linux-kernel - Re: [PATCH] arm64/fpsimd: Exit streaming mode when flushing tasks

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <ZIH4/kLTkvIN34FL@FVFF77S0Q05N>
Date:   Thu, 8 Jun 2023 16:51:26 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Mark Brown <broonie@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Anders Roxell <anders.roxell@...aro.org>,
        Naresh Kamboju <naresh.kamboju@...aro.org>
Subject: Re: [PATCH] arm64/fpsimd: Exit streaming mode when flushing tasks

On Wed, Jun 07, 2023 at 09:30:51PM +0100, Mark Brown wrote:
> Ensure there is no path where we might attempt to save SME state after we
> flush a task by updating the SVCR register state as well as updating our
> in memory state. I haven't seen a specific case where this is happening or
> seen a path where it might happen but for the cost of a single low overhead
> instruction it seems sensible to close the potential gap.
> 
> Signed-off-by: Mark Brown <broonie@...nel.org>
> ---
>  arch/arm64/kernel/fpsimd.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> index 2fbafa5cc7ac..1627e0efe39a 100644
> --- a/arch/arm64/kernel/fpsimd.c
> +++ b/arch/arm64/kernel/fpsimd.c
> @@ -1649,6 +1649,7 @@ void fpsimd_flush_thread(void)
>  
>  		fpsimd_flush_thread_vl(ARM64_VEC_SME);
>  		current->thread.svcr = 0;
> +		sme_smstop_sm();

I don't think we should blindly do this if we never expect to get here in that
state; this is just going to mask bugs and make them harder to debug going
forwards.

If we need this, it'd be better to have something like:

	if (WARN_ON_ONCE(sme_is_in_streaming_mode()))
		sme_smstop_sm();

... so that we can identify this case and fix it.

Thanks,
Mark.