On Sun, 08 Jun 2008 19:37:32 +0400
Dmitri Monakhov <dmonakhov@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
> 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(¤t->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.
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