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    Subject[patch 23/38] Be more robust about bad arguments in get_user_pages()
    2.6.24-stable review patch.  If anyone has any objections, please let us
    know.

    ------------------
    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
    @@ -980,6 +980,8 @@ 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.
    --


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