Re: [PATCH] Avoid buffer overflows in get_user_pages()

Robert Hancock
2008-02-12T03:20:22Z

Nick Piggin wrote:
> On Tuesday 12 February 2008 10:17, Jonathan Corbet wrote:
>> Avoid buffer overflows in get_user_pages()
>>
>> So I spent a while pounding my head against my monitor trying to figure
>> out the vmsplice() vulnerability - how could a failure to check for
>> *read* access turn into a root exploit? It turns out that it's a buffer
>> overflow problem which is made easy by the way get_user_pages() is
>> coded.
>>
>> In particular, "len" is a signed int, and it is only checked at the
>> *end* of a do {} while() loop. So, if it is passed in as zero, the loop
>> will execute once and decrement len to -1. At that point, the loop will
>> proceed until the next invalid address is found; in the process, it will
>> likely overflow the pages array passed in to get_user_pages().
>>
>> I think that, if get_user_pages() has been asked to grab zero pages,
>> that's what it should do. Thus this patch; it is, among other things,
>> enough to block the (already fixed) root exploit and any others which
>> might be lurking in similar code. I also think that the number of pages
>> should be unsigned, but changing the prototype of this function probably
>> requires some more careful review.
>>
>> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index e5628a5..7f50fd8 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -989,6 +989,8 @@ int get_user_pages(struct task_struct *tsk, struct
>> mm_struct *mm, int i;
>> unsigned int vm_flags;
>>
>> + if (len <= 0)
>> + return 0;
>
> BUG_ON()?

Well, not if the code involved in the exploit can pass a zero value,
otherwise it's just turning it into a DoS..