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    SubjectRe: [PATCH] amd64-agp: fix arbitrary kernel memory writes
    On Wed, May 29, 2019 at 12:52:01PM +0800, Young Xiao wrote:
    > pg_start is copied from userspace on AGPIOC_BIND and AGPIOC_UNBIND ioctl
    > cmds of agp_ioctl() and passed to agpioc_bind_wrap(). As said in the
    > comment, (pg_start + mem->page_count) may wrap in case of AGPIOC_BIND,
    > and it is not checked at all in case of AGPIOC_UNBIND. As a result, user
    > with sufficient privileges (usually "video" group) may generate either
    > local DoS or privilege escalation.
    >
    > See commit 194b3da873fd ("agp: fix arbitrary kernel memory writes")
    > for details.
    >
    > Signed-off-by: Young Xiao <92siuyang@gmail.com>
    > ---
    > drivers/char/agp/amd64-agp.c | 3 ++-
    > 1 file changed, 2 insertions(+), 1 deletion(-)
    >
    > diff --git a/drivers/char/agp/amd64-agp.c b/drivers/char/agp/amd64-agp.c
    > index c69e39f..5daa0e3 100644
    > --- a/drivers/char/agp/amd64-agp.c
    > +++ b/drivers/char/agp/amd64-agp.c
    > @@ -60,7 +60,8 @@ static int amd64_insert_memory(struct agp_memory *mem, off_t pg_start, int type)
    >
    > /* Make sure we can fit the range in the gatt table. */
    > /* FIXME: could wrap */
    > - if (((unsigned long)pg_start + mem->page_count) > num_entries)
    > + if (((pg_start + mem->page_count) > num_entries) ||
    > + ((pg_start + mem->page_count) < pg_start))

    Why did you take off the cast for the first test?

    And if this really does fix this issue, should you remove the FIXME
    line?

    thanks,

    greg k-h

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