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

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Message-ID: <20180220233008.55rfm7zw62hrao5p@agluck-desk>
Date:   Tue, 20 Feb 2018 15:30:09 -0800
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Joe Konno <joe.konno@...ux.intel.com>, linux-efi@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Matthew Garrett <matthew.garrett@...ula.com>,
        Jeremy Kerr <jk@...abs.org>, Andi Kleen <ak@...ux.intel.com>,
        Matthew Garrett <mjg59@...gle.com>,
        Peter Jones <pjones@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        James Bottomley <james.bottomley@...senpartnership.com>
Subject: Re: [PATCH 1/2] fs/efivarfs: restrict inode permissions

On Tue, Feb 20, 2018 at 02:01:51PM -0800, Linus Torvalds wrote:
> And just on general principlies, I don't want to see weasel-wordy
> commit messages like
> 
>  "Reading certain EFI variables trigger SMIs"
> 
> I understand *writing* them causing SMI's due to some flash protection
> scheme. What's the reading thing? And why aren't we calling that
> garbage out?

Too much weasel there.  Should say:

EFI[1] stinks. Reading any file in /sys/firmware/efi/efivars/ generates
4 (yes FOUR!) SMIs.

# rdmsr 0x34
14e2
# cat /sys/firmware/efi/efivars/ConIn-8be4df61-93ca-11d2-aa0d-00e098032b8c > /dev/null
# rdmsr 0x34
14e6

-Tony

[1] I didn't dig through the Linux code to check whether we manage to
get those four SMIs from a single EFI call, or whether we make multiple
EFI calls to open/read/close one file. It is possible that we stink a
bit too if we are doing more EFI calls than required.