linux-kernel - RE: [PATCH] efivarfs: Limit the rate for non-root to read files

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Message-ID: <3908561D78D1C84285E8C5FCA982C28F7B37FE28@ORSMSX110.amr.corp.intel.com>
Date:   Thu, 22 Feb 2018 05:34:01 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
CC:     Andi Kleen <ak@...ux.intel.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Joe Konno <joe.konno@...ux.intel.com>,
        "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        Jeremy Kerr <jk@...abs.org>,
        Matthew Garrett <mjg59@...gle.com>,
        Peter Jones <pjones@...hat.com>,
        "Andy Lutomirski" <luto@...nel.org>,
        James Bottomley <james.bottomley@...senpartnership.com>
Subject: RE: [PATCH] efivarfs: Limit the rate for non-root to read files

>  - just make it return -EAGAIN instead of sleeping (which probably
> just works fine and doesn't break anything and is simple)

It is very simple. But it does break things :-(. If I read one of these files
using "dd bs=1", that used to read the whole file (while generating
lots of SMI). With the -EAGAIN it just reads 100 bytes and says:

dd: error reading 'DefSetup-e8a99903-302c-4851-a6be-ab2731873b2f': Resource temporarily unavailable
100+0 records in
100+0 records out
100 bytes copied, 0.153943 s, 0.6 kB/s

>  - add a per-user mutex, and do the usleep inside of it, so that
> anybody who tries to do a thousand threads will just be serialized by
> the mutex.
>
> Note that the mutex needs to be per-user, because otherwise it will be
> a DoS for the other users.

I can try that tomorrow (adding the per-user mutex to struct user_struct
right next to the ratelimit I added.

-Tony