From: Jonathan Corbet Date: Mon, 11 Feb 2008 23:17:33 +0000 (-0700) Subject: Be more robust about bad arguments in get_user_pages() X-Git-Tag: v2.6.25-rc2~83 X-Git-Url: https://err.no/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=900cf086;p=linux-2.6 Be more robust about bad arguments 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 Signed-off-by: Linus Torvalds --- diff --git a/mm/memory.c b/mm/memory.c index e5628a5fd6..717aa0e3be 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; /* * Require read or write permissions. * If 'force' is set, we only require the "MAY" flags.