On Thu, Jul 24, 2025 at 11:39 AM Casey Schaufler <ca...@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 <p...@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?
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;
> > }
--
paul-moore.com
