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

Bodo Eggert
2008-02-12T08:34:35Z

Andrew Morton <akpm@linux-foundation.org> wrote:
> On Mon, 11 Feb 2008 16:17:33 -0700 Jonathan Corbet <corbet@lwn.net> 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().

[...]

> Can we just convert
>
> do {
> ...
> } while (len);
>
> into
>
> while (len) {

while (len > 0), if I understand this patch correctly.