[RFC PATCH 08/29] lsm: get rid of the lsm_names list and do some cleanup

[Paul Moore]

On Wed, Apr 9, 2025 at 7:13 PM Kees Cook <kees at kernel.org> wrote:
>
> On Wed, Apr 09, 2025 at 02:49:53PM -0400, 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 an as-needed basis when
> > userspace reads "/sys/kernel/security/lsm".
> >
> > Signed-off-by: Paul Moore <paul at paul-moore.com>
> > ---
> >  include/linux/lsm_hooks.h |  1 -
> >  security/inode.c          | 27 +++++++++++++++++++--
> >  security/lsm_init.c       | 49 ---------------------------------------
> >  3 files changed, 25 insertions(+), 52 deletions(-)

...

> > @@ -343,8 +345,29 @@ static struct dentry *lsm_dentry;
> >  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 rc, len = 0;
> > +
> > +     for (i = 0; i < lsm_count; i++)
> > +             /* the '+ 1' accounts for either a comma or a NUL terminator */
> > +             len += strlen(lsm_order[i]->id->name) + 1;
> > +
> > +     str = kmalloc(len, GFP_KERNEL);
> > +     if (!str)
> > +             return -ENOMEM;
> > +     str[0] = '\0';
> > +
> > +     i = 0;
> > +     while (i < lsm_count) {
> > +             strcat(str, lsm_order[i]->id->name);
> > +             if (++i < lsm_count)
> > +                     strcat(str, ",");
> > +     }
> > +
> > +     rc = simple_read_from_buffer(buf, count, ppos, str, len);
> > +     kfree(str);
> > +     return rc;
>
> Hrm, at least cache it?

Are you aware of a performance critical use of this?

> Better yet, do this whole thing in a initcall after LSMs are loaded, and
> both can gain __ro_after_init...

I *really* disliked all the stuff we were having to do during boot,
and all the redundant global state we were keeping around.  I'll go
ahead and cache the lsm_read() result local to the function but that's
probably all I'm going to accept at this point in time.

-- 
paul-moore.com