Re: Linux kernels DoSable by file-max limit

[Aleksander Adamowski <olo () altkom com pl>]

Kurt Seifried wrote:

> The available limits are:
>
>  core -- Limits the core file size (KB); usually set to 0 for most users to
> prevent core dumps.
>  data -- Maximum data size (KB).
>  fsize -- Maximum file size (KB).
>  memlock -- Maximum locked-in-memory address space (KB).
>  nofile -- Maximum number of open files.
>  rss -- Maximum resident set size (KB).
>  stack -- Maximum stack size (KB).
>  cpu -- Maximum CPU time (MIN).
>  nproc -- Maximum number of processes.
>  as -- Address space limit.
>  maxlogins -- Maximum number of logins for this user or group.
>  priority -- The priority to run user process with.
>  
from bash manual:
"The value of limit can be a number in the unit specified for the 
resource, or the value unlimited"

Having a fixed, absolute limit on: number, size, amount of resources 
isn't very flexible - it's not dependent on current usage by other users.

Now imagine there are 100 users of a system, all of them shouldn't be 
trusted and belong more or less to the same hash bucket (so you can't 
differentiate using per-group limits).
Now, suppose that some of them use the system frequently, some of them 
sporadically, some of them require as much resources as possible for 
their work, some don't need that much. You can't determine beforehand 
who will need what.

If you can't specify those limits so that they are relative to amount of 
resources available at the time of limit check, you're in problem - 
either you leave limits too high and one user can bring the machine to 
its knees, or you set the absolute limits and you start getting calls 
from frustrated users whose software doesn't work because of those 
limits being enforced.

Best regards,

--
   Olo
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