linux-kernel - Re: [PATCH v3] char: tty3270: fix a missing check on list iterator

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Message-ID: <02741f84-ee8f-2f4c-e2eb-5f74183f60f8@kernel.org>
Date:   Tue, 29 Mar 2022 08:07:16 +0200
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Xiaomeng Tong <xiam0nd.tong@...il.com>
Cc:     agordeev@...ux.ibm.com, borntraeger@...ux.ibm.com,
        dsterba@...e.com, elder@...aro.org, gor@...ux.ibm.com,
        gregkh@...uxfoundation.org, hca@...ux.ibm.com, jcmvbkbc@...il.com,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        stable@...r.kernel.org, svens@...ux.ibm.com
Subject: Re: [PATCH v3] char: tty3270: fix a missing check on list iterator

On 28. 03. 22, 12:27, Xiaomeng Tong wrote:
> On Mon, 28 Mar 2022 12:09:59 +0200, Jiri Slaby wrote:
>> On 28. 03. 22, 11:35, Xiaomeng Tong wrote:
>>> The bug is here:
>>> 	if (s->len != flen) {
>>>
>>> The list iterator 's' will point to a bogus position containing
>>> HEAD if the list is empty or no element is found.
>>
>> Could you also explain how that can happen?
>>
> 
> When list_for_each_entry_* do not early exits (if the list is empty
> or no break/goto/return hit inside the loop), it will set pos ('s' here)
> with a bogus pointer that point to a invalid struct computed based
> on &HEAD using container_of.
> 
> #define list_for_each_entry(pos, head, member)                          \
>          for (pos = list_first_entry(head, typeof(*pos), member);        \
>               !list_entry_is_head(pos, head, member);                    \
>               pos = list_next_entry(pos, member))

No, I didn't mean what happens on that site on the code level. I think 
everyone understands that. Instead, I meant: what circumstances lead to 
this _situation_ in reality?

thanks,
-- 
js
suse labs