Re: [PATCH] zram: restrict add/remove attributes to root only

[Sergey Senozhatsky]

On (12/04/16 12:28), Greg KH wrote:
> Date: Sun, 4 Dec 2016 12:28:20 +0100
> From: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
> To: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, Minchan Kim
>  <minchan@xxxxxxxxxx>, Steven Allen <steven@xxxxxxxxxxxxx>,
>  linux-kernel@xxxxxxxxxxxxxxx, stable@xxxxxxxxxxxxxxx, Sergey Senozhatsky
>  <sergey.senozhatsky.work@xxxxxxxxx>
> Subject: Re: [PATCH] zram: restrict add/remove attributes to root only
> User-Agent: Mutt/1.7.1 (2016-10-04)
> 
> On Sun, Dec 04, 2016 at 07:52:08PM +0900, Sergey Senozhatsky wrote:
> > On (12/04/16 11:28), Greg KH wrote:
> > > On Sun, Dec 04, 2016 at 11:35:15AM +0900, Sergey Senozhatsky wrote:
> > [..]
> > 
> > > Why can't a normal user read the attribute?  Does a read actually modify
> > > something?
> > 
> > yes, it does.

to clarify a bit more:

we allocate a new device ID using idr_alloc(). so the IDs are limited
and, thus, the number of devices is limited as well - signed int. each
new device has NO:
 -- zspoll (zsmalloc pool in zram case)
 -- compression per-CPU backends (working-mem/scratch buffers, etc.)
 -- meta table

so no big memory allocations. (a 'normal' user can't init the device,
he/she can just create it. which is the problem here: we don't want a
'normal' user be able to do this).

every device has:
 -- blk queue
 -- sysfs attrs
 -- gendisk
 -- zram structure allocated.

so each new device consumes some memory, but not insane amounts of it.


> Oh that's totally and completely broken then.
> 
> Reading from a sysfs file should NEVER cause side affects to the system.
> Please fix up this api.

some history. we started with a 'loop device'-like scheme, but
ended up with a sysfs approach

 [1] https://marc.info/?l=linux-kernel&m=142495984002611
 [2] https://marc.info/?l=linux-kernel&m=142507747808572
 [3] https://marc.info/?l=linux-kernel&m=142530591720172
 [4] https://marc.info/?l=linux-kernel&m=142509446812318
 [5] https://marc.info/?l=linux-kernel&m=142509782112819


> > reading from a hot_add file creates a new zram device and returns a new
> > device's device_id. not initialized device (so it does not eat the memory
> > for handle table, etc.), but with its own set of sysfs attrs, etc. which
> > consumes memory after all. so a 'normal' user, doing a simple read from a
> > hot_add file in a loop just for fun, can create a lot of devices and,
> > quite likely, cause some troubles (as reported by Steven Allen).
> 
> Please switch this to be a char device node if you wish to "write and
> get a device handle back".  I don't know how I missed that in the
> original api review, sorry about that.
>
> For now, you need to document the heck out of this in the attribute
> declaration that this is what is going on.  Otherwise someone like me
> will come along and "fix up" the file to use ATTR_RO again in the
> future and you will have the same problem again.


I believe we have a documentation

	Documentation/ABI/testing/sysfs-class-zram
and
	Documentation/blockdev/zram.txt

both explain this attr.

	-ss