Re: [PATCH 1/2] fs/efivarfs: restrict inode permissions

[Linus Torvalds <torvalds@linux-foundation.org>]

On Wed, Feb 21, 2018 at 11:58 AM, Luck, Tony <tony.luck@intel.com> wrote:
>
> How are you envisioning this rate-limiting to be implemented? Are
> you going to fail an EFI call if the rate is too high?  I'm thinking that
> we just add a delay to each call so that we can't exceed the limit.

Delaying sounds ok, I guess.

But the "obvious" implementation may be simple:

    static void efi_ratelimit(void)
    {
        static DEFINE_RATELIMIT_STATE(ratelimit, HZ, 100);

        if (!__ratelimit(&ratelimit))
                msleep(10);
        }
    }

but the above is actually completely broken.

Why? If you have multiple processes, they can each only do a hundred
per second, but globally they can do millions per second by just
having a few thousand threads. They all sleep, but..

So how do you restrict it *globally*?

If you put this all inside a lock like a mutex, you can generate
basically arbitrary delays, and you're back to the DoS schenario. A
fair lock will allow thousands of waiters to line up and make the
delay be

But maybe I'm missing some really obvious way. You *can* make the
msleep be a spinning wait instead, and rely on the scheduler, I guess.

Or maybe I'm just stupid and am overlooking the obvious case.

Again, making the ratelimiting be per-user makes all of these issues
go away. Then one user cannot delay another one.

             Linus