Re: [PATCH] configfs-tsm-report: Fix NULL dereference of tsm_ops

On 4/23/25 2:01 PM, Dan Williams wrote:

Unlike sysfs, the lifetime of configfs objects is controlled by userspace. There is no mechanism for the kernel to find and delete all created config-items. Instead, the configfs-tsm-report mechanism has an expectation that tsm_unregister() can happen at any time and cause established config-item access to start failing.

That expectation is not fully satisfied. While tsm_report_read(), tsm_report_{is,is_bin}_visible(), and tsm_report_make_item() safely fail if tsm_ops have been unregistered, tsm_report_privlevel_store() tsm_report_provider_show() fail to check for ops registration. Add the missing checks for tsm_ops having been removed.

Now, in supporting the ability for tsm_unregister() to always succeed, it leaves the problem of what to do with lingering config-items. The expectation is that the admin that arranges for the ->remove() (unbind) of the ${tsm_arch}-guest driver is also responsible for deletion of all open config-items. Until that deletion happens, ->probe() (reload / bind) of the ${tsm_arch}-guest driver fails.

This allows for emergency shutdown / revocation of attestation interfaces, and requires coordinated restart.

Fixes: 70e6f7e2b985 ("configfs-tsm: Introduce a shared ABI for attestation reports") Cc: stable@vger.kernel.org Cc: Suzuki K Poulose suzuki.poulose@arm.com Cc: Steven Price steven.price@arm.com Cc: Sami Mujawar sami.mujawar@arm.com Cc: Borislav Petkov (AMD) bp@alien8.de Cc: Tom Lendacky thomas.lendacky@amd.com Cc: Kuppuswamy Sathyanarayanan sathyanarayanan.kuppuswamy@linux.intel.com Reported-by: Cedric Xing cedric.xing@intel.com Signed-off-by: Dan Williams dan.j.williams@intel.com

---

Looks good to me

Reviewed-by: Kuppuswamy Sathyanarayanan sathyanarayanan.kuppuswamy@linux.intel.com

drivers/virt/coco/tsm.c | 26 +++++++++++++++++++++++++- 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/drivers/virt/coco/tsm.c b/drivers/virt/coco/tsm.c index 9432d4e303f1..096f4f7c0c11 100644 --- a/drivers/virt/coco/tsm.c +++ b/drivers/virt/coco/tsm.c @@ -15,6 +15,7 @@ static struct tsm_provider { const struct tsm_ops *ops; void *data;

• atomic_t count; } provider; static DECLARE_RWSEM(tsm_rwsem);

@@ -92,6 +93,10 @@ static ssize_t tsm_report_privlevel_store(struct config_item *cfg, if (rc) return rc;

• guard(rwsem_write)(&tsm_rwsem);
• if (!provider.ops)

• return -ENXIO;
  

• /*
  • The valid privilege levels that a TSM might accept, if it accepts a
  • privilege level setting at all, are a max of TSM_PRIVLEVEL_MAX (see

@@ -101,7 +106,6 @@ static ssize_t tsm_report_privlevel_store(struct config_item *cfg, if (provider.ops->privlevel_floor > val || val > TSM_PRIVLEVEL_MAX) return -EINVAL;

• guard(rwsem_write)(&tsm_rwsem); rc = try_advance_write_generation(report); if (rc) return rc;

@@ -115,6 +119,10 @@ static ssize_t tsm_report_privlevel_floor_show(struct config_item *cfg, char *buf) { guard(rwsem_read)(&tsm_rwsem);

• if (!provider.ops)

• return -ENXIO;
  

• return sysfs_emit(buf, "%u\n", provider.ops->privlevel_floor); } CONFIGFS_ATTR_RO(tsm_report_, privlevel_floor);

@@ -217,6 +225,9 @@ CONFIGFS_ATTR_RO(tsm_report_, generation); static ssize_t tsm_report_provider_show(struct config_item *cfg, char *buf) { guard(rwsem_read)(&tsm_rwsem);

• if (!provider.ops)

• return -ENXIO;
  

• return sysfs_emit(buf, "%s\n", provider.ops->name); } CONFIGFS_ATTR_RO(tsm_report_, provider);

@@ -421,12 +432,20 @@ static struct config_item *tsm_report_make_item(struct config_group *group, if (!state) return ERR_PTR(-ENOMEM);

• atomic_inc(&provider.count); config_item_init_type_name(&state->cfg, name, &tsm_report_type); return &state->cfg; }

+static void tsm_report_drop_item(struct config_group *group, struct config_item *item) +{

• config_item_put(item);
• atomic_dec(&provider.count);

+}

• static struct configfs_group_operations tsm_report_group_ops = { .make_item = tsm_report_make_item,
• .drop_item = tsm_report_drop_item, };

static const struct config_item_type tsm_reports_type = { @@ -459,6 +478,11 @@ int tsm_register(const struct tsm_ops *ops, void *priv) return -EBUSY; }

• if (atomic_read(&provider.count)) {

• pr_err("configfs/tsm not empty\n");
  

Nit: I think adding the provider ops name will make the debug log clear.

• return -EBUSY;
  

• }
• provider.ops = ops; provider.data = priv; return 0;