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

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Date:   Fri, 16 Feb 2018 17:05:37 -0500
From:   Peter Jones <pjones@...hat.com>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     James Bottomley <James.Bottomley@...senPartnership.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        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>,
        "linux-efi@...r.kernel.org" <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>,
        Benjamin Drung <benjamin.drung@...fitbricks.com>
Subject: Re: [PATCH 0/2] efivars: reading variables can generate SMIs

On Fri, Feb 16, 2018 at 09:09:30PM +0000, Luck, Tony wrote:
> > That said, I'm not sure how many non-root users run the toolkit to
> > extract their EFI certificates or check on the secure boot status of
> > the system, but I suspect it might be non-zero: I can see the tinfoil
> > hat people wanting at least to check the secure boot status when they
> > log in.
> 
> Another fix option might be to rate limit EFI calls for non-root users (on X86
> since only we have the SMI problem). That would:
> 
> 1) Avoid using memory to cache all the variables
> 2) Catch any other places where non-root users can call EFI

I could get behind that as well.  Currently the things I maintain do
approximately this many normal accesses with invocations you can do as a
user:

"efibootmgr -v" - six files we always try to read, plus one per Boot####
		  entry.
"fwupdate --info" - one file it always tries to read, one file for each
                    ESRT entry.
"dbxtool -l" - one file it always reads.
"mokutil --sb-state" - reads the same file twice.  I don't maintain
                       this, but I'll send a patch to Gary to make it
		       only read it once.  AFAICS all of the other
		       invocations you can currently do as a user
		       /legitimately/ read two files, though.

Some systems seem to *love* making a pile of Boot#### entries; I think
the most I've seen is something like 16.  So on that machine, one
"efibootmgr -v" invocation is ~22 efivars files read.  I've never seen a
machine that advertised more than 2 ESRT entries, but maybe we'll get
there some day.

-- 
  Peter