linux-kernel - Re: [PATCH 0/2] efivars: reading variables can generate SMIs

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Message-ID: <CAKv+Gu80pJ5tbGoJqBm8CCKrEZXdkE83c944383KbQ5jREUC0Q@mail.gmail.com>
Date:   Fri, 16 Feb 2018 10:41:45 +0000
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Joe Konno <joe.konno@...ux.intel.com>,
        Matthew Garrett <mjg59@...gle.com>,
        Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>
Cc:     linux-efi@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jeremy Kerr <jk@...abs.org>, Andi Kleen <ak@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Benjamin Drung <benjamin.drung@...fitbricks.com>,
        Peter Jones <pjones@...hat.com>
Subject: Re: [PATCH 0/2] efivars: reading variables can generate SMIs

On 15 February 2018 at 18:22, Joe Konno <joe.konno@...ux.intel.com> wrote:
> From: Joe Konno <joe.konno@...el.com>
>
> It was pointed out that normal, unprivileged users reading certain EFI
> variables (through efivarfs) can generate SMIs. Given these nodes are created
> with 0644 permissions, normal users could generate a lot of SMIs. By
> restricting permissions a bit (patch 1), we can make it harder for normal users
> to generate spurious SMIs.
>
> A normal user could generate lots of SMIs by reading the efivarfs in a trivial
> loop:
>
> ```
> while true; do
>     cat /sys/firmware/efi/evivars/* > /dev/null
> done
> ```
>
> Patch 1 in this series limits read and write permissions on efivarfs to the
> owner/superuser. Group and world cannot access.
>
> Patch 2 is for consistency and hygiene. If we restrict permissions for either
> efivarfs or efi/vars, the other interface should get the same treatment.
>

I am inclined to apply this as a fix, but I will give the x86 guys a
chance to respond as well.


> Joe Konno (2):
>   fs/efivarfs: restrict inode permissions
>   efi: restrict top-level attribute permissions
>
>  drivers/firmware/efi/efi.c | 10 ++++++----
>  fs/efivarfs/super.c        |  4 ++--
>  2 files changed, 8 insertions(+), 6 deletions(-)
>
> --
> 2.14.1
>