Re: [PATCH 5/5] efi: Make efivarfs entries immutable by default. (v5)

[Matt Fleming <matt-mF/unelCI9GS6iBeEJttW/XRex20P6io@public.gmane.org>]

On Mon, 15 Feb, at 12:02:16PM, Peter Jones wrote:
> On Mon, Feb 15, 2016 at 10:48:01AM +0000, Matt Fleming wrote:
> > On Fri, 12 Feb, at 10:09:49AM, Peter Jones wrote:
> > > On Fri, Feb 12, 2016 at 02:36:28PM +0100, Laszlo Ersek wrote:
> > > > On 02/04/16 16:34, Peter Jones wrote:
> > > > > "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.
> > > > 
> > > > I think that the whitelist should include the following pattern
> > > > (permitting deletion):
> > > > - GUID:                  LINUX_EFI_CRASH_GUID
> > > > - variable name pattern: "dump-type*"
> > > > 
> > > > This is because the pstore filesystem can be backed by UEFI variables,
> > > > and (for example) a crash might dump the last kilobytes of the dmesg
> > > > into a number of pstore entries, each entry backed by a separate UEFI
> > > > variable in the above GUID namespace, and with a variable name according
> > > > to the above pattern.
> > > > 
> > > > Please see "drivers/firmware/efi/efi-pstore.c".
> > > > 
> > > > While this patch series will not prevent the user from deleting those
> > > > UEFI variables via the pstore filesystem (i.e., deleting a pstore fs
> > > > entry will continue to delete the backing UEFI variable), I think it
> > > > would be nice to preserve the possibility for the sysadmin to delete
> > > > Linux-created UEFI variables that carry portions of the crash log,
> > > > *without* having to mount the pstore filesystem.
> > > 
> > > They still have that ability with this patch - they have to chattr -i it
> > > manually, or use libefivar's efi_del_variable() with a suitably new
> > > libefivar, but we aren't ever actually stopping deletion.
> > > 
> > > Which doesn't mean you're wrong, mind you, but it does remove urgency.
> > 
> > Well, we should avoid having different whitelists in different kernel
> > versions, so I think there is some urgency in fixing the efi-pstore
> > case now.
> > 
> > Remember, we're breaking userspace ABI with these changes, which is a
> > major downside that is only offset by the risk of bricking the
> > machine. There's no chance of bricking by deleting the efi-pstore
> > variables, so we'd be breaking userspace ABI for no reason.
> > 
> > Would something like this allow efi-pstore variables to be deleted
> > (and anything else that uses LINUX_EFI_CRASH_GUID in future)?
> 
> Yes, this patch works as expected:
> 
> Acked-by: Peter Jones <pjones-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> Tested-by: Peter Jones <pjones-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>

Thanks Peter. This is what I've got queued up,

---

>From e246eb568bc4cbbdd8a30a3c11151ff9b7ca7312 Mon Sep 17 00:00:00 2001
From: Matt Fleming <matt-mF/unelCI9GS6iBeEJttW/XRex20P6io@public.gmane.org>
Date: Mon, 15 Feb 2016 10:34:05 +0000
Subject: [PATCH] efi: Add pstore variables to the deletion whitelist

Laszlo explains why this is a good idea,

 'This is because the pstore filesystem can be backed by UEFI variables,
  and (for example) a crash might dump the last kilobytes of the dmesg
  into a number of pstore entries, each entry backed by a separate UEFI
  variable in the above GUID namespace, and with a variable name
  according to the above pattern.

  Please see "drivers/firmware/efi/efi-pstore.c".

  While this patch series will not prevent the user from deleting those
  UEFI variables via the pstore filesystem (i.e., deleting a pstore fs
  entry will continue to delete the backing UEFI variable), I think it
  would be nice to preserve the possibility for the sysadmin to delete
  Linux-created UEFI variables that carry portions of the crash log,
  *without* having to mount the pstore filesystem.'

There's also no chance of causing machines to become bricked by
deleting these variables, which is the whole purpose of excluding
things from the whitelist.

Use the LINUX_EFI_CRASH_GUID guid and a wildcard '*' for the match so
that we don't have to update the string in the future if new variable
name formats are created for crash dump variables.

Reported-by: Laszlo Ersek <lersek-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Acked-by: Peter Jones <pjones-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Tested-by: Peter Jones <pjones-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Cc: Matthew Garrett <mjg59-1xO5oi07KQx4cg9Nei1l7Q@public.gmane.org>
Cc: "Lee, Chun-Yi" <jlee-IBi9RG/b67k@public.gmane.org>
Signed-off-by: Matt Fleming <matt-mF/unelCI9GS6iBeEJttW/XRex20P6io@public.gmane.org>
---
 drivers/firmware/efi/vars.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/firmware/efi/vars.c b/drivers/firmware/efi/vars.c
index 50f10bad2604..7f2ea21c730d 100644
--- a/drivers/firmware/efi/vars.c
+++ b/drivers/firmware/efi/vars.c
@@ -198,6 +198,7 @@ static const struct variable_validate variable_validate[] = {
 	{ EFI_GLOBAL_VARIABLE_GUID, "OsIndications", NULL },
 	{ EFI_GLOBAL_VARIABLE_GUID, "PlatformLang", validate_ascii_string },
 	{ EFI_GLOBAL_VARIABLE_GUID, "Timeout", validate_uint16 },
+	{ LINUX_EFI_CRASH_GUID, "*", NULL },
 	{ NULL_GUID, "", NULL },
 };
 
-- 
2.6.2