Re: [RFC PATCH v3 0/1] kern/dl: Add module version check

[Pete Batard]

I'm sorry but that's NO_GO for me.

This is a major step back from v2, where the check was never applicable 
for BIOS and I believe I explained why a module check that can be 
enforced for BIOS is going to create a major headache for end-users.

It seems pretty obvious that, if distro maintainers start to use 
--enable-modver-check=enforce for UEFI they are also going to use the 
same option for BIOS (because why should they use a different 
compilation options if it applies for both?), and you *ARE* going to 
leave Windows users stranded through them not trying to boot GRUB based 
media because they will be forced to use DD to create a workable media 
(even though, and this is the major point here, in the very vast 
majority of cases there is not issue with mixing versions if you're on 
the same major/minor base for GRUB), and consider that, because they 
cannot see its content, or can only see the ESP, their media was not 
properly created and is not worth trying to boot.

This is a *VERY REAL* problem, for which I have provided many examples 
(see [1]), and could provide even more, which is why utilities like 
Rufus try to steer away from using DD mode where possible.

However, if people can use --enable-modver-check=enforce for BIOS, then, 
under the aim of solving one minor issue (that from looking the Google 
links for "452: out of range pointer" seem to be a problem that is 
mostly encountered in UEFI environments rather than BIOS, or at least is 
not a BIOS-only issue, and therefore something that will be sorted out 
for the majority of folks experiencing it *even* if 
--enable-modver-check cannot apply for BIOS), you will be adding to an 
already existing a major one, and you are going to leave people, and 
especially Windows users who are trying to switch to Linux, stranded.

In short, GRUB would be doing a major disservice to its end users if it 
is to allow module version checks to be enforceable for BIOS, and, 
considering how there is little benefit in enforcing this in an 
environment that cannot be secured in the first place, it's hard to see 
a justification for what is essentially a *MAJOR* design regression from 
the v2 of this patch. Or at the very least, a better approach is to 
introduce it for UEFI only, then, once that has been integrated with 
UEFI for a while, see if BIOS users are actually reporting issues due to 
module version mismatch, and reconvene on whether we really want to 
allow this check to also apply to BIOS.

Thus, in my view, the proper approach for the sake of the end users is 
leave default BIOS boot behaviour as it currently stands, with no 
warning and no possibility of failing the boot process on version 
mismatch (because, and I am well placed to know, both from experience as 
well as the feedback I get from Rufus users, that in the very vast 
majority of cases, as long as you use modules and core.img based on the 
same major+minor, there is no issue, and producing a warning that hints 
that there may be is only going to be detrimental to the end user).

And, once again, I have to ask this mailing list to please try not to 
consider issues only from a Linux/UNIX based maintainer's perspective, 
but also consider them from the end-users who are actually going to be 
creating GRUB based media on Windows, where using ISOHybrid as DD is far 
from the panacea it's supposed to be.

Finally, if you are interested in seeing how end users actually struggle 
with boot media and boot media creation, I *strongly* invite you to 
spend some time on reddit, askubuntu, superuser and other end-user 
places, where you should be able to validate that, as I have pointed 
out, Windows end users really do struggle with ISOHybrdids that are 
written in DD mode, which will hopefully help you reach the conclusion 
that, if this v3 proposal gets integrated, it will cause more harm for 
BIOS users than it will benefit them on account that distro maintainers 
are bound to start using --enable-modver-check=enforce everywhere, if it 
can apply to both BIOS and UEFI.

Regards,

/Pete

[1] https://lists.gnu.org/archive/html/grub-devel/2022-06/msg00024.html

On 2022.12.22 16:38, Zhang Boyang wrote:
> Hi,
>
> This is the V3 of my patch.
>
> V2 is at:
> https://lists.gnu.org/archive/html/grub-devel/2022-12/msg00234.html
>
> V1 is at:
> https://lists.gnu.org/archive/html/grub-devel/2022-12/msg00213.html
>
>
> [ TD;LR ]
>
> 1) The check is always enforced when GRUB is locked down, i.e. modules 
> will be refused to load if they have mismatched version
>
> 2) If built with "--disable-modver-check", modules can always be loaded 
> even if they have mismatched version, and no message will be displayed.
>
> 3) If built with "--enable-modver-check=audit", modules can always be 
> loaded even if they have mismatched version, but if mismatch is found, a 
> warning message will be displayed. This is the default.
>
> 4) If built with "--enable-modver-check=enforce", the behavior is same 
> as 1)
>
>
> [ Why this patch is useful, even for BIOS boot ]
>
> Because it helps people diagnose broken (or improper) GRUB installations.
>
> For example, if you google "452: out of range pointer", you will got a 
> lot of results in 2022. I think the most of them are related to 
> mismatched modules. However, these problem are often not properly 
> diagnosed because they disappear magically, e.g. update whole system 
> (which triggers grub reinstall). There are several people even suspect 
> there are problems with their hard disk / BIOS. However, the root cause 
> is 052e6068be62 ("mm: When adding a region, merge with region after as 
> well as before") changed the layout of `struct grub_mm_region`, which is 
> both used in main program and "relocator.mod", so the module reads the 
> wrong field and crashes GRUB. Please the commit did nothing wrong 
> because there is no API/ABI compatibility guarantees in GRUB.
>
> If there are warning messages about mismatched modules, user will easily 
> notice there are problems with their GRUB installation.
>
>
> [ Why not enforce this check to prevent crashes ]
>
> As Glenn & Pete said, most mismatched modules isn't harmful. At most 
> times, GRUB with mismatched modules can boot Linux happily, even if 
> these modules come from another Linux distribution. This enables user to 
> fix his/her GRUB installation without using a boot/rescue disk, because 
> the user can boot the existing Linux using the existing (but improperly 
> installed) GRUB.
>
>
> [ Why warning can be disabled ]
>
> Some tools like Rufus relies on mismatched modules. Some advanced users 
> also doesn't like redundant warnings for their existing known-to-work 
> configurations.
>
> However, it's highly unrecommended to disable this warning.
>
>
> [ Why this patch is a prerequisite for external signed module support ]
>
> Consider this scenario:
>
> 1) GRUB 2.XX is free of vulnerabilities
>
> 2) GRUB 2.YY is also free of vulnerabilities
>
> 3) So GRUB 2.XX shares same SBAT numbers with GRUB 2.YY, therefore SBAT 
> can't help in version check
>
> 4) If there is no version check, it's possible to load GRUB 2.YY modules 
> into GRUB 2.XX (and vice versa)
>
> 5) However, due to some changes in API or ABI, although unlikely, there 
> is possibility that there are vulnerabilities when using GRUB 2.YY 
> modules with GRUB 2.XX  (and vice versa)
>
> 6) So we must enforce version check to prevent this from happening
>
> However, because version string is only consisted of PACKAGE_VERSION, it 
> must be unique for one given vendor (signer). For example, version 
> string need to be different for Debian 10 and Debian 11 even they both 
> use GRUB 2.06, and no two build in Debian 10 (or Debian 11) have same 
> version string.
>
>
> Best Regards,
> Zhang Boyang