| From 900cf086fd2fbad07f72f4575449e0d0958f860f Mon Sep 17 00:00:00 2001 |
| From: Jonathan Corbet <corbet@lwn.net> |
| Date: Mon, 11 Feb 2008 16:17:33 -0700 |
| Subject: [PATCH] Be more robust about bad arguments in get_user_pages() |
| |
| From: Jonathan Corbet <corbet@lwn.net> |
| |
| patch 900cf086fd2fbad07f72f4575449e0d0958f860f in mainline. |
| |
| 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> |
| Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> |
| Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de> |
| |
| --- |
| mm/memory.c | 2 ++ |
| 1 file changed, 2 insertions(+) |
| |
| --- a/mm/memory.c |
| +++ b/mm/memory.c |
| @@ -981,6 +981,8 @@ int get_user_pages(struct task_struct *t |
| 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. |
