linux-kernel - Re: [PATCH v6 3/8] firmware_loader: Move module refcounts to allow unloading

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Message-ID: <20241115172843.jqdhggfgotl2vtuz@4VRSMR2-DT.corp.robot.car>
Date: Fri, 15 Nov 2024 09:28:43 -0800
From: Russ Weight <russ.weight@...ux.dev>
To: Tom Lendacky <thomas.lendacky@....com>
Cc: Dan Williams <dan.j.williams@...el.com>,
	Dionna Glaze <dionnaglaze@...gle.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, Luis Chamberlain <mcgrof@...nel.org>,
	Danilo Krummrich <dakr@...hat.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Tianfei zhang <tianfei.zhang@...el.com>, linux-coco@...ts.linux.dev,
	Sean Christopherson <seanjc@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Ashish Kalra <ashish.kalra@....com>,
	John Allen <john.allen@....com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S. Miller" <davem@...emloft.net>,
	Michael Roth <michael.roth@....com>,
	Alexey Kardashevskiy <aik@....com>,
	Russ Weight <russell.h.weight@...el.com>
Subject: Re: [PATCH v6 3/8] firmware_loader: Move module refcounts to allow
 unloading

On Thu, Nov 14, 2024 at 01:30:16PM -0600, Tom Lendacky wrote:
> On 11/14/24 12:17, Dan Williams wrote:
> > Russ Weight wrote:
> > [..]
> >> Clearly this would be an unexpected/unusual case. Someone with root
> >> access would have to remove the device driver. I'm not sure how much
> >> effort should be expended in preventing it - but this is the reasoning
> >> behind the incrementing/decrementing of the module reference counts.
> > 
> > The module reference needs to be held only if the producer of those
> > symbols can be removed without triggering some coordinated removal with
> > action consumer. A driver that fails to call
> > firmware_upload_unregister() in its module removal path is simply a driver
> > with a memory-leak and use-after-free bug, not something the firmware
> > upload core needs to worry about.
> > 
> > So, the prevention mechanism is "thou shalt use
> > firmware_upload_unregister() correctly", and when that is in place
> > explicit module references are not only redundant, but trying to
> > implement them causes circular dependency loops.

Thanks for the explanation. Makes total sense to me. I agree that the
module reference counts can/should be removed.

- Russ
> 
> I believe that is how other similar services, like debugfs, work, the
> module is responsible for cleaning up.
> 
> Thanks,
> Tom