Re: [PATCH v5 5/8] LSM: Create lsm_module_list system call

[Casey Schaufler]

On 1/11/2023 1:07 PM, Paul Moore wrote:
> On Mon, Jan 9, 2023 at 1:09 PM Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote:
>> Create a system call to report the list of Linux Security Modules
>> that are active on the system. The list is provided as an array
>> of LSM ID numbers.
>>
>> The calling application can use this list determine what LSM
>> specific actions it might take. That might include chosing an
>> output format, determining required privilege or bypassing
>> security module specific behavior.
>>
>> Signed-off-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
>> ---
>>  Documentation/userspace-api/lsm.rst |  3 +++
>>  include/linux/syscalls.h            |  1 +
>>  kernel/sys_ni.c                     |  1 +
>>  security/lsm_syscalls.c             | 41 +++++++++++++++++++++++++++++
>>  4 files changed, 46 insertions(+)
> ..
>
>> diff --git a/security/lsm_syscalls.c b/security/lsm_syscalls.c
>> index 55e8bf61ac8a..92af1fcaa654 100644
>> --- a/security/lsm_syscalls.c
>> +++ b/security/lsm_syscalls.c
>> @@ -180,3 +180,44 @@ SYSCALL_DEFINE3(lsm_get_self_attr,
>>         kfree(final);
>>         return rc;
>>  }
>> +
>> +/**
>> + * sys_lsm_module_list - Return a list of the active security modules
>> + * @ids: the LSM module ids
>> + * @size: size of @ids, updated on return
>> + * @flags: reserved for future use, must be zero
>> + *
>> + * Returns a list of the active LSM ids. On success this function
>> + * returns the number of @ids array elements. This value may be zero
>> + * if there are no LSMs active. If @size is insufficient to contain
>> + * the return data -E2BIG is returned and @size is set to the minimum
>> + * required size. In all other cases a negative value indicating the
>> + * error is returned.
>> + */
>> +SYSCALL_DEFINE3(lsm_module_list,
>> +               u32 __user *, ids,
>> +               size_t __user *, size,
>> +               u64, flags)
>> +{
>> +       size_t total_size = lsm_active_cnt * sizeof(*ids);
>> +       size_t usize;
>> +       int i;
>> +
>> +       if (flags)
>> +               return -EINVAL;
>> +
>> +       if (get_user(usize, size))
>> +               return -EFAULT;
>> +
>> +       if (put_user(total_size, size) != 0)
>> +               return -EFAULT;
>> +
>> +       if (usize < total_size)
>> +               return -E2BIG;
>> +
>> +       for (i = 0; i < lsm_active_cnt; i++)
>> +               if (put_user(lsm_idlist[i]->id, ids++))
>> +                       return -EFAULT;
>> +
>> +       return lsm_active_cnt;
>> +}
> Similar to my comments in 4/8, I would probably create a new LSM hook
> for this syscall so that the lsm_ctx is passed through the LSM layer
> directly to the target LSM:
>
>   int security_sys_setselfattr(u64 attr, struct lsm_ctx __user *ctx,
> size_t len);

That seems like a whole lot of work when you can just look it up
in an existing table.

> --
> paul-moore.com