Re: [PATCH] RDMA/nldev: Fix multiple potential NULL pointer dereferences

[Gustavo A. R. Silva]

Quoting Leon Romanovsky <leon@xxxxxxxxxx>:

> On Fri, Feb 09, 2018 at 09:56:00AM -0600, Gustavo A. R. Silva wrote:
> > Quoting Leon Romanovsky <leon@xxxxxxxxxx>:
> >
> > > On Fri, Feb 09, 2018 at 07:36:49AM -0600, Gustavo A. R. Silva wrote:
> > > > Hi Leon,
> > > >
> > > > Quoting Leon Romanovsky <leon@xxxxxxxxxx>:
> > > >
> > > > > On Fri, Feb 09, 2018 at 12:37:02AM -0600, Gustavo A. R. Silva wrote:
> > > > > > In case the message header and payload cannot be stored, function
> > > > > > nlmsg_put returns null.
> > > > > >
> > > > > > Fix this by adding multiple sanity checks and avoid a potential
> > > > > > null dereference on _nlh_ when calling nlmsg_end.
> > > > > >
> > > > > > Addresses-Coverity-ID: 1454215 ("Dereference null return value")
> > > > > > Addresses-Coverity-ID: 1454223 ("Dereference null return value")
> > > > > > Addresses-Coverity-ID: 1454224 ("Dereference null return value")
> > > > > > Addresses-Coverity-ID: 1464669 ("Dereference null return value")
> > > > > > Addresses-Coverity-ID: 1464670 ("Dereference null return value")
> > > > > > Addresses-Coverity-ID: 1464672 ("Dereference null return value")
> > > > > > Fixes: e5c9469efcb1 ("RDMA/netlink: Add nldev device doit
> > > > implementation")
> > > > > > Fixes: c3f66f7b0052 ("RDMA/netlink: Implement nldev port  
> > doit callback")
> > > > > > Fixes: 7d02f605f0dc ("RDMA/netlink: Add nldev port dumpit
> > > > implementation")
> > > > > > Fixes: b5fa635aab8f ("RDMA/nldev: Provide detailed QP information")
> > > > > > Fixes: bf3c5a93c523 ("RDMA/nldev: Provide global resource  
> > utilization")
> > > > > > Signed-off-by: Gustavo A. R. Silva <gustavo@xxxxxxxxxxxxxx>
> > > > > > ---
> > > > > >  drivers/infiniband/core/nldev.c | 26 +++++++++++++++++++++++++-
> > > > > >  1 file changed, 25 insertions(+), 1 deletion(-)
> > > > > >
> > > > >
> > > > > It will be much better to fix the tool instead of fixing ghost case.
> > > > > This scenario is impossible for all those flows.
> > > > > We can receive the skv/msg in two ways:
> > > > >  * First by allocating new message with NLMSG_DEFAULT_SIZE, which has
> > > > > more room
> > > > >    than nlmsg_total_size(payload), payload is 0.
> > > > >  * Second by getting from netlink.c and it will be at least "struct
> > > > > nlmsghdr" too.
> > > > >
> > > > > Can you please add this info to the commit message?
> > > > >
> > > >
> > > > Actually, I was planing to send a new version of this patch.  
> > This time using
> > > > the unlikely macro for all the null checks on nlh.
> > > >
> > > > What do you think?
> > >
> > > It is not datapath, so "unlikely" is not needed. Let's assume that smart
> > > enough
> > > compiler will optimize such flow anyway, because nlmsg_put returns NULL
> > > in unlikely scenario, so this check will be unlikely automatically too.
> > >
> >
> > I'm curious about why the return value of nlmsg_put is null checked 118 out
> > of 129 times (based on Coverity reports) in the last linux-next tree.
> >
> > So based on what you mention, do you think all those checks are actually
> > unnecessary and, maybe they should be removed?
>
> I honestly don't know about all cases, but if message is allocated with
> NLMSG_DEFAULT_SIZE and payload is 0, this check won't be needed.

I got it.

> So go ahead, add check if (!...) in all places, but be cautious with
> "potential null dereference" claims, it is not always true.

You are right. I will update the subject and commit message.

Thanks for the feedback, Leon.
I appreciate it.
--
Gustavo