linux-stable/Documentation/filesystems/efivarfs.txt
Peter Jones ed8b0de5a3 efi: Make efivarfs entries immutable by default
"rm -rf" is bricking some peoples' laptops because of variables being
used to store non-reinitializable firmware driver data that's required
to POST the hardware.

These are 100% bugs, and they need to be fixed, but in the mean time it
shouldn't be easy to *accidentally* brick machines.

We have to have delete working, and picking which variables do and don't
work for deletion is quite intractable, so instead make everything
immutable by default (except for a whitelist), and make tools that
aren't quite so broad-spectrum unset the immutable flag.

Signed-off-by: Peter Jones <pjones@redhat.com>
Tested-by: Lee, Chun-Yi <jlee@suse.com>
Acked-by: Matthew Garrett <mjg59@coreos.com>
Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>
2016-02-10 16:25:52 +00:00

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efivarfs - a (U)EFI variable filesystem
The efivarfs filesystem was created to address the shortcomings of
using entries in sysfs to maintain EFI variables. The old sysfs EFI
variables code only supported variables of up to 1024 bytes. This
limitation existed in version 0.99 of the EFI specification, but was
removed before any full releases. Since variables can now be larger
than a single page, sysfs isn't the best interface for this.
Variables can be created, deleted and modified with the efivarfs
filesystem.
efivarfs is typically mounted like this,
mount -t efivarfs none /sys/firmware/efi/efivars
Due to the presence of numerous firmware bugs where removing non-standard
UEFI variables causes the system firmware to fail to POST, efivarfs
files that are not well-known standardized variables are created
as immutable files. This doesn't prevent removal - "chattr -i" will work -
but it does prevent this kind of failure from being accomplished
accidentally.