linux-hardening - Re: [PATCH] Revert "ACPI, APEI, use raw spinlock in ERST"

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Date:   Tue, 22 Aug 2023 17:45:22 -0600
From:   Uday Shankar <ushankar@...estorage.com>
To:     Jörn Engel <joern@...estorage.com>
Cc:     "Huang, Ying" <ying.huang@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Tony Luck <tony.luck@...el.com>,
        "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>, James Morse <james.morse@....com>,
        Borislav Petkov <bp@...en8.de>,
        Len Brown <len.brown@...el.com>,
        linux-hardening@...r.kernel.org, linux-acpi@...r.kernel.org,
        Qiuxu Zhuo <qiuxu.zhuo@...el.com>
Subject: Re: [PATCH] Revert "ACPI, APEI, use raw spinlock in ERST"

On Mon, Aug 21, 2023 at 08:16:05PM -0700, Jörn Engel wrote:
> On Tue, Aug 22, 2023 at 09:56:38AM +0800, Huang, Ying wrote:
> > 
> > ERST is mainly used to log the hardware error.  While, hardware error
> > may be reported via NMI (e.g., ACPI APEI GHES NMI), so we need to call
> > ERST functions in NMI handlers.  Where normal spinlock cannot be used
> > because they will be converted to sleepable rt_mutex in RT kernel.
> 
> Non-sleeping spinlocks cannot be used in NMI context either.
> raw_spin_lock_irqsave() will prevent regular interrupts, but not NMI.
> So taking a spinlock inside an NMI can trigger a deadlock.
> 
> Am I missing something here?
> 
> Jörn
> 
> --
> All art is but imitation of nature.
> -- Lucius Annaeus Seneca

Also want to point out that both before and after this commit, we only
use trylock from erst_write, which looks like the only function touched
by this patch which can be called from NMI context. Trylock should be
safe in NMI context regardless of the type of synchronization used
(raw_spinlock, spinlock, or otherwise).