[PATCH v2 1/2] bpf: add bpf_init_inode_xattr kfunc for atomic inode labeling

[Song Liu]

On Mon, May 4, 2026 at 10:14 PM Paul Moore <paul at paul-moore.com> wrote:
[...]
> > diff --git a/fs/bpf_fs_kfuncs.c b/fs/bpf_fs_kfuncs.c
> > index 9d27be058494..193accc00796 100644
> > --- a/fs/bpf_fs_kfuncs.c
> > +++ b/fs/bpf_fs_kfuncs.c
> > @@ -10,6 +10,7 @@
> >  #include <linux/fsnotify.h>
> >  #include <linux/file.h>
> >  #include <linux/kernfs.h>
> > +#include <linux/lsm_hooks.h>
> >  #include <linux/mm.h>
> >  #include <linux/xattr.h>
> >
> > @@ -353,6 +354,97 @@ __bpf_kfunc int bpf_cgroup_read_xattr(struct cgroup *cgroup, const char *name__s
> >  }
> >  #endif /* CONFIG_CGROUPS */
> >
> > +static int bpf_xattrs_used(const struct lsm_xattr_ctx *ctx)
> > +{
> > +       const size_t prefix_len = sizeof(XATTR_BPF_LSM_SUFFIX) - 1;
> > +       int i, n = 0;
> > +
> > +       for (i = 0; i < *ctx->xattr_count; i++) {
> > +               const char *name = ctx->xattrs[i].name;
> > +
> > +               if (name && !strncmp(name, XATTR_BPF_LSM_SUFFIX, prefix_len))
> > +                       n++;
> > +       }
> > +       return n;
> > +}
[...]
> > +
> >  static int bpf_fs_kfuncs_filter(const struct bpf_prog *prog, u32 kfunc_id)
> >  {
> >         if (!btf_id_set8_contains(&bpf_fs_kfunc_set_ids, kfunc_id) ||
> > -           prog->type == BPF_PROG_TYPE_LSM)
> > +           prog->type == BPF_PROG_TYPE_LSM) {
> > +               /* bpf_init_inode_xattr only attaches to inode_init_security. */
> > +               if (kfunc_id == bpf_init_inode_xattr_btf_ids[0] &&
> > +                   prog->aux->attach_btf_id != bpf_lsm_inode_init_security_btf_ids[0])
> > +                       return -EACCES;

We need to mark bpf_init_inode_xattr with KF_RCU (requires a trusted
pointer), then we can remove this check above.

> >                 return 0;
> > +       }
> >         return -EACCES;
> >  }
>
> Perhaps I'm simply not seeing it, but is there a check to ensure that
> there is only one BPF LSM calling into security_inode_init_security()
> at any given time?  With the BPF LSM only reserving a single xattr
> slot, multiple loaded BPF LSM programs providing
> security_inode_init_security() callbacks will be a problem.

I don't think there is such a check. Also, a single BPF LSM function
may call the kfunc multiple times, which is also problematic.

I think we will need to make the default bigger, and also introduce
some realloc mechanism for the worst case scenario. This should
work, but the code might be a bit messy.

Thanks,
Song

>
> > diff --git a/include/linux/security.h b/include/linux/security.h
> > index 41d7367cf403..a2fc72e63ada 100644
> > --- a/include/linux/security.h
> > +++ b/include/linux/security.h
> > @@ -68,6 +68,11 @@ struct watch;
> >  struct watch_notification;
> >  struct lsm_ctx;
> >
[...]