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    SubjectRe: [PATCH] Avoid buffer overflows in get_user_pages()
    On Mon, 11 Feb 2008 16:17:33 -0700 Jonathan Corbet <corbet@lwn.net> wrote:

    > Avoid buffer overflows 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
    > 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().

    Sounds convincing.

    > 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>
    >
    > diff --git a/mm/memory.c b/mm/memory.c
    > index e5628a5..7f50fd8 100644
    > --- a/mm/memory.c
    > +++ b/mm/memory.c
    > @@ -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.

    Can we just convert

    do {
    ...
    } while (len);

    into

    while (len) {
    ...
    }

    ?


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