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

[Paul Moore <paul@paul-moore.com>]

On Fri, Jul 25, 2025 at 10:27 AM Casey Schaufler <casey@schaufler-ca.com> wrote:
> On 7/24/2025 7:28 PM, Paul Moore wrote:
> > On Thu, Jul 24, 2025 at 11:39 AM Casey Schaufler <casey@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@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.

Ah, yes, gotcha.  Thanks for catching this, the fix will be in the
next revision.

-- 
paul-moore.com