Re: [PATCH] firmware: export x86_64 platform flash bios region via sysfs

[Mauro Lima]

On Tue, Nov 9, 2021 at 9:42 AM Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, Nov 09, 2021 at 01:32:48PM +0100, Hans-Gert Dahmen wrote:
> > On Tue, 9 Nov 2021, 11:28 Greg KH, <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > On Tue, Nov 09, 2021 at 09:52:53AM +0100, Hans-Gert Dahmen wrote:
> > > > On 09.11.21 07:16, Greg KH wrote:
> > > > > On Tue, Nov 09, 2021 at 01:01:30AM +0100, Hans-Gert Dahmen wrote:
> > > > > > Make the 16MiB long memory-mapped BIOS region of the platform SPI flash
> > > > > > on X86_64 system available via /sys/kernel/firmware/flash_mmap/bios_region
> > > > > > for pen-testing, security analysis and malware detection on kernels
> > > > > > which restrict module loading and/or access to /dev/mem.
> > > > >
> > > > > That feels like a big security hole we would be opening up for no good
> > > > > reason.
> > > > >
> > > > > > It will be used by the open source Converged Security Suite.
> > > > > > https://github.com/9elements/converged-security-suite
> > > > >
> > > > > What is the reason for this, and what use is this new interface?
> > > > Because it is very hard to access the SPI flash to read the BIOS contents
> > > > for (security) analysis and this works without the more complex and
> > > > unfinished SPI drivers and it does so on a system where we may not access
> > > > the full /dev/mem.
> > >
> > > Why not fix the spi drivers to do this properly?  What is preventing you
> > > from doing that instead of adding a new user/kernel api that you will
> > > have to support for the next 20+ years?
> > >
> >
> > Because this interface we want to use is inherently tied to the
> > workings of x86_64 CPUs. It is very stable and easy to use. The SPI
> > drivers in contrast have a history of being complex, buggy and in
> > general not reliable.
>
> Do you have a pointer to these complex and buggy drivers anywhere?

Right now the intel spi driver has a _(DANGEROUS)_ tag [1], this is
preventing the kernel engineers from compiling the driver into
different distros.
A couple of months ago I tried to modify the driver and make it RO [2]
 but the drivers author told me that they don't want to remove the
tag, even if the driver is compiled without any write/erase
functionality as I proposed.
The driver is fixed as the author claims but, as I said, they want to
preserve the tag.
This patch is close to what we need if we can't use the intel spi mechanism.

>
> > This module will require very little maintenance
> > and will probably work in the future without needing modification for
> > newer platforms. It generally is meant as a fallback to the real SPI
> > flash drivers so that userspace programs are able to provide essential
> > functionality.
>
> If it is a "fallback", how is it going to interact on a system where the
> SPI driver is loaded?
>
> > > > > > +static int __init flash_mmap_init(void)
> > > > > > +{
> > > > > > + int ret;
> > > > > > +
> > > > > > + pdev = platform_device_register_simple("flash_mmap", -1, NULL, 0);
> > > > > > + if (IS_ERR(pdev))
> > > > > > +         return PTR_ERR(pdev);
> > > > > > +
> > > > > > + ret = sysfs_create_group(&pdev->dev.kobj, &flash_mmap_group);
> > > > >
> > > > > You just raced with userspace and lost  >
> > > > > Please set the attribute to the platform driver before you create the
> > > > > device.
> > > > >
> > > >
> > > > Sorry, but I went through tons of code and could not find a single instance
> > > > where I can use a default group for creation without using a probe function
> > > > that does the magic for me. Please help me find the correct way of doing
> > > > this without manually creating and adding kobjects.
> > >
> > > The problem here is that you are abusing the platform driver code and
> > > making a "fake" device that is not attached to anything real, and
> > > without a driver.  That should not be done, as you do not know what
> > > hardware this driver is going to be run on.
> > >
> > > Bind to the real hardware resource please.
> >
> > In a previous mail in June you suggested this is some sort of platform
> > device, that is why I rewrote it as such.
>
> That's fine, but bind to the real platform device that describes this
> hardware!  You are not doing anything like this at all here, you are
> just creating a random device in the sysfs tree and instantly allowing
> userspace access to raw memory.  You are not actually controlling the
> platform device at all.
>
> > The hardware resource here
> > is the south bridge for x86_64 CPUs and the module dependencies will
> > compile it only for these platforms.
>
> What "module dependencies"?  You do realize that distro kernels enable
> all modules to be built. We have an "autoload only the modules that this
> hardware needs" infrastructure that you need to tie into here, you are
> totally ignoring it (despite it being present for almost 20 years
> now...)
>
> > The situation is like, if you
> > have that CPU, you have the hardware, so it is implicitly bound or it
> > just will not execute on a machine code level.
>
> What do you mean "implicitly bound"?  How does this tie into the driver
> model such that it will only be autoloaded, and have the driver bind to
> the hardware, that it knows it can control?
>
> That is what needs to be fixed here, you are just claiming that it can
> talk to the hardware without having any check at all for this.
>
> > I feel like your
> > requirement is somewhat impossible to satisfy and I really don't know
> > what to do. Please, can anyone help me by pointing out a proper
> > example elsewhere in the kernel?
>
> What resource in the system describes the hardware you wish to bind to?
> Write a driver for _that_ resource, and have it be properly autoloaded
> when that resource is present in the system.
>
> You have hundreds of examples running on your system today, but as I do
> not know where this hardware is described specifically, it's hard to be
> able to point you to an example that works like that.
>
> So again, what hardware signature describes the resource you wish to
> bind to?
>
> thanks,
>
> greg k-h

Thanks, Mauro.