[PATCH] Be more robust about bad arguments in get_user_pages()

[Jonathan Corbet]

MAINLINE: 900cf086fd2fbad07f72f4575449e0d0958f860f

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@xxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
CC: Oliver Pinter <oliver.pntr@xxxxxxxxx>

diff --git a/mm/memory.c b/mm/memory.c
index f64cbf9..538f054 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -983,6 +983,8 @@ 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.