Re: [PATCH] arm64: rsi: Add automatic arm-cca-guest module loading

[Jeremy Linton]

Hi Gavin,

Thanks for looking at this!

On 10/29/24 7:23 PM, Gavin Shan wrote:
> Hi Jeremy,
>
> On 10/30/24 12:11 AM, Jeremy Linton wrote:
> > The TSM module provides both guest identification as well as
> > attestation when a guest is run in CCA mode. Lets assure by creating a
> > dummy platform device that the module is automatically loaded during
> > boot. Once it is in place it can be used earlier in the boot process
> > to say decrypt a LUKS rootfs.
> >
> > Signed-off-by: Jeremy Linton <jeremy.linton@xxxxxxx>
> > ---
> >   arch/arm64/include/asm/rsi.h                    |  2 ++
> >   arch/arm64/kernel/rsi.c                         | 15 +++++++++++++++
> >   drivers/virt/coco/arm-cca-guest/arm-cca-guest.c |  7 +++++++
> >   3 files changed, 24 insertions(+)
>
> I don't understand how the TSM module is automatically loaded and 
> arm_cca_guest_init()
> is triggered because of the newly introduced platform device. Could you 
> please provide
> more details? Apart from it, some nick-picks as below.

I think your asking how the module boilerplate here works, AKA how the 
standard uevent/udev/modalias/kmod stuff works? The short version is 
that the platform bus uevents an add device with a modalias and 
userspace udev + kmod finds matching modules, and their dependencies, 
and loads them which triggers the module_init() calls.

The suse folks have a detailed description of how this works:
https://doc.opensuse.org/documentation/leap/reference/html/book-reference/cha-udev.html#sec-udev-kernel

So, this is a fairly common misuse of the platform bus, in this case to 
avoid needing a HWCAP. Assuring the module exists in the initrd will 
then result in it being loaded along any other modules required for the 
rootfs pivot.


> > diff --git a/arch/arm64/include/asm/rsi.h b/arch/arm64/include/asm/rsi.h
> > index 188cbb9b23f5..1b14a4c4257a 100644
> > --- a/arch/arm64/include/asm/rsi.h
> > +++ b/arch/arm64/include/asm/rsi.h
> > @@ -10,6 +10,8 @@
> >   #include <linux/jump_label.h>
> >   #include <asm/rsi_cmds.h>
> > +#define ARMV9_RSI_PDEV_NAME "arm-cca-dev"
> > +
>
> Maybe 'ARMV9' can be avoided since RSI is the specific feature to ARMv9.
> Besides, we already had @rsi_present there. So I would suggest to rename
> it to RSI_PDEV_NAME

This and the remainder of the comments below look reasonable to me, thanks!
> >   DECLARE_STATIC_KEY_FALSE(rsi_present);
> >   void __init arm64_rsi_init(void);
> > diff --git a/arch/arm64/kernel/rsi.c b/arch/arm64/kernel/rsi.c
> > index 3031f25c32ef..ad963eb12921 100644
> > --- a/arch/arm64/kernel/rsi.c
> > +++ b/arch/arm64/kernel/rsi.c
> > @@ -8,6 +8,7 @@
> >   #include <linux/psci.h>
> >   #include <linux/swiotlb.h>
> >   #include <linux/cc_platform.h>
> > +#include <linux/platform_device.h>
> >   #include <asm/io.h>
> >   #include <asm/mem_encrypt.h>
> > @@ -140,3 +141,17 @@ void __init arm64_rsi_init(void)
> >       static_branch_enable(&rsi_present);
> >   }
> > +static struct platform_device rsi_dev = {
> > +    .name = ARMV9_RSI_PDEV_NAME,
> > +    .id = -1
> > +};
> > +
>
>          .id = PLATFORM_DEVID_NONE,
>
> > +static int __init rsi_init(void)
> > +{
> > +    if (is_realm_world())
> > +        if (platform_device_register(&rsi_dev))
> > +            pr_err("failed to register rsi platform device");
> > +    return 0;
> > +}
> > +
>
> Those two checks can be connected with '&&' and '\n' seems missed in the
> error message.
>
>          if (is_realm_world() && platform_device_register(&rsi_dev))
>              pr_err("Failed to register RSI platform device\n");
>
> > +arch_initcall(rsi_init)
> > diff --git a/drivers/virt/coco/arm-cca-guest/arm-cca-guest.c b/ 
> > drivers/virt/coco/arm-cca-guest/arm-cca-guest.c
> > index 488153879ec9..e7ef3b83d5d9 100644
> > --- a/drivers/virt/coco/arm-cca-guest/arm-cca-guest.c
> > +++ b/drivers/virt/coco/arm-cca-guest/arm-cca-guest.c
> > @@ -6,6 +6,7 @@
> >   #include <linux/arm-smccc.h>
> >   #include <linux/cc_platform.h>
> >   #include <linux/kernel.h>
> > +#include <linux/mod_devicetable.h>
> >   #include <linux/module.h>
> >   #include <linux/smp.h>
> >   #include <linux/tsm.h>
> > @@ -219,6 +220,12 @@ static void __exit arm_cca_guest_exit(void)
> >   }
> >   module_exit(arm_cca_guest_exit);
> > +static const struct platform_device_id arm_cca_match[] = {
> > +    { ARMV9_RSI_PDEV_NAME, 0},
> > +    { }
> > +};
> > +
> > +MODULE_DEVICE_TABLE(platform, arm_cca_match);
>
>
> /* Comments here to explain why @arm_cca_dev_ids[] is needed */
> static const struct platform_device_id arm_cca_dev_ids[] = {
>         ...
> };
>
> MODULE_DEVICE_TABLE(platform, arm_cca_dev_ids);
>
> >   MODULE_AUTHOR("Sami Mujawar <sami.mujawar@xxxxxxx>");
> >   MODULE_DESCRIPTION("Arm CCA Guest TSM Driver");
> >   MODULE_LICENSE("GPL");
>
> Thanks,
> Gavin