[PATCH v2 1/2] drm: adv7511: Fix use-after-free in adv7533_attach_dsi()

[Biju Das]

Hi Laurent,

Thanks for the feedback.

> -----Original Message-----
> From: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> Sent: 05 November 2024 16:02
> Subject: Re: [PATCH v2 1/2] drm: adv7511: Fix use-after-free in adv7533_attach_dsi()
> 
> Hi Biju,
> 
> Thank you for the patch.
> 
> On Tue, Nov 05, 2024 at 11:12:18AM +0000, Biju Das wrote:
> > The host_node pointer assigned and freed in adv7533_parse_dt() and
> > later adv7533_attach_dsi() uses the same. Fix this issue by freeing
> > the host_node in adv7533_attach_dsi() instead of adv7533_parse_dt().
> >
> > Fixes: 1e4d58cd7f88 ("drm/bridge: adv7533: Create a MIPI DSI device")
> > Cc: stable at vger.kernel.org
> > Signed-off-by: Biju Das <biju.das.jz at bp.renesas.com>
> > ---
> > Changes in v2:
> >  - Added the tag "Cc: stable at vger.kernel.org" in the sign-off area.
> >  - Dropped Archit Taneja invalid Mail address
> > ---
> >  drivers/gpu/drm/bridge/adv7511/adv7533.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/bridge/adv7511/adv7533.c
> > b/drivers/gpu/drm/bridge/adv7511/adv7533.c
> > index 4481489aaf5e..3e57ba838e5e 100644
> > --- a/drivers/gpu/drm/bridge/adv7511/adv7533.c
> > +++ b/drivers/gpu/drm/bridge/adv7511/adv7533.c
> > @@ -133,6 +133,7 @@ int adv7533_patch_cec_registers(struct adv7511
> > *adv)
> >
> >  int adv7533_attach_dsi(struct adv7511 *adv)  {
> > +	struct device_node *np __free(device_node) = adv->host_node;
> 
> This raises so many red flags. The function will free the node while the
> adv->host_node variable still points to it, opening the door to

I agree, adv->host_node still points to it and open again the door to use-after-free.

> use-after-free. Furthermore, there's nothing in the function name that hints it will do this, callers
> can get surprised by this behaviour.

By looking at [1], it can free the node pointer when it returns from the function. As you
said there is no hints from function name. __free(device_node) just tells to free the 
random device node assigned to the local variable.

[1]
https://elixir.bootlin.com/linux/v6.12-rc5/source/include/linux/of.h#L138


> 
> I'm sure you can do better than this and fix the problem with readable code, not cryptic and error-
> prone constructs :-)

Agreed. Will fix the problem with readable code.

Cheers,
Biju

> 
> >  	struct device *dev = &adv->i2c_main->dev;
> >  	struct mipi_dsi_host *host;
> >  	struct mipi_dsi_device *dsi;
> > @@ -181,8 +182,6 @@ int adv7533_parse_dt(struct device_node *np, struct adv7511 *adv)
> >  	if (!adv->host_node)
> >  		return -ENODEV;
> >
> > -	of_node_put(adv->host_node);
> > -
> >  	adv->use_timing_gen = !of_property_read_bool(np,
> >  						"adi,disable-timing-generator");
> >
> 
> --
> Regards,
> 
> Laurent Pinchart