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Date:   Wed, 28 Feb 2018 10:32:39 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     Seunghun Han <kkamagui@gmail.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Tony Luck <tony.luck@intel.com>, linux-edac@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] x86: mce: fix kernel panic when check_interval is changed
Message-ID: <20180228093238.GA3769@pd.tnic>
References: <20180223101350.8344-1-kkamagui@gmail.com>
 <20180223105220.GA12058@kroah.com>
 <CAHjaAcTVUxm6LnD_3sp2BwGgR9ustuctoHUBFkMD1nZh4o6bTA@mail.gmail.com>
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On Mon, Feb 26, 2018 at 05:05:04AM +0900, Seunghun Han wrote:
> >> It is a critical security problem because the attacker can make kernel panic
> >> by writing a value to the check_interval file in userspace, and it can be
> >> used for Denial-of-Service (DoS) attack.
> >
> > As only root can write to that file, it's not that critical of an issue,
> > but yes, this is a problem.  Nice find and fix.

This is still the wrong fix. You need to:

1. check the old value of check_interval in store_int_with_restart() and
exit early if it is the same.

2. have mce_restart() grab a newly defined mutex, say, mce_sysfs_mutex
or so, which synchronizes all CPUs so that their timers get deleted and
reinitialized in the proper order.

Thx.

-- 
Regards/Gruss,
    Boris.

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