linux-kernel - RE: [PATCH v3 2/4] x86/microcode: Fix __user annotations around generic_load

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Date:   Tue, 2 Apr 2019 10:26:30 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Borislav Petkov' <bp@...en8.de>, Jann Horn <jannh@...gle.com>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Qiaowei Ren <qiaowei.ren@...el.com>,
        Mukesh Ojha <mojha@...eaurora.org>
Subject: RE: [PATCH v3 2/4] x86/microcode: Fix __user annotations around
 generic_load_microcode()

From: Borislav Petkov
> Sent: 02 April 2019 11:02
> 
> On Mon, Apr 01, 2019 at 07:53:46PM +0200, Jann Horn wrote:
> > Hm. request_microcode_fw() gets that buffer from
> > request_firmware_direct(), which does this:
> >
> >         __module_get(THIS_MODULE);
> >         ret = _request_firmware(firmware_p, name, device, NULL, 0,
> >                                 FW_OPT_UEVENT | FW_OPT_NO_WARN |
> >                                 FW_OPT_NOFALLBACK);
> >         module_put(THIS_MODULE);
> >         return ret;
> >
> > What is that module_get()/module_put() supposed to be good for? Are we
> > expecting that caller to do something ridiculous like calling
> > module_put() on us? This doesn't seem to make any sense.
> 
> Yah, the microcode thing used to be a module. Not anymore.

That probably makes diddly-squit difference.
There has to be a 'hold' on THIS_MODULE in order to be executing
code from it.
You only (usually) need an extra reference for a kernel thread,
and that needs to exit with module_put_and_exit().

	David

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