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SubjectRe: [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@rere.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

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