Re: [RFC PATCH v2 1/3] x86: cpu/bugs: update SpectreRSB comments for AMD

[Pawan Gupta]

On Mon, Nov 11, 2024 at 05:46:44PM -0800, Josh Poimboeuf wrote:
> +	 * 1) RSB underflow ("Intel Retbleed")
>  	 *
>  	 *    Some Intel parts have "bottomless RSB".  When the RSB is empty,
>  	 *    speculated return targets may come from the branch predictor,
>  	 *    which could have a user-poisoned BTB or BHB entry.
>  	 *
> -	 *    AMD has it even worse: *all* returns are speculated from the BTB,
> -	 *    regardless of the state of the RSB.
> +	 *    When IBRS or eIBRS is enabled, the "user -> kernel" attack is
> +	 *    mitigated by the IBRS branch prediction isolation properties, so
> +	 *    the RSB buffer filling wouldn't be necessary to protect against
> +	 *    this type of attack.
>  	 *
> -	 *    When IBRS or eIBRS is enabled, the "user -> kernel" attack
> -	 *    scenario is mitigated by the IBRS branch prediction isolation
> -	 *    properties, so the RSB buffer filling wouldn't be necessary to
> -	 *    protect against this type of attack.
> +	 *    The "user -> user" attack is mitigated by RSB filling on context
> +	 *    switch.

user->user SpectreRSB is also mitigated by IBPB, so RSB filling is
unnecessary when IBPB is issued. Also, when an appication does not opted-in
for IBPB at context switch, spectre-v2 for that app is not mitigated,
filling RSB is only a half measure in that case.

Is RSB filling really serving any purpose for userspace?