Re: [patch 1/1] Drop CAP_SYS_RAWIO requirement for FIBMAP

Mike Waychison
2007-10-26T00:36:20Z

Alan Cox wrote:
> On Thu, 25 Oct 2007 16:06:40 -0700
> Mike Waychison <mikew@google.com> wrote:
>
>> Remove the need for having CAP_SYS_RAWIO when doing a FIBMAP call on an open file descriptor.
>>
>> It would be nice to allow users to have permission to see where their data is landing on disk, and there really isn't a good reason to keep them from getting at this information.
>
> Historically this was done because people felt it was more secure. It
> also allows you to make some deductions about other activities on the
> disk but thats probably only a concern for very very security crazed
> compartmentalised boxes
>
> Also historically at least FIBMAP could be abused to crash the system.
> Now if you can verify that has been fixed I have no problem, but given
> that I can find no record of that being fixed it would be wise to audit
> it first and review Chris Evans and other reports about what occurs when
> FIBMAP is passed random block numbers.
>
> FIBMAP has another problem for this general use as well - it takes an int
> but the block number can now be bigger for very large files on 32bit.
>
> Alan

I found Chris's comment about negative block numbers, I'll send a patch
out for that.

You mentioned back in 99 about racing with ftruncate. Is it sufficient
to mutex_lock(i_mutex) and down_read(i_alloc_sem)?

Mike Waychison