linux-kernel - Re: [PATCH] ipv4: fib: avoid NULL dereference

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Date:   Fri, 6 Jul 2018 17:33:40 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        "David S . Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        netdev@...r.kernel.org, arnd@...db.de
Subject: Re: [PATCH] ipv4: fib: avoid NULL dereference

On Fri, Jul 06, 2018 at 09:20:44AM -0700, Eric Dumazet wrote:
> 
> 
> On 07/06/2018 08:54 AM, Mark Rutland wrote:
> > On Fri, Jul 06, 2018 at 08:39:11AM -0700, Eric Dumazet wrote:
> >>
> >>
> >> On 07/06/2018 07:57 AM, Mark Rutland wrote:
> >>> On Fri, Jul 06, 2018 at 07:47:04AM -0700, Eric Dumazet wrote:
> >>>>
> >>>>
> >>>> On 07/06/2018 07:28 AM, Mark Rutland wrote:
> >>>>> In tnode_free() we iterate over a callback_head list with a while loop.
> >>>>> At the start of the loop body we generate the next head pointer, and at
> >>>>> the end of the loop body we generate the tn pointer for the next
> >>>>> iteration of the loop by using container_of() on the head pointer to
> >>>>> find the tnode, and deriving the kv pointer from this.
> >>>>>
> >>>>> In the final iteration of the loop, this means that we derive a pointer
> >>>>> from NULL, which is undefined behaviour, which UBSAN detects:
> >>>>
> >>>> There is no dereference, your patch title is misleading.
> >>>>
> >>>> UBSAN might be fooled, not the C compiler.
> >>>
> >>> I'm happy to change the title to "avoid undefined behaviour".
> >>>
> >>
> >> Are you planning to change this as well ?
> >>
> >> include/linux/stddef.h:19:#define offsetof(TYPE, MEMBER)	((size_t)&((TYPE *)0)->MEMBER)
> > 
> > No, because __builtin_offsetof() is used these days (since GCC 4),
> > avoiding the undefined behaviour.
> 
> Ah... should we remove the line and declare linux must be compiled
> with GCC 4 at least ?

Good question -- IIUC x86 already mandates at least GCC 4.6, as this is
necessary for jump labels.

There was a push to mandate a more recent compiler generally, but I'm
not sure of the current state of things.

Arnd, were we planning to mandate at least GCC 4.x?

> > I do concede that if this is everywhere it's not worth the effort, and
> > from the looks of things, the gnaliest cases are where we do things
> > like:
> > 
> >   get_user(var, &struct->field)
> > 
> > ... where the user could validly pass a NULL pointer if it wished.
> > 
> > So I guess I'll give up.
> 
> There is value to your patch, since it makes UBSAN happy.
> 
> But please change the title and changelog accordingly.

Sure thing. Are you happy with the "deriving a pointer from NULL"
wording in the explanation? That wasn't intended to mean a dereference.

I'll change the title to:

  ipv4: fib: avoid undefined behaviour

Thanks,
Mark.