Re: [PATCH 2/3] x86/sev-es: Check if regs->sp is trusted before adjusting #VC IST stack

[Borislav Petkov]

On Wed, Feb 17, 2021 at 01:01:42PM +0100, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@xxxxxxx>
> 
> The code in the NMI handler to adjust the #VC handler IST stack is
> needed in case an NMI hits when the #VC handler is still using its IST
> stack.
> But the check for this condition also needs to look if the regs->sp
> value is trusted, meaning it was not set by user-space. Extend the
> check to not use regs->sp when the NMI interrupted user-space code or
> the SYSCALL gap.
> 
> Reported-by: Andy Lutomirski <luto@xxxxxxxxxx>
> Fixes: 315562c9af3d5 ("x86/sev-es: Adjust #VC IST Stack on entering NMI handler")
> Cc: stable@xxxxxxxxxxxxxxx # 5.10+
> Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
> ---
>  arch/x86/kernel/sev-es.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/sev-es.c b/arch/x86/kernel/sev-es.c
> index 84c1821819af..0df38b185d53 100644
> --- a/arch/x86/kernel/sev-es.c
> +++ b/arch/x86/kernel/sev-es.c
> @@ -144,7 +144,9 @@ void noinstr __sev_es_ist_enter(struct pt_regs *regs)
>  	old_ist = __this_cpu_read(cpu_tss_rw.x86_tss.ist[IST_INDEX_VC]);
>  
>  	/* Make room on the IST stack */
> -	if (on_vc_stack(regs->sp))
> +	if (on_vc_stack(regs->sp) &&
> +	    !user_mode(regs) &&
> +	    !from_syscall_gap(regs))

Why not add those checks to on_vc_stack() directly? Because in it, you
can say:

on_vc_stack():

	/* user mode rSP is not trusted */
	if (user_mode())
		return false;

	/* ditto */
	if (ip_within_syscall_gap())
		return false;

	...

?

-- 
Regards/Gruss,
    Boris.

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