linux-kernel - Re: [BUG] Use of probe_kernel_address() in task_rcu_dereference() without checking return value

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Message-ID: <20190830152419.GB2634@redhat.com>
Date:   Fri, 30 Aug 2019 17:24:19 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Chris Metcalf <cmetcalf@...hip.com>,
        Christoph Lameter <cl@...ux.com>,
        Kirill Tkhai <tkhai@...dex.ru>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mike Galbraith <efault@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vladimir Davydov <vdavydov@...allels.com>,
        Kirill Tkhai <ktkhai@...allels.com>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [BUG] Use of probe_kernel_address() in task_rcu_dereference()
 without checking return value

On 08/30, Russell King - ARM Linux admin wrote:
>
> which means that when probe_kernel_address() returns -EFAULT, the
> destination is left uninitialised.  In the case of
> task_rcu_dereference(), this means that "siginfo" can be used without
> having been initialised,

Yes, but this is fine, please see the long comment below (case 2).

If probe_kernel_address() fails, "sighand" is not initialized. but this
doesn't differ from the case when we inspect the random value if this
task_struct was freed, then reallocated as another thing, then freed and
reallocated as task_struct again.

Oleg.