On Tue, Dec 20, 2022 at 2:03 PM Luis Chamberlain mcgrof@kernel.org wrote:
On Tue, Dec 20, 2022 at 01:49:04PM -0600, Allen Webb wrote:
I took another stab at clarifying (and also dropped the ifdev since the same macro works both for separate and built-in modules:
/*
- Creates an alias so file2alias.c can find device table.
- Use this in cases where a device table is used to match devices because it
- surfaces match-id based module aliases to userspace for:
- Automatic module loading.
- Tools like USBGuard which allow or block devices based on policy such as
which modules match a device.- The module name is included in the alias for two reasons:
- It avoids creating two aliases with the same name for built-in modules.
Historically MODULE_DEVICE_TABLE was a no-op for built-in modules, sothere was nothing to stop different modules from having the same devicetable name and consequently the same alias when building as a module.
- The module name is needed by files2alias.c to associate a particular
device table with its associated module for built-in modules sincefiles2alias would otherwise see the module name as `vmlinuz.o`.*/
This is still weak in light of the questions I had. It does not make it easy for a driver developer who is going to support only built-in only if they need to define this or not. And it seems we're still discussing the merits of this, so I'd wait until this is fleshed out, but I think we are on the right track finally.
The deciding factor in whether it makes sense to remove these vs fix them seems to be, "How complete do we want modules.builtin.alias to be?"
Arguably we should just drop these in cases where there isn't an "authorized" sysfs attribute but following that logic there is not any reason to generate built-in aliases for anything except USB and thunderbolt.
There we go, now we have a *real* use case for this for built-in stuff to consider. Is USBGuard effective even for built-in stuff?
Yes, just because a module is loaded doesn't mean a specific device has probed the driver yet.
Given everything discussed so far I'd like to get clarification if it even help for built-in USB / thunderbolt. Does it? If so how? What could userspace do with this information if the driver is already built-in?
We are not trying to stop the module from being loaded (which is always the case for built-in modules) and in fact it is possible to have devices already using the module and still not authorize (and by extension probe the module for) newly connected devices.
For example someone might have an unattended computer downloading installation media to a USB drive. Presumably this computer would be locked to make it more difficult for a bad actor to access the computer. Since USB storage devices are not needed to interact with the lock screen, we can use the authorized_default sysfs attribute to not allow new USB devices to probe modules by default and have USBGuard vet the devices. Mice, keyboards, etc can be allowed so that the lock screen can still be used (this important in cases like suspend+resume or docks).
On the flip side, if we are going to the effort to make this a generic solution that covers everything, the built-in aliases are only as useful as they are complete, so we would want everything that defines a device table to call the macro correctly.
It is the ambiguity which is terrible to add. If the only use case is for USB and Thunderbolt then we can spell it out, then only those driver developers would care to consider it if the driver is bool. And, a respective tooling would scrape only those drivers to verify if the table is missing for built-in too.
I was aiming to write it so that it wouldn't easily become obsolete by later changes, so tying it to the authorized and authorized_default sysfs attributes is probably the ideal deciding factor and listing USB and thunderbolt as examples makes sense.
It definitely is needed for never-tristate modules that match devices in subsystems that surface the authorized attribute.
What is this "authorized attribute" BTW exactly? Do have some documentation reference?
There are sysfs attributes called authorized and authorized_default that together can prevent devices from being fully enumerated and probed. authorized_default gets set to 0 for the hub and any devices connected after that will show in sysfs, but not fully enumerate or probe until the device's authorized attribute is set to 1. There are some edge cases like internal devices which have some extra complexity.
As for documentation, I wasn't able to find much other than: https://github.com/torvalds/linux/blob/v6.1/drivers/usb/core/hcd.c#L370 /* authorized_default behaviour: * -1 is authorized for all devices except wireless (old behaviour) * 0 is unauthorized for all devices * 1 is authorized for all devices * 2 is authorized for internal devices */ ... and https://github.com/torvalds/linux/blob/v6.1/Documentation/admin-guide/kernel... usbcore.authorized_default= [USB] Default USB device authorization: (default -1 = authorized except for wireless USB, 0 = not authorized, 1 = authorized, 2 = authorized if device connected to internal port) ...
The feature looks like it was originally introduced for wireless USB in: https://www.mail-archive.com/linux-usb-devel@lists.sourceforge.net/msg54289.... and later adapted for use cases like USBGuard here: https://github.com/torvalds/linux/commit/c4fc2342cb611f945fa468e742759e25984...
Luis