[RFC PATCH v2 11/34] lsm: get rid of the lsm_names list and do some cleanup

[Casey Schaufler]

On 7/24/2025 7:28 PM, Paul Moore wrote:
> On Thu, Jul 24, 2025 at 11:39 AM Casey Schaufler <casey at schaufler-ca.com> wrote:
>> On 7/21/2025 4:21 PM, Paul Moore wrote:
>>> The LSM currently has a lot of code to maintain a list of the currently
>>> active LSMs in a human readable string, with the only user being the
>>> "/sys/kernel/security/lsm" code.  Let's drop all of that code and
>>> generate the string on first use and then cache it for subsequent use.
>>>
>>> Signed-off-by: Paul Moore <paul at paul-moore.com>
>>> ---
>>>  include/linux/lsm_hooks.h |  1 -
>>>  security/inode.c          | 59 +++++++++++++++++++++++++++++++++++++--
>>>  security/lsm_init.c       | 49 --------------------------------
>>>  3 files changed, 57 insertions(+), 52 deletions(-)
> ..
>
>>> +/* NOTE: we never free the string below once it is set. */
>>> +static DEFINE_SPINLOCK(lsm_read_lock);
>>> +static char *lsm_read_str = NULL;
>>> +static ssize_t lsm_read_len = 0;
>>> +
>>>  static ssize_t lsm_read(struct file *filp, char __user *buf, size_t count,
>>>                       loff_t *ppos)
>>>  {
>>> -     return simple_read_from_buffer(buf, count, ppos, lsm_names,
>>> -             strlen(lsm_names));
>>> +     int i;
>>> +     char *str;
>>> +     ssize_t len;
>>> +
>>> +restart:
>>> +
>>> +     rcu_read_lock();
>>> +     if (!lsm_read_str) {
>>> +             /* we need to generate the string and try again */
>>> +             rcu_read_unlock();
>>> +             goto generate_string;
>>> +     }
>>> +     len = simple_read_from_buffer(buf, count, ppos,
>>> +                                   rcu_dereference(lsm_read_str),
>>> +                                   lsm_read_len);
>>> +     rcu_read_unlock();
>>> +     return len;
>>> +
>>> +generate_string:
>>> +
>>> +     for (i = 0; i < lsm_active_cnt; i++)
>>> +             /* the '+ 1' accounts for either a comma or a NUL */
>>> +             len += strlen(lsm_idlist[i]->name) + 1;
>>> +
>>> +     str = kmalloc(len, GFP_KERNEL);
>>> +     if (!str)
>>> +             return -ENOMEM;
>>> +     str[0] = '\0';
>>> +
>>> +     for (i = 0; i < lsm_active_cnt; i++) {
>>> +             if (i > 0)
>>> +                     strcat(str, ",");
>>> +             strcat(str, lsm_idlist[i]->name);
>>> +     }
>>> +
>>> +     spin_lock(&lsm_read_lock);
>>> +     if (lsm_read_str) {
>>> +             /* we raced and lost */
>>> +             spin_unlock(&lsm_read_lock);
>>> +             kfree(str);
>>> +             goto restart;
>>> +     }
>>> +     lsm_read_str = str;
>>> +     lsm_read_len = len;
>> You're going to get a nul byte at the end of the string because
>> you accounted for the ',' above, but there isn't one at the end
>> of the string.
> I'm not sure I understand your concern here, can you phrase it differently?

"lockdown,capability,...,evm\0" You get the '\0' because you always expect
a trailing ','. On the last element there is no ',' but the length is added
as if there is.

+	lsm_read_len = len - 1;

will fix the problem.

>
> If you're worried about lsm_read_str potentially not being terminated
> with a NUL byte, the strcat() should copy over the trailing NUL.
>
>>> +     spin_unlock(&lsm_read_lock);
>>> +
>>> +     goto restart;
>>>  }