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

From: Masami Hiramatsu <mhiramat@kernel.org>
To: Nadav Amit <namit@vmware.com>
Cc: Rick Edgecombe <rick.p.edgecombe@intel.com>,
	Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
	the arch/x86 maintainers <x86@kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	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 <ard.biesheuvel@linaro.org>,
	"kristen@linux.intel.com" <kristen@linux.intel.com>,
	"deneen.t.dock@intel.com" <deneen.t.dock@intel.com>
Subject: Re: [PATCH 17/17] module: Prevent module removal racing with text_poke()
Date: Fri, 18 Jan 2019 17:23:34 +0900	[thread overview]
Message-ID: <20190118172334.d7b1bcd580c3f6c4ed388160@kernel.org> (raw)
In-Reply-To: <B48C6E93-AD57-4FF8-BBE8-887A5E965793@vmware.com>

On Thu, 17 Jan 2019 18:07:03 +0000
Nadav Amit <namit@vmware.com> wrote:

> > On Jan 16, 2019, at 11:54 PM, Masami Hiramatsu <mhiramat@kernel.org> 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?
> 
> 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 agreed, but in that case, this is just a fool proof. I think we should
prevent this kind of bug by review, and should comment it on text_poke(),
instead of locking text_mutex.

What I thought was even if we take text_mutex here, such user can modify
the (released) module code right after we exit this section.

Maybe we'd better make text_poke() more smart?

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

I think we don't need to care about kgdb. It is a tool which should be able
to shoot your feet and we can not prevent it. Only expert can avoid it. :)

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>