linux-kernel - Re: [PATCH v5] x86/sgx: Fix use-after-free in sgx_mmu_notifier

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Date:   Wed, 3 Feb 2021 23:54:42 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     linux-sgx@...r.kernel.org, stable@...r.kernel.org,
        Sean Christopherson <seanjc@...gle.com>,
        Haitao Huang <haitao.huang@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Jethro Beekman <jethro@...tanix.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5] x86/sgx: Fix use-after-free in
 sgx_mmu_notifier_release()

On Wed, Feb 03, 2021 at 07:46:48AM -0800, Dave Hansen wrote:
> On 1/30/21 11:20 AM, Jarkko Sakkinen wrote:
> ...
> > Example scenario would such that all removals "side-channel" through
> > the notifier callback. Then mmu_notifier_unregister() gets called
> > exactly zero times. No MMU notifier srcu sync would be then happening.
> > 
> > NOTE: There's bunch of other examples, I'm just giving one.
> 
> Could you flesh this out a bit?  I don't quite understand the scenario
> from what you describe above.
> 
> In any case, I'm open to other implementations that fix the race we know
> about.  If you think you have a better fix, I'm happy to review it and
> make sure it closes the other race.

I'll bake up a new patch. Generally speaking, I think why this has been so
difficult, is because of a chicken-egg-problem. The whole issue should be
sorted when a new entry is first added to the mm_list, i.e. increase the
refcount for each added entry.

/Jarkko