Re: [PATCH 17/17] module: Prevent module removal racing with text_poke()

From: Nadav Amit <nadav.amit@gmail.com>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>,
	Rick Edgecombe <rick.p.edgecombe@intel.com>,
	Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Damian Tometzki <linux_dti@icloud.com>,
	linux-integrity <linux-integrity@vger.kernel.org>,
	LSM List <linux-security-module@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>,
	Linux-MM <linux-mm@kvack.org>, Will Deacon <will.deacon@arm.com>,
	ard.biesheuvel@linaro.org, kristen@linux.intel.com,
	deneen.t.dock@intel.com
Subject: Re: [PATCH 17/17] module: Prevent module removal racing with text_poke()
Date: Thu, 17 Jan 2019 17:15:27 -0800	[thread overview]
Message-ID: <69EA2C81-826F-46BA-8D80-241C39B0B70B@gmail.com> (raw)
In-Reply-To: <e865e051-be79-adce-7275-72abf2173bdb@zytor.com>

> On Jan 17, 2019, at 3:58 PM, H. Peter Anvin <hpa@zytor.com> wrote:
> 
> On 1/16/19 11:54 PM, Masami Hiramatsu wrote:
>> On Wed, 16 Jan 2019 16:32:59 -0800
>> Rick Edgecombe <rick.p.edgecombe@intel.com> wrote:
>> 
>>> From: Nadav Amit <namit@vmware.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.

Yes, that what the mutex was supposed to achieve. It’s not supposed just
to check whether it is a code page, but also that it is the same code
page that you wanted to patch. 

> 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.

This will not tell you that you have the *right* code-page. The module
notifiers help to do so, since they synchronize the text poking with
the module removal.

> (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.)

I really need to read your patches more carefully to see what you mean.

Anyhow, so what do you prefer? I’m ok with either one:
	1. Keep this patch
	2. Remove this patch and change into a comment on text_poke()
	3. Just drop the patch