Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:
> 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.
Ok, i'll resend updated version, but we sill have to protect dio_defill_pages
against fcntl(,F_SETFL,) hack.
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