linux-kernel - Re: [PATCH 17/17] module: Prevent module removal racing with text

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Date:   Thu, 17 Jan 2019 18:07:03 +0000
From:   Nadav Amit <namit@...are.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
CC:     Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Damian Tometzki <linux_dti@...oud.com>,
        linux-integrity <linux-integrity@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux-MM <linux-mm@...ck.org>, Will Deacon <will.deacon@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        "kristen@...ux.intel.com" <kristen@...ux.intel.com>,
        "deneen.t.dock@...el.com" <deneen.t.dock@...el.com>
Subject: Re: [PATCH 17/17] module: Prevent module removal racing with
 text_poke()

> On Jan 16, 2019, at 11:54 PM, Masami Hiramatsu <mhiramat@...nel.org> wrote:
> 
> On Wed, 16 Jan 2019 16:32:59 -0800
> Rick Edgecombe <rick.p.edgecombe@...el.com> wrote:
> 
>> From: Nadav Amit <namit@...are.com>
>> 
>> It seems dangerous to allow code modifications to take place
>> concurrently with module unloading. So take the text_mutex while the
>> memory of the module is freed.
> 
> At that point, since the module itself is removed from module list,
> it seems no actual harm. Or would you have any concern?

So it appears that you are right and all the users of text_poke() and
text_poke_bp() do install module notifiers, and remove the module from their
internal data structure when they are done (*). As long as they prevent
text_poke*() to be called concurrently (e.g., using jump_label_lock()),
everything is fine.

Having said that, the question is whether you “trust” text_poke*() users to
do so. text_poke() description does not day explicitly that you need to
prevent modules from being removed.

What do you say?


(*) I am not sure about kgdb, but it probably does not matter much