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

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Date:   Thu, 17 Jan 2019 15:58:31 -0800
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Nadav Amit <nadav.amit@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>, linux_dti@...oud.com,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, akpm@...ux-foundation.org,
        kernel-hardening@...ts.openwall.com, linux-mm@...ck.org,
        will.deacon@....com, ard.biesheuvel@...aro.org,
        kristen@...ux.intel.com, deneen.t.dock@...el.com,
        Nadav Amit <namit@...are.com>
Subject: Re: [PATCH 17/17] module: Prevent module removal racing with
 text_poke()

On 1/16/19 11:54 PM, Masami Hiramatsu 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?
> 

The issue isn't the module list, but rather when it is safe to free the
contents, so we don't clobber anything. We absolutely need to enforce
that we can't text_poke() something that might have already been freed.

That being said, we *also* really would prefer to enforce that we can't
text_poke() memory that doesn't actually contain code; as far as I can
tell we don't currently do that check.

This, again, is a good use for a separate mm context. We can enforce
that that context will only ever contain valid page mappings for actual
code pages.

(Note: in my proposed algorithm, with a separate mm, replace INVLPG with
switching CR3 if we have to do a rollback or roll forward in the
breakpoint handler.)

	-hpa