[PATCH net-next] net: ethtool: Fix suspicious rcu_dereference usage

[PATCH net-next] net: ethtool: Fix suspicious rcu_dereference usage

[Kory Maincent]

The __ethtool_get_ts_info function can be called with or without the
rtnl lock held. When the rtnl lock is not held, using rtnl_dereference()
triggers a warning due to improper lock context.

Replace rtnl_dereference() with rcu_dereference_rtnl() to safely
dereference the RCU pointer in both scenarios, ensuring proper handling
regardless of the rtnl lock state.

Reported-by: syzbot+a344326c05c98ba19682@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/676147f8.050a0220.37aaf.0154.GAE@google.com/
Fixes: b9e3f7dc9ed9 ("net: ethtool: tsinfo: Enhance tsinfo to support several hwtstamp by net topology")
Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
---
 net/ethtool/common.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ethtool/common.c b/net/ethtool/common.c
index 02f941f667dd..ec6f2e2caaf9 100644
--- a/net/ethtool/common.c
+++ b/net/ethtool/common.c
@@ -870,7 +870,7 @@ int __ethtool_get_ts_info(struct net_device *dev,
 {
 	struct hwtstamp_provider *hwprov;
 
-	hwprov = rtnl_dereference(dev->hwprov);
+	hwprov = rcu_dereference_rtnl(dev->hwprov);
 	/* No provider specified, use default behavior */
 	if (!hwprov) {
 		const struct ethtool_ops *ops = dev->ethtool_ops;
-- 
2.34.1

Re: [PATCH net-next] net: ethtool: Fix suspicious rcu_dereference usage

[Eric Dumazet]

On Tue, Dec 17, 2024 at 3:03 PM Kory Maincent <kory.maincent@bootlin.com> wrote:
>
> The __ethtool_get_ts_info function can be called with or without the
> rtnl lock held. When the rtnl lock is not held, using rtnl_dereference()
> triggers a warning due to improper lock context.
>
> Replace rtnl_dereference() with rcu_dereference_rtnl() to safely
> dereference the RCU pointer in both scenarios, ensuring proper handling
> regardless of the rtnl lock state.
>
> Reported-by: syzbot+a344326c05c98ba19682@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/netdev/676147f8.050a0220.37aaf.0154.GAE@google.com/
> Fixes: b9e3f7dc9ed9 ("net: ethtool: tsinfo: Enhance tsinfo to support several hwtstamp by net topology")
> Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
> ---
>  net/ethtool/common.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/ethtool/common.c b/net/ethtool/common.c
> index 02f941f667dd..ec6f2e2caaf9 100644
> --- a/net/ethtool/common.c
> +++ b/net/ethtool/common.c
> @@ -870,7 +870,7 @@ int __ethtool_get_ts_info(struct net_device *dev,
>  {
>         struct hwtstamp_provider *hwprov;
>
> -       hwprov = rtnl_dereference(dev->hwprov);
> +       hwprov = rcu_dereference_rtnl(dev->hwprov);
>         /* No provider specified, use default behavior */
>         if (!hwprov) {
>                 const struct ethtool_ops *ops = dev->ethtool_ops;

I have to ask : Can you tell how this patch has been tested ?

rcu is not held according to syzbot report.

If rtnl and rcu are not held, lockdep will still complain.

Re: [PATCH net-next] net: ethtool: Fix suspicious rcu_dereference usage

[Kory Maincent]

On Tue, 17 Dec 2024 16:47:07 +0100
Eric Dumazet <edumazet@google.com> wrote:

> On Tue, Dec 17, 2024 at 3:03 PM Kory Maincent <kory.maincent@bootlin.com>
> wrote:
> >
> > The __ethtool_get_ts_info function can be called with or without the
> > rtnl lock held. When the rtnl lock is not held, using rtnl_dereference()
> > triggers a warning due to improper lock context.
> >
> > Replace rtnl_dereference() with rcu_dereference_rtnl() to safely
> > dereference the RCU pointer in both scenarios, ensuring proper handling
> > regardless of the rtnl lock state.
> >
> > Reported-by: syzbot+a344326c05c98ba19682@syzkaller.appspotmail.com
> > Closes:
> > https://lore.kernel.org/netdev/676147f8.050a0220.37aaf.0154.GAE@google.com/
> > Fixes: b9e3f7dc9ed9 ("net: ethtool: tsinfo: Enhance tsinfo to support
> > several hwtstamp by net topology") Signed-off-by: Kory Maincent
> > <kory.maincent@bootlin.com> --- net/ethtool/common.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/net/ethtool/common.c b/net/ethtool/common.c
> > index 02f941f667dd..ec6f2e2caaf9 100644
> > --- a/net/ethtool/common.c
> > +++ b/net/ethtool/common.c
> > @@ -870,7 +870,7 @@ int __ethtool_get_ts_info(struct net_device *dev,
> >  {
> >         struct hwtstamp_provider *hwprov;
> >
> > -       hwprov = rtnl_dereference(dev->hwprov);
> > +       hwprov = rcu_dereference_rtnl(dev->hwprov);
> >         /* No provider specified, use default behavior */
> >         if (!hwprov) {
> >                 const struct ethtool_ops *ops = dev->ethtool_ops;  
> 
> I have to ask : Can you tell how this patch has been tested ?

Oh, it was not at all sufficiently tested. Sorry!
I thought I had spotted the issue but I hadn't.

> rcu is not held according to syzbot report.
> 
> If rtnl and rcu are not held, lockdep will still complain.

You are totally right.
I may be able to see it with the timestamping kselftest. I will try.

Regards,
-- 
Köry Maincent, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com

Re: [PATCH net-next] net: ethtool: Fix suspicious rcu_dereference usage

[Eric Dumazet]

On Tue, Dec 17, 2024 at 6:17 PM Kory Maincent <kory.maincent@bootlin.com> wrote:
>
> On Tue, 17 Dec 2024 16:47:07 +0100
> Eric Dumazet <edumazet@google.com> wrote:
>
> > On Tue, Dec 17, 2024 at 3:03 PM Kory Maincent <kory.maincent@bootlin.com>
> > wrote:
> > >
> > > The __ethtool_get_ts_info function can be called with or without the
> > > rtnl lock held. When the rtnl lock is not held, using rtnl_dereference()
> > > triggers a warning due to improper lock context.
> > >
> > > Replace rtnl_dereference() with rcu_dereference_rtnl() to safely
> > > dereference the RCU pointer in both scenarios, ensuring proper handling
> > > regardless of the rtnl lock state.
> > >
> > > Reported-by: syzbot+a344326c05c98ba19682@syzkaller.appspotmail.com
> > > Closes:
> > > https://lore.kernel.org/netdev/676147f8.050a0220.37aaf.0154.GAE@google.com/
> > > Fixes: b9e3f7dc9ed9 ("net: ethtool: tsinfo: Enhance tsinfo to support
> > > several hwtstamp by net topology") Signed-off-by: Kory Maincent
> > > <kory.maincent@bootlin.com> --- net/ethtool/common.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/net/ethtool/common.c b/net/ethtool/common.c
> > > index 02f941f667dd..ec6f2e2caaf9 100644
> > > --- a/net/ethtool/common.c
> > > +++ b/net/ethtool/common.c
> > > @@ -870,7 +870,7 @@ int __ethtool_get_ts_info(struct net_device *dev,
> > >  {
> > >         struct hwtstamp_provider *hwprov;
> > >
> > > -       hwprov = rtnl_dereference(dev->hwprov);
> > > +       hwprov = rcu_dereference_rtnl(dev->hwprov);
> > >         /* No provider specified, use default behavior */
> > >         if (!hwprov) {
> > >                 const struct ethtool_ops *ops = dev->ethtool_ops;
> >
> > I have to ask : Can you tell how this patch has been tested ?
>
> Oh, it was not at all sufficiently tested. Sorry!
> I thought I had spotted the issue but I hadn't.
>
> > rcu is not held according to syzbot report.
> >
> > If rtnl and rcu are not held, lockdep will still complain.
>
> You are totally right.
> I may be able to see it with the timestamping kselftest. I will try.

syzbot has a repro that you can compile and run.

Make sure to build and use a kernel with

CONFIG_PROVE_LOCKING=y

Re: [PATCH net-next] net: ethtool: Fix suspicious rcu_dereference usage

[Kory Maincent]

On Tue, 17 Dec 2024 18:28:16 +0100
Eric Dumazet <edumazet@google.com> wrote:

> On Tue, Dec 17, 2024 at 6:17 PM Kory Maincent <kory.maincent@bootlin.com>
> wrote:
> >
> > On Tue, 17 Dec 2024 16:47:07 +0100
> > Eric Dumazet <edumazet@google.com> wrote:
> >  
> > > On Tue, Dec 17, 2024 at 3:03 PM Kory Maincent <kory.maincent@bootlin.com>
> > > wrote:  
>  [...]  
> > >
> > > I have to ask : Can you tell how this patch has been tested ?  
> >
> > Oh, it was not at all sufficiently tested. Sorry!
> > I thought I had spotted the issue but I hadn't.
> >  
> > > rcu is not held according to syzbot report.
> > >
> > > If rtnl and rcu are not held, lockdep will still complain.  
> >
> > You are totally right.
> > I may be able to see it with the timestamping kselftest. I will try.  
> 
> syzbot has a repro that you can compile and run.

Oh indeed, thanks for the pointer.

> Make sure to build and use a kernel with
> 
> CONFIG_PROVE_LOCKING=y

Yes I did.

Regards,
-- 
Köry Maincent, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com