normal
page fault handler couldn't use it because it needs to be able to block.

Ah I get it -- you handle it in the double fault handler?  So every
stack grow will be a #DF too? That's scary. 


-Andi