Re: PROBLEM: brcmfmac's DMI-based fw file names break built-in fw loader

[Arend Van Spriel]

On May 4, 2019 9:44:51 PM Victor Bravo <1905@xxxxxxxxxx> wrote:

> On Sat, May 04, 2019 at 09:11:09PM +0200, Arend Van Spriel wrote:
> > + Hans, Luis
> >
> > On 5/4/2019 6:26 PM, Victor Bravo wrote:
> > > The brcmfmac driver seems to have partially fixed problems which
> > > prevented it to be used in shared system/kernel images for multiple
> > > hardware by trying to load it's <config>.txt as
> > > <config>.<dmi_sys_vendor>.<dmi_product_name>.txt first and then
> > > falling back to <config>.txt. Real-life example:
> > >
> > > brcmfmac mmc1:0001:1: Direct firmware load for 
> > brcm/brcmfmac43340-sdio.ASUSTeK COMPUTER INC.-T100HAN.txt failed with
> > > error -2
> > > brcmfmac: brcmf_fw_alloc_request: using brcm/brcmfmac43340-sdio for chip
> > > BCM43340/2
> > >
> > > Unfortunately this doesn't really help on systems which use static
> > > kernel with firmware blobs (and also text configuration files in case of
> > > brcmfmac) built-in using CONFIG_EXTRA_FIRMWARE, as CONFIG_EXTRA_FIRMWARE
> > > doesn't support spaces in file names - kernel build fails with
> > >
> > > CONFIG_EXTRA_FIRMWARE="brcm/brcmfmac43340-sdio.bin 
> > brcm/brcmfmac43340-sdio.ASUSTeK COMPUTER INC.-T100HAN.txt"
> > >
> > > for obvious reasons. So the only way here is to stay with good old
> > > brcmfmac43340-sdio.txt and support at most one brcmfmac-equipped machine
> > > per kernel image.
> > >
> > > Please consider filtering the DMI strings and replacing spaces and
> > > possibly other invalid characters with underscores, and/or adding module
> > > parameter to allow passing the string from command line (using
> > > brcmfmac.tag=t100 or brcmfmac.board=t100 to make the module load
> > > brcmfmac43340-sdio.t100.txt seems nicer to me, and isn't prone to
> > > breaking when DMI strings change on BIOS update).
> >
> > The intent of the DMI approach was to avoid end-users from passing module
> > parameters for this. As to fixing DMI string usage patches are welcome.
>
> Well I think I could also provide a patch to fix, this can be easily
> done by adding a string of allowed characters and then replacing
> unknown ones with underscores.
>
> > > My brief grep-based research also suggest that strings retrieved
> > > by dmi_get_system_info() are passed to firmware loader without any
> > > checks for special character, /../ etc. I'm not sure whether this is
> > > considered to be proper & safe use, but if it's not, it may also have
> > > some security implications, as it allows attacker with access to DMI
> > > strings (using root rights/other OS/BIOS/physical access) to mess
> > > with kernel space or secure boot.
> >
> > Hmm. Attackers with that kind of access can do bad is a gazillion ways.
>
> Agreed. It will be definitely easier to make filenames contain only safe
> characters than to discuss those ways.
>
> > > I would also really appreciate not allowing future brcm (and other)
> > > drivers to leave staging area before they fully support =y.
> >
> > Define fully support. At the time we moved into the wireless tree (almost a
> > decade ago) we did support =y. As such you could consider the DMI approach a
> > regression, but I find that a bit harsh to say. Hans made a honest attempt
> > and it is something that can be fixed. It can be you providing just that ;-)
>
> Well... I agree that the idea wasn't really complete ;).
>
> As for the patches, I also realized that the txt config file actually
> comes from EFI/BIOS, so it's quite possible that it may differ between
> BIOS versions. So I'm thinking of 3 patches here:
>
>   1) Character filtering as described above.
>
>   2) Adding bios_version next to board_type, and changing load order to
>
>     <config>.<dmi_sys_vendor>.<dmi_product_name>.<dmi_bios_version>.txt
>     <config>.<dmi_sys_vendor>.<dmi_product_name>.txt
>     <config>.txt
>
>   3) Adding command-line parameters to override these on problems.
>
> 1) breaks backward compatibility, but the DMI code seems to be quite
> new so hopefully many people don't rely on it yet.
>
> 2) & 3) are backward compatible.

Actually, the configuration file, or nvram file as we tend to call it, does 
not come from EFI/BIOS. There are a few platforms that have the nvram file 
stored in EFI and it's name is well-defined. It does assume there is only 
one brcmfmac device in the system.

Regards,
Arend