svn commit: r263755 - head/sys/kern

[Warner Losh]

On Mar 29, 2014, at 6:31 AM, David Xu <davidxu at FreeBSD.org> wrote:

> 
> On 2014/03/29 15:52, Don Lewis wrote:
>> On 29 Mar, Mateusz Guzik wrote:
>>> On Sat, Mar 29, 2014 at 11:52:09AM +0800, David Xu wrote:
>>>>> If fsetown handling like this is insecure this would bite us in that
>>>>> scenario (and few others). In short, if we can avoid giving another way
>>>>> to corrupt stuff in the kernel to userspace, we should.
>>>>> 
>>>> I can not see what you said, where is the security problem with fsetown ?
>>>> if you have per-jail instance of devsoftc, they all are operating on their
>>>> own instance. but I don't think this patch should address jail now, there
>>>> are many things are not jail ready.
>>>> 
>>> I asked if multpiple concurrent calls to fsetown(.., &pointer) could
>>> result in some corruption in the kernel - if they could, we would have a
>>> problem in the future.
>>> 
>>> I decided to read the code once more and fsetown seems to be safe in
>>> this regard after all and with that in mind the patch looks good to me.
>>  
>> The fsetown() implementation does sufficient locking to prevent the
>> kernel from getting into a bad state.  The issue is that the device can
>> only have at most one owner (which may be a process group).  If multiple
>> processes are allowed to open the device, or if a process that opened
>> the device shares the descriptor with another process, the last call to
>> fsetown() wins.  That means that one process could steal ownership from
>> another if they both have the same device open.
>> 
>> The reason that I suggested checking ownership when handling FIOASYNC is
>> that in the case of two processes sharing access to a device, there is
>> currently nothing that prevents a non-owner of the device from enabling
>> this mode and causing SIGIO signals to be sent to the owner, which might
>> not be expecting to receive them.
> I think if you add ownership checking, it will be incompatible with
> other code,  people have to change their mind when dealing with
> this special file descriptor, my recommendation is people don't need
> to refresh their brain.
> OTOH, if it is a problem, we should have already been flooded by
> the problem, but in the past years, I saw zero complaining in the mailing
> lists.

I believe that the SIGIO code was cut and pasted from a driver I was working
on in the 4.x time frame. devd is the only consumer, and it doesn’t do the
FIOASYNC stuff at all.

So I’d be strongly biased to either (a) remove support for this or (b) make
the support correct, even at the cost of speed or performance.

Warner