linux-kernel - Re: [PATCH v6 48/76] x86/entry/64: Add entry code for #VC handler

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Date:   Mon, 31 Aug 2020 13:30:02 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Joerg Roedel <joro@...tes.org>
Cc:     x86@...nel.org, Joerg Roedel <jroedel@...e.de>, hpa@...or.com,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Slaby <jslaby@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Juergen Gross <jgross@...e.com>,
        Kees Cook <keescook@...omium.org>,
        David Rientjes <rientjes@...gle.com>,
        Cfir Cohen <cfir@...gle.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mike Stunes <mstunes@...are.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Martin Radev <martin.b.radev@...il.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v6 48/76] x86/entry/64: Add entry code for #VC handler

On Mon, Aug 24, 2020 at 10:54:43AM +0200, Joerg Roedel wrote:
> @@ -446,6 +448,82 @@ _ASM_NOKPROBE(\asmsym)
>  SYM_CODE_END(\asmsym)
>  .endm
>  

ifdeffery pls...

> +/**
> + * idtentry_vc - Macro to generate entry stub for #VC
> + * @vector:		Vector number
> + * @asmsym:		ASM symbol for the entry point
> + * @cfunc:		C function to be called
> + *
> + * The macro emits code to set up the kernel context for #VC. The #VC handler
> + * runs on an IST stack and needs to be able to cause nested #VC exceptions.
> + *
> + * To make this work the #VC entry code tries its best to pretend it doesn't use
> + * an IST stack by switching to the task stack if coming from user-space (which
> + * includes early SYSCALL entry path) or back to the stack in the IRET frame if
> + * entered from kernel-mode.
> + *
> + * If entered from kernel-mode the return stack is validated first, and if it is
> + * not safe to use (e.g. because it points to the entry stack) the #VC handler
> + * will switch to a fall-back stack (VC2) and call a special handler function.
> + *
> + * The macro is only used for one vector, but it is planned to extend it in the
								^^^^^^^^^^^

"... to be extended..."

...

> @@ -674,6 +675,56 @@ asmlinkage __visible noinstr struct pt_regs *sync_regs(struct pt_regs *eregs)
>  	return regs;
>  }
>  
> +#ifdef CONFIG_AMD_MEM_ENCRYPT
> +asmlinkage __visible noinstr struct pt_regs *vc_switch_off_ist(struct pt_regs *eregs)
> +{
> +	unsigned long sp, *stack;
> +	struct stack_info info;
> +	struct pt_regs *regs;

Let's call those "regs_ret" or so, so that the argument can be "regs" by
convention and for better differentiation.

> +	/*
> +	 * In the SYSCALL entry path the RSP value comes from user-space - don't
> +	 * trust it and switch to the current kernel stack
> +	 */
> +	if (eregs->ip >= (unsigned long)entry_SYSCALL_64 &&
> +	    eregs->ip <  (unsigned long)entry_SYSCALL_64_safe_stack) {
> +		sp = this_cpu_read(cpu_current_top_of_stack);
> +		goto sync;
> +	}
> +
> +	/*
> +	 * From here on the the RSP value is trusted - more RSP sanity checks
> +	 * need to happen above.
> +	 *
> +	 * Check whether entry happened from a safe stack.
> +	 */
> +	sp    = eregs->sp;
> +	stack = (unsigned long *)sp;
> +	get_stack_info_noinstr(stack, current, &info);
> +
> +	/*
> +	 * Don't sync to entry stack or other unknown stacks - use the fall-back
> +	 * stack instead.
> +	 */
> +	if (info.type == STACK_TYPE_UNKNOWN || info.type == STACK_TYPE_ENTRY ||

AFAICT, that STACK_TYPE_UNKNOWN gets set only by the plain
get_stack_info() function - not by the _noinstr() variant so you'd need
to check the return value of latter...

> +	    info.type >= STACK_TYPE_EXCEPTION_LAST)
> +		sp = __this_cpu_ist_top_va(VC2);
> +
> +sync:
> +	/*
> +	 * Found a safe stack - switch to it as if the entry didn't happen via
> +	 * IST stack. The code below only copies pt_regs, the real switch happens
> +	 * in assembly code.
> +	 */
> +	sp = ALIGN_DOWN(sp, 8) - sizeof(*regs);
> +
> +	regs = (struct pt_regs *)sp;
> +	*regs = *eregs;
> +
> +	return regs;
> +}
> +#endif
> +
>  struct bad_iret_stack {
>  	void *error_entry_ret;
>  	struct pt_regs regs;
> -- 

-- 
Regards/Gruss,
    Boris.

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