Re: [PATCH] x86/bhi: BHI mitigation can trigger warning in #DB handler

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 23/05/2024 1:33 pm, Alexandre Chartre wrote:
> diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
> index 11c9b8efdc4c..7fa04edc87e9 100644
> --- a/arch/x86/entry/entry_64_compat.S
> +++ b/arch/x86/entry/entry_64_compat.S
> @@ -91,7 +91,6 @@ SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)
>  
>  	IBRS_ENTER
>  	UNTRAIN_RET
> -	CLEAR_BRANCH_HISTORY
>  
>  	/*
>  	 * SYSENTER doesn't filter flags, so we need to clear NT and AC
> @@ -116,6 +115,12 @@ SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)
>  	jnz	.Lsysenter_fix_flags
>  .Lsysenter_flags_fixed:
>  
> +	/*
> +	 * CLEAR_BRANCH_HISTORY can call other functions. It should be invoked
> +	 * after making sure TF is cleared because single-step is ignored only
> +	 * for instructions inside the entry_SYSENTER_compat function.
> +	 */
> +	CLEAR_BRANCH_HISTORY

Exactly the same is true of UNTRAIN_RET, although it will only manifest
in i386 builds running on AMD hardware (SYSENTER is #UD on AMD hardware
in Long mode.)

#DB is IST so does handle it's own speculation safety.  It should be
safe to move all the speculation safety logic in the sysenter handler to
after .Lsysenter_flags_fixed:, I think?

~Andrew