Commit 900cf086 authored by Jonathan Corbet's avatar Jonathan Corbet Committed by Linus Torvalds
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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

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: default avatarJonathan Corbet <>
Signed-off-by: default avatarLinus Torvalds <>
parent b1292b17
......@@ -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.
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