Re: [PATCH] prevent get_user_pages call from kernel thread

[Dmitri Monakhov <dmonakhov@openvz.org>]

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.