Re: [PATCH v8 4/5] efivarfs: automatically update super block flag

Hi Sumit,

[...]

...

...

...

Wouldn't it be better to detect that the tee device of the supplicant closes and use that to switch the permissions? I get why we need that for the TPM, the entire subsystem needs to send TPM commands *before* the supplicant dies. But this is not needed for the EFI variables case, we could just remount the FS as RO the moment the supplicant dies.

As we discussed offline, we should have a unified approach to notify kernel TEE client drivers. So the approach implemented as part of [0] should address the needs for fTPM as well as EFI.

Ideally yes, we should have a unified approach. But this is a bit different IMHO. In the majority of the cases, the supplicant goes away, we lose access to storage and that's the only thing we care about.

What do you think about in-flight secure storage operations while the tee-supplicant goes away? They can fail abruptly if the tee-supplicant goes away without waiting for them to complete. IMO, the graceful handling should be that the tee-supplicant should be alive when the dependent devices are being closed.

For an EFI variable store case as well isn't that a possible scenario?

I think it is, but that would involve issuing a write and then killing the supplicant immediately. But that would lead to a weird state as well with the graceful exit. IOW we don't block until the operation is finished, if you want to shoot yourself in the foot you can start a write and then unbind. That would lead to a partial EFI variable being written as well no?

...

Only the TPM subsystem is 'special' because it has to perform a shutdown of the device as well. On top of that, I think we should try to avoid the kernel depending on userspace apps as much as possible.

There is only tee-supplicant user-space dependency which has to be handled carefully. I am very much up for RPMB fastpath via kernel and get rid of this dependency as well.

Yes, that is without a doubt our best solution. Jens is already working on that, but the supplicant has an entire ecosystem built around it. There might be cases where using the in-kernel replacement might not be possible.

...

I think it's best if we support both of these and add documentation on why this is happening. Would it be hard to have a combination of both of your patches?

It won't be hard but there can be corner cases as mentioned above.

TBH I am fine with merging it as-is. Both cases aren't ideal so it's mostly a matter of documenting them properly. I can update the docs once we decide what to pull and mention the caveats in detail. I am mostly interested to see what Jan is thinking since he pointed out the behavior initially

Thanks /Ilias

-Sumit

...

Thanks /Ilias

...

-Sumit

...

[0] https://lore.kernel.org/all/20230728134832.326467-1-sumit.garg@linaro.org/

Regards /Ilias

...

Signed-off-by: Masahisa Kojima masahisa.kojima@linaro.org

drivers/firmware/efi/efi.c | 6 ++++++ drivers/firmware/efi/vars.c | 8 ++++++++ fs/efivarfs/super.c | 33 +++++++++++++++++++++++++++++++++ include/linux/efi.h | 8 ++++++++ 4 files changed, 55 insertions(+)

diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c index 53ae25bbb6ac..d2eec5ed8e5e 100644 --- a/drivers/firmware/efi/efi.c +++ b/drivers/firmware/efi/efi.c @@ -32,6 +32,7 @@ #include <linux/ucs2_string.h> #include <linux/memblock.h> #include <linux/security.h> +#include <linux/notifier.h>

#include <asm/early_ioremap.h>

@@ -187,6 +188,9 @@ static const struct attribute_group efi_subsys_attr_group = { .is_visible = efi_attr_is_visible, };

+struct blocking_notifier_head efivar_ops_nh; +EXPORT_SYMBOL_GPL(efivar_ops_nh);

static struct efivars generic_efivars; static struct efivar_operations generic_ops;

@@ -427,6 +431,8 @@ static int __init efisubsys_init(void) platform_device_register_simple("efivars", 0, NULL, 0); }

•   BLOCKING_INIT_NOTIFIER_HEAD(&efivar_ops_nh);
  

•   error = sysfs_create_group(efi_kobj, &efi_subsys_attr_group);
    if (error) {
            pr_err("efi: Sysfs attribute export failed with error %d.\n",
  

diff --git a/drivers/firmware/efi/vars.c b/drivers/firmware/efi/vars.c index e9dc7116daf1..f654e6f6af87 100644 --- a/drivers/firmware/efi/vars.c +++ b/drivers/firmware/efi/vars.c @@ -63,6 +63,7 @@ int efivars_register(struct efivars *efivars, const struct efivar_operations *ops) { int rv;

•   int event;
  
    if (down_interruptible(&efivars_lock))
            return -EINTR;
  

@@ -77,6 +78,13 @@ int efivars_register(struct efivars *efivars,

    __efivars = efivars;

•   if (efivar_supports_writes())
  

•           event = EFIVAR_OPS_RDWR;
  

•   else
  

•           event = EFIVAR_OPS_RDONLY;
  

•   blocking_notifier_call_chain(&efivar_ops_nh, event, NULL);
  

•   pr_info("Registered efivars operations\n");
    rv = 0;
  

out: diff --git a/fs/efivarfs/super.c b/fs/efivarfs/super.c index e028fafa04f3..0f6e4d223aea 100644 --- a/fs/efivarfs/super.c +++ b/fs/efivarfs/super.c @@ -14,11 +14,36 @@ #include <linux/slab.h> #include <linux/magic.h> #include <linux/statfs.h> +#include <linux/notifier.h>

#include "internal.h"

LIST_HEAD(efivarfs_list);

+struct efivarfs_info {

•   struct super_block *sb;
  

•   struct notifier_block nb;
  

+};

+static struct efivarfs_info info;

+static int efivarfs_ops_notifier(struct notifier_block *nb, unsigned long event,

•                            void *data)
  

+{

•   switch (event) {
  

•   case EFIVAR_OPS_RDONLY:
  

•           info.sb->s_flags |= SB_RDONLY;
  

•           break;
  

•   case EFIVAR_OPS_RDWR:
  

•           info.sb->s_flags &= ~SB_RDONLY;
  

•           break;
  

•   default:
  

•           return NOTIFY_DONE;
  

•   }
  

•   return NOTIFY_OK;
  

+}

static void efivarfs_evict_inode(struct inode *inode) { clear_inode(inode); @@ -255,6 +280,12 @@ static int efivarfs_fill_super(struct super_block *sb, struct fs_context *fc) if (!root) return -ENOMEM;

•   info.sb = sb;
  

•   info.nb.notifier_call = efivarfs_ops_notifier;
  

•   err = blocking_notifier_chain_register(&efivar_ops_nh, &info.nb);
  

•   if (err)
  

•           return err;
  

•   INIT_LIST_HEAD(&efivarfs_list);
  
    err = efivar_init(efivarfs_callback, (void *)sb, true, &efivarfs_list);
  

@@ -281,6 +312,8 @@ static int efivarfs_init_fs_context(struct fs_context *fc)

static void efivarfs_kill_sb(struct super_block *sb) {

•   blocking_notifier_chain_unregister(&efivar_ops_nh, &info.nb);
  

•   info.sb = NULL;
    kill_litter_super(sb);
  
    if (!efivar_is_available())
  

diff --git a/include/linux/efi.h b/include/linux/efi.h index 603bba2d6437..17cd628b5c42 100644 --- a/include/linux/efi.h +++ b/include/linux/efi.h @@ -1365,6 +1365,14 @@ bool efi_config_table_is_usable(const efi_guid_t *guid, unsigned long table)

umode_t efi_attr_is_visible(struct kobject *kobj, struct attribute *attr, int n);

+/*

• • efivar ops event type
• */

+#define EFIVAR_OPS_RDONLY 0 +#define EFIVAR_OPS_RDWR 1

+extern struct blocking_notifier_head efivar_ops_nh;

void efivars_generic_ops_register(void); void efivars_generic_ops_unregister(void);

-- 2.30.2