Be more robust about bad arguments in get_user_pages()

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>

diff --git a/mm/memory.c b/mm/memory.c
index 4b0144b24c123681dcd9e95e715ae56b009355d7..da8b74b73f548b32762d3c1b997974f58e3c9f9f 100644 (file)
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -980,6 +980,8 @@ 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.