On 8/20/25 16:21, Mickaël Salaün wrote:
> On Wed, Jul 09, 2025 at 10:00:55AM +0200, Maxime Bélair wrote:
>> Define two new LSM hooks: security_lsm_config_self_policy and
>> security_lsm_config_system_policy and wire them into the corresponding
>> lsm_config_*_policy() syscalls so that LSMs can register a unified
>> interface for policy management. This initial, minimal implementation
>> only supports the LSM_POLICY_LOAD operation to limit changes.
>>
>> Signed-off-by: Maxime Bélair <[email protected]>
>> ---
>> include/linux/lsm_hook_defs.h | 4 +++
>> include/linux/security.h | 20 ++++++++++++
>> include/uapi/linux/lsm.h | 8 +++++
>> security/lsm_syscalls.c | 17 ++++++++--
>> security/security.c | 60 +++++++++++++++++++++++++++++++++++
>> 5 files changed, 107 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
>> index bf3bbac4e02a..fca490444643 100644
>> --- a/include/linux/lsm_hook_defs.h
>> +++ b/include/linux/lsm_hook_defs.h
>> @@ -464,3 +464,7 @@ LSM_HOOK(int, 0, bdev_alloc_security, struct
>> block_device *bdev)
>> LSM_HOOK(void, LSM_RET_VOID, bdev_free_security, struct block_device *bdev)
>> LSM_HOOK(int, 0, bdev_setintegrity, struct block_device *bdev,
>> enum lsm_integrity_type type, const void *value, size_t size)
>> +LSM_HOOK(int, -EINVAL, lsm_config_self_policy, u32 lsm_id, u32 op,
>> + void __user *buf, size_t size, u32 flags)
>> +LSM_HOOK(int, -EINVAL, lsm_config_system_policy, u32 lsm_id, u32 op,
>> + void __user *buf, size_t size, u32 flags)
>> diff --git a/include/linux/security.h b/include/linux/security.h
>> index cc9b54d95d22..54acaee4a994 100644
>> --- a/include/linux/security.h
>> +++ b/include/linux/security.h
>> @@ -581,6 +581,11 @@ void security_bdev_free(struct block_device *bdev);
>> int security_bdev_setintegrity(struct block_device *bdev,
>> enum lsm_integrity_type type, const void *value,
>> size_t size);
>> +int security_lsm_config_self_policy(u32 lsm_id, u32 op, void __user *buf,
>> + size_t size, u32 flags);
>> +int security_lsm_config_system_policy(u32 lsm_id, u32 op, void __user *buf,
>> + size_t size, u32 flags);
>> +
>> #else /* CONFIG_SECURITY */
>>
>> /**
>> @@ -1603,6 +1608,21 @@ static inline int security_bdev_setintegrity(struct
>> block_device *bdev,
>> return 0;
>> }
>>
>> +static inline int security_lsm_config_self_policy(u32 lsm_id, u32 op,
>> + void __user *buf,
>> + size_t size, u32 flags)
>> +{
>> +
>> + return -EOPNOTSUPP;
>> +}
>> +
>> +static inline int security_lsm_config_system_policy(u32 lsm_id, u32 op,
>> + void __user *buf,
>> + size_t size, u32 flags)
>> +{
>> +
>> + return -EOPNOTSUPP;
>> +}
>> #endif /* CONFIG_SECURITY */
>>
>> #if defined(CONFIG_SECURITY) && defined(CONFIG_WATCH_QUEUE)
>> diff --git a/include/uapi/linux/lsm.h b/include/uapi/linux/lsm.h
>> index 938593dfd5da..2b9432a30cdc 100644
>> --- a/include/uapi/linux/lsm.h
>> +++ b/include/uapi/linux/lsm.h
>> @@ -90,4 +90,12 @@ struct lsm_ctx {
>> */
>> #define LSM_FLAG_SINGLE 0x0001
>>
>> +/*
>> + * LSM_POLICY_XXX definitions identify the different operations
>> + * to configure LSM policies
>> + */
>> +
>> +#define LSM_POLICY_UNDEF 0
>> +#define LSM_POLICY_LOAD 100
>
> Why the gap between 0 and 100?
>
>> +
>> #endif /* _UAPI_LINUX_LSM_H */
>> diff --git a/security/lsm_syscalls.c b/security/lsm_syscalls.c
>> index a3cb6dab8102..dd016ba6976c 100644
>> --- a/security/lsm_syscalls.c
>> +++ b/security/lsm_syscalls.c
>> @@ -122,11 +122,24 @@ SYSCALL_DEFINE3(lsm_list_modules, u64 __user *, ids,
>> u32 __user *, size,
>> SYSCALL_DEFINE5(lsm_config_self_policy, u32, lsm_id, u32, op, void __user *,
>> buf, u32 __user *, size, u32, flags)
>
> Given these are a multiplexor syscalls, I'm wondering if they should not
> have common flags and LSM-specific flags. Alternatively, the op
> argument could also contains some optional flags. In either case, the
> documentation should guide LSM developers for flags that may be shared
> amongst LSMs.
>
> Examples of such flags could be to restrict the whole process instead of
> the calling thread.
>
Indeed, in v6 I used both common_flags and flags. For now I didn't
support any of them to keep this patchset simple but we could discuss
which flags we want to support.
>> {
>> - return 0;
>> + size_t usize;
>> +
>> + if (get_user(usize, size))
>
> Size should just be u32, not a pointer.
Indeed
>
>> + return -EFAULT;
>> +
>> + return security_lsm_config_self_policy(lsm_id, op, buf, usize, flags);
>> }
>>
>> SYSCALL_DEFINE5(lsm_config_system_policy, u32, lsm_id, u32, op, void __user
>> *,
>> buf, u32 __user *, size, u32, flags)
>> {
>> - return 0;
>> + size_t usize;
>> +
>> + if (!capable(CAP_SYS_ADMIN))
>> + return -EPERM;
>
> I like this mandatory capability check for this specific syscall. This
> makes the semantic clearer. However, to avoid the superpower of
> CAP_SYS_ADMIN, I'm wondering how we could use the CAP_MAC_ADMIN instead.
> This syscall could require CAP_MAC_ADMIN, and current LSMs (relying on a
> filesystem interface for policy configuration) could also enforce
> CAP_SYS_ADMIN for compatibility reasons.
I agree and lsm_config_system_policy is now restricted to CAP_MAC_ADMIN
in v6.
>
> In fact, this "system" syscall could be a "namespace" syscall, which
> would take a security/LSM namespace file descriptor as argument. If the
> namespace is not the initial namespace, any CAP_SYS_ADMIN implemented by
> current LSMs could be avoided. See
> https://lore.kernel.org/r/CAHC9VhRGMmhxbajwQNfGFy+ZFF1uN=uebjqqzq4uby7yds3...@mail.gmail.com
I would appreciate additional feedback on the best way to handle
namespaces for this syscall.
Possible approaches include:
- Passing a value in buf (as I did patch v6 3/5 for AppArmor). This is
simple and let individual LSM handle namespaces as see fit. However,
it may slightly complicate the policy format.
- Passing a file descriptor as a syscall argument. This offers a cleaner
interface but couples the pseudofs to this syscall, reducing some of
its advantages.
- Providing no support for namespaces at this time.
I tend to prefer the first approach here but I'm open to suggestions
>
>> +
>> + if (get_user(usize, size))
>
> ditto
>
>> + return -EFAULT;
>> +
>> + return security_lsm_config_system_policy(lsm_id, op, buf, usize, flags);
>> }
>> diff --git a/security/security.c b/security/security.c
>> index fb57e8fddd91..166d7d9936d0 100644
>> --- a/security/security.c
>> +++ b/security/security.c
>> @@ -5883,6 +5883,66 @@ int security_bdev_setintegrity(struct block_device
>> *bdev,
>> }
>> EXPORT_SYMBOL(security_bdev_setintegrity);
>>
>> +/**
>> + * security_lsm_config_self_policy() - Configure caller's LSM policies
>> + * @lsm_id: id of the LSM to target
>> + * @op: Operation to perform (one of the LSM_POLICY_XXX values)
>> + * @buf: userspace pointer to policy data
>> + * @size: size of @buf
>> + * @flags: lsm policy configuration flags
>> + *
>> + * Configure the policies of a LSM for the current domain/user. This notably
>> + * allows to update them even when the lsmfs is unavailable or restricted.
>> + * Currently, only LSM_POLICY_LOAD is supported.
>> + *
>> + * Return: Returns 0 on success, error on failure.
>> + */
>> +int security_lsm_config_self_policy(u32 lsm_id, u32 op, void __user *buf,
>> + size_t size, u32 flags)
>> +{
>> + int rc = LSM_RET_DEFAULT(lsm_config_self_policy);
>> + struct lsm_static_call *scall;
>> +
>> + lsm_for_each_hook(scall, lsm_config_self_policy) {
>> + if ((scall->hl->lsmid->id) == lsm_id) {
>> + rc = scall->hl->hook.lsm_config_self_policy(lsm_id, op,
>> buf, size, flags);
>
> The lsm_id should not be passed to the hook.
Indeed
>
> The LSM syscall should manage the argument copy and buffer allocation
> instead of duplicating this code in each LSM hook implementation (see
> other LSM syscalls).
I get your point but methods used internally by LSMs already handle the
allocation themselves through a char __user * parameter.
- smack: smk_write_rules_list
- selinux: sel_write_load
- apparmor: policy_update
Hence, I think that it's actually better to let LSMs handle allocations
>
>> + break;
>> + }
>> + }
>> +
>> + return rc;
>> +}
>> +
>> +/**
>> + * security_lsm_config_system_policy() - Configure system LSM policies
>> + * @lsm_id: id of the lsm to target
>> + * @op: Operation to perform (one of the LSM_POLICY_XXX values)
>> + * @buf: userspace pointer to policy data
>> + * @size: size of @buf
>> + * @flags: lsm policy configuration flags
>> + *
>> + * Configure the policies of a LSM for the whole system. This notably allows
>> + * to update them even when the lsmfs is unavailable or restricted.
>> Currently,
>> + * only LSM_POLICY_LOAD is supported.
>> + *
>> + * Return: Returns 0 on success, error on failure.
>> + */
>> +int security_lsm_config_system_policy(u32 lsm_id, u32 op, void __user *buf,
>> + size_t size, u32 flags)
>> +{
>> + int rc = LSM_RET_DEFAULT(lsm_config_system_policy);
>> + struct lsm_static_call *scall;
>> +
>> + lsm_for_each_hook(scall, lsm_config_system_policy) {
>> + if ((scall->hl->lsmid->id) == lsm_id) {
>> + rc = scall->hl->hook.lsm_config_system_policy(lsm_id,
>> op, buf, size, flags);
>
> ditto
>
>> + break;
>> + }
>> + }
>> +
>> + return rc;
>> +}
>> +
>> #ifdef CONFIG_PERF_EVENTS
>> /**
>> * security_perf_event_open() - Check if a perf event open is allowed
>> --
>> 2.48.1
>>
>>
