linux-kernel - Re: [PATCH] x86: also disable FSRM if ERMS is disabled

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Message-ID: <Yy2U2BW6Tx0imGpK@zn.tnic>
Date:   Fri, 23 Sep 2022 13:13:28 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Daniel Verkamp <dverkamp@...omium.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Tony Luck <tony.luck@...el.com>
Subject: Re: [PATCH] x86: also disable FSRM if ERMS is disabled

On Thu, Sep 22, 2022 at 05:58:27PM -0700, Daniel Verkamp wrote:
> In the "Fast Short REP MOVSB" path of memmove, if we take the path where
> the FSRM flag is enabled but the ERMS flag is not, there is no longer a
> check for length >= 0x20 (both alternatives will be replaced with NOPs).
> If a memmove() requiring a forward copy of less than 0x20 bytes happens
> in this case, the `sub $0x20, %rdx` will cause the length to roll around
> to a huge value and the copy will eventually hit a page fault.
> 
> This is not intended to happen, as the comment above the alternatives
> mentions "FSRM implies ERMS".
> 
> However, there is a check in early_init_intel() that can disable ERMS,
> so we should also be disabling FSRM in this path to maintain correctness
> of the memmove() optimization.

Is this something you hit in a real-world scenario? If so, how exactly?

Thx.

-- 
Regards/Gruss,
    Boris.

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