Re: [PATCH] USB: chipidea: msm: fix ulpi-node lookup

[Peter Chen]

On Tue, Dec 12, 2017 at 01:59:29PM +0100, Johan Hovold wrote:
> On Tue, Dec 12, 2017 at 11:08:17AM +0800, Peter Chen wrote:
> > On Mon, Nov 13, 2017 at 11:12:58AM +0100, Johan Hovold wrote:
> > > Fix child-node lookup during probe, which ended up searching the whole
> > > device tree depth-first starting at the parent rather than just matching
> > > on its children.
> > > 
> > > Note that the original premature free of the parent node has already
> > > been fixed separately, but that fix was apparently never backported to
> > > stable.
> > > 
> > > Fixes: 47654a162081 ("usb: chipidea: msm: Restore wrapper settings after reset")
> > > Fixes: b74c43156c0c ("usb: chipidea: msm: ci_hdrc_msm_probe() missing of_node_get()")
> > > Cc: stable <stable@xxxxxxxxxxxxxxx>     # 4.10: b74c43156c0c
> > > Cc: Stephen Boyd <stephen.boyd@xxxxxxxxxx>
> > > Cc: Frank Rowand <frank.rowand@xxxxxxxx>
> > > Signed-off-by: Johan Hovold <johan@xxxxxxxxxx>
> > > ---
> > >  drivers/usb/chipidea/ci_hdrc_msm.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/usb/chipidea/ci_hdrc_msm.c b/drivers/usb/chipidea/ci_hdrc_msm.c
> > > index 3593ce0ec641..880009987460 100644
> > > --- a/drivers/usb/chipidea/ci_hdrc_msm.c
> > > +++ b/drivers/usb/chipidea/ci_hdrc_msm.c
> > > @@ -247,7 +247,7 @@ static int ci_hdrc_msm_probe(struct platform_device *pdev)
> > >  	if (ret)
> > >  		goto err_mux;
> > >  
> > > -	ulpi_node = of_find_node_by_name(of_node_get(pdev->dev.of_node), "ulpi");
> > > +	ulpi_node = of_get_child_by_name(pdev->dev.of_node, "ulpi");
> > 
> > Stephen, would you comment on it? I am afraid I can't find the benefit
> > for this change.
> 
> The general problem is that several drivers were using the wrong
> OF-helper when looking up child nodes. This meant that instead of just
> matching on child nodes, a tree-wide, depth-first search was done,
> something which could end up matching an unrelated node elsewhere in the
> device tree.
> 
> To make things worse, most erroneous users of of_find_node_by_name(),
> also failed to notice that that function drops a reference to its first
> argument, something which can lead to use-after-free and crashes, for
> example, after probe deferrals.
> 
> In this case, it looks like the child node is looked-up to determine
> whether to enable a hardware quirk. The potential use-after-free has
> already been fixed up (by adding the missing of_node_get()), but that
> fix was never backported to stable.
> 
> This patch addresses both issues (tree-wide search + unbalanced put in
> stable), while removing buggy code which could otherwise end up being
> reproduced in yet another driver.
> 
> Johan

Thanks, if Stephen does not have comments, I will send Greg it next
Monday.

-- 

Best Regards,
Peter Chen