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

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Date:   Fri, 6 Jul 2018 09:20:44 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Mark Rutland <mark.rutland@....com>,
        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 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 ?

> 
>> (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.

There is value to your patch, since it makes UBSAN happy.

But please change the title and changelog accordingly.

Thanks.