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

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Date:   Fri, 6 Jul 2018 16:54:36 +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
Subject: Re: [PATCH] ipv4: fib: avoid NULL dereference

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.

> (And probably dozens of other locations)

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.

Thanks,
Mark.