linux-kernel - Re: [PATCH] i2c: core: fix potential use-after-free on adapter removal

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Date:   Wed, 15 Jun 2022 20:01:41 +0200
From:   Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:     Wolfram Sang <wsa@...nel.org>, linux-i2c@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] i2c: core: fix potential use-after-free on adapter
 removal

On Tue, Jun 14, 2022 at 09:45:42PM +0200, Wolfram Sang wrote:
> Hi Michał,
> 
> I finally had a look at your patch...
> 
> > put_device(&adap->dev) might free the memory pointed to by `adap`,
> > so we shouldn't read adap->owner after that.
> > 
> > Fix by saving module pointer before calling put_device().
> 
> ... and found a different approach for this problem from 2019:
> 
> http://patchwork.ozlabs.org/project/linux-i2c/patch/1577439272-10362-1-git-send-email-vulab@iscas.ac.cn/
> 
> I think this is also proper. I found other subsystems in the kernel
> first putting the module, then the device. Do you see problems with the
> above patch?
> 
> Thanks for looking into the issue!

Hi!

I looked briefly at the kobject machinery and it seems to ignore module
dependencies. So while both approaches might work, I'd usually reverse
the order the init code is using: in this case module_get+device_get,
so on release: device_put+module_put. I don't know what keeps the kernel
from unloading the module after module_put() and before the function
returns, but I assume that would blow up for both patches.

Best Regards
Michał Mirosław

> > 
> > Signed-off-by: Michał Mirosław <mirq-linux@...e.qmqm.pl>
> > ---
> >  drivers/i2c/i2c-core-base.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/i2c/i2c-core-base.c b/drivers/i2c/i2c-core-base.c
> > index 2c59dd748a49..5d694f8ce9ef 100644
> > --- a/drivers/i2c/i2c-core-base.c
> > +++ b/drivers/i2c/i2c-core-base.c
> > @@ -2464,11 +2464,14 @@ EXPORT_SYMBOL(i2c_get_adapter);
> >  
> >  void i2c_put_adapter(struct i2c_adapter *adap)
> >  {
> > +	struct module *owner;
> > +
> >  	if (!adap)
> >  		return;
> >  
> > +	owner = adap->owner;
> >  	put_device(&adap->dev);
> > -	module_put(adap->owner);
> > +	module_put(owner);
> >  }
> >  EXPORT_SYMBOL(i2c_put_adapter);
> >  
> > -- 
> > 2.30.2
> > 



-- 
Michał Mirosław