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    SubjectRe: [PATCH] prevent get_user_pages call from kernel thread
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    Andrew Morton <akpm@linux-foundation.org> writes:

    > On Sun, 08 Jun 2008 19:37:32 +0400
    > Dmitri Monakhov <dmonakhov@openvz.org> wrote:
    >
    >> Yes, yes.. everybody know that it is bad to write from kernel thread, and it
    >> is madness to do it with O_DIRECT. But occasionally file with O_DIRECT flag
    >> may be passed to loop device via LOOP_SET_FD. So if system hasn't address
    >> space ops, or simply hide it like GFS, it is possible to kill kernel via
    >> two lines program. In fact we can't effectively guard kernel space by
    >> disabling O_DIRECT in loop's code, because user space can change it back
    >> via fcntl(,F_SETFL,). Let's simply add sanity check mm related logic.
    >>
    >> Signed-off-by: Dmitri Monakhov <dmonakhov@openvz.org>
    >> ---
    >> drivers/block/loop.c | 1 +
    >> fs/direct-io.c | 4 ++++
    >> 2 files changed, 5 insertions(+), 0 deletions(-)
    >>
    >> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
    >> index d3a25b0..bb2a262 100644
    >> --- a/drivers/block/loop.c
    >> +++ b/drivers/block/loop.c
    >> @@ -753,6 +753,7 @@ static int loop_set_fd(struct loop_device *lo, struct file *lo_file,
    >>
    >> mapping = file->f_mapping;
    >> inode = mapping->host;
    >> + file->f_mode &= ~O_DIRECT;
    >>
    >> if (!(file->f_mode & FMODE_WRITE))
    >> lo_flags |= LO_FLAGS_READ_ONLY;
    >> diff --git a/fs/direct-io.c b/fs/direct-io.c
    >> index 9e81add..8e17224 100644
    >> --- a/fs/direct-io.c
    >> +++ b/fs/direct-io.c
    >> @@ -149,6 +149,10 @@ static int dio_refill_pages(struct dio *dio)
    >> int ret;
    >> int nr_pages;
    >>
    >> + if (unlikely(!current->mm)) {
    >> + WARN_ON_ONCE(1);
    >> + return -EINVAL;
    >> + }
    >> nr_pages = min(dio->total_pages - dio->curr_page, DIO_PAGES);
    >> down_read(&current->mm->mmap_sem);
    >> ret = get_user_pages(
    >
    > It's a bit rude to silently turn off O_DIRECT in this case.
    >
    > I think it would be better to simply fail the LOOP_SET_FD if the file
    > has O_DIRECT and the relevant address_space_operations vectors are
    > NULL.
    Ok, i'll resend updated version, but we sill have to protect dio_defill_pages
    against fcntl(,F_SETFL,) hack.


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