[PATCH v5 4/5] bpf: Only enable BPF LSM hooks when an LSM program is attached

KP Singh kpsingh at kernel.org
Thu Oct 5 16:07:28 UTC 2023


On Thu, Oct 5, 2023 at 3:52 PM Jiri Olsa <olsajiri at gmail.com> wrote:
>
> On Thu, Oct 05, 2023 at 03:27:35PM +0200, KP Singh wrote:
> > On Thu, Oct 5, 2023 at 3:26 PM KP Singh <kpsingh at kernel.org> wrote:
> > >
> > > On Thu, Oct 5, 2023 at 10:09 AM Jiri Olsa <olsajiri at gmail.com> wrote:
> > > >
> > > > On Thu, Sep 28, 2023 at 10:24:09PM +0200, KP Singh wrote:
> > > >
> > > > SNIP
> > > >
> > > > > diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> > > > > index e97aeda3a86b..df9699bce372 100644
> > > > > --- a/kernel/bpf/trampoline.c
> > > > > +++ b/kernel/bpf/trampoline.c
> > > > > @@ -13,6 +13,7 @@
> > > > >  #include <linux/bpf_verifier.h>
> > > > >  #include <linux/bpf_lsm.h>
> > > > >  #include <linux/delay.h>
> > > > > +#include <linux/bpf_lsm.h>
> > > > >
> > > > >  /* dummy _ops. The verifier will operate on target program's ops. */
> > > > >  const struct bpf_verifier_ops bpf_extension_verifier_ops = {
> > > > > @@ -514,7 +515,7 @@ static int __bpf_trampoline_link_prog(struct bpf_tramp_link *link, struct bpf_tr
> > > > >  {
> > > > >       enum bpf_tramp_prog_type kind;
> > > > >       struct bpf_tramp_link *link_exiting;
> >
> > I think this is a typo here. It should be existing, no?
>
> yes, I was wondering about that as well ;-)
>
> jirka
>
> >
> > > > > -     int err = 0;
> > > > > +     int err = 0, num_lsm_progs = 0;
> > > > >       int cnt = 0, i;
> > > > >
> > > > >       kind = bpf_attach_type_to_tramp(link->link.prog);
> > > > > @@ -545,8 +546,14 @@ static int __bpf_trampoline_link_prog(struct bpf_tramp_link *link, struct bpf_tr
> > > > >                       continue;
> > > > >               /* prog already linked */
> > > > >               return -EBUSY;
> > > > > +
> > > > > +             if (link_exiting->link.prog->type == BPF_PROG_TYPE_LSM)
> > > > > +                     num_lsm_progs++;
> > > >
> > > > this looks wrong, it's never reached.. seems like we should add separate
> > > > hlist_for_each_entry loop over trampoline's links for this check/init of
> > > > num_lsm_progs ?
> > > >
> > > > jirka
> > >
> > > Good catch, I missed this during my rebase, after
> > > https://lore.kernel.org/bpf/20220510205923.3206889-2-kuifeng@fb.com/
> > > this condition is basically never reached. I will do a general loop
> > > over to count LSM programs and toggle the hook to true (and same for
> > > unlink).

So, there is something that is unclear about this code, i.e. what
happens when there is an error from bpf_trampoline_update fails and
the link and unlink seem to have different expectations:

* link seems to go back to the linked list and removes the trampoline
and restores the refcount:

[...]
 err = bpf_trampoline_update(tr, true /* lock_direct_mutex */);
 if (err) {
    hlist_del_init(&link->tramp_hlist);
    tr->progs_cnt[kind]--;
  }
 return err;
}


* unlink does restore the side effect (i.e. it does not put the
removed trampoline back and increments the refcount).

hlist_del_init(&link->tramp_hlist);
tr->progs_cnt[kind]--;
return bpf_trampoline_update(tr, true /* lock_direct_mutex */);

However, I think I will make it simpler and enforce the invariant that
if an LSM program is attached, the hook is enabled and vice versa. How
about:

diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
index df9699bce372..4f31384b5637 100644
--- a/kernel/bpf/trampoline.c
+++ b/kernel/bpf/trampoline.c
@@ -511,11 +511,30 @@ static enum bpf_tramp_prog_type
bpf_attach_type_to_tramp(struct bpf_prog *prog)
        }
 }

+static void bpf_trampoline_toggle_lsm(struct bpf_trampoline *tr,
+                                     enum bpf_tramp_prog_type kind)
+{
+       struct bpf_tramp_link *link;
+       volatile bool found = false;
+
+       /* Loop through the links and if any LSM program is attached, ensure
+        * that the hook is enabled.
+        */
+       hlist_for_each_entry(link, &tr->progs_hlist[kind], tramp_hlist) {
+               if (link->link.prog->type == BPF_PROG_TYPE_LSM) {
+                       found  = true;
+                       break;
+               }
+       }
+
+       bpf_lsm_toggle_hook(tr->func.addr, found);
+}
+
 static int __bpf_trampoline_link_prog(struct bpf_tramp_link *link,
struct bpf_trampoline *tr)
 {
        enum bpf_tramp_prog_type kind;
        struct bpf_tramp_link *link_exiting;
-       int err = 0, num_lsm_progs = 0;
+       int err = 0;
        int cnt = 0, i;

        kind = bpf_attach_type_to_tramp(link->link.prog);
@@ -547,15 +566,14 @@ static int __bpf_trampoline_link_prog(struct
bpf_tramp_link *link, struct bpf_tr
                /* prog already linked */
                return -EBUSY;

-               if (link_exiting->link.prog->type == BPF_PROG_TYPE_LSM)
-                       num_lsm_progs++;
        }

-       if (!num_lsm_progs && link->link.prog->type == BPF_PROG_TYPE_LSM)
-               bpf_lsm_toggle_hook(tr->func.addr, true);
-
        hlist_add_head(&link->tramp_hlist, &tr->progs_hlist[kind]);
        tr->progs_cnt[kind]++;
+
+       if (link->link.prog->type == BPF_PROG_TYPE_LSM)
+               bpf_trampoline_toggle_lsm(tr, kind);
+
        err = bpf_trampoline_update(tr, true /* lock_direct_mutex */);
        if (err) {
                hlist_del_init(&link->tramp_hlist);
@@ -578,7 +596,6 @@ static int __bpf_trampoline_unlink_prog(struct
bpf_tramp_link *link, struct bpf_
 {
        struct bpf_tramp_link *link_exiting;
        enum bpf_tramp_prog_type kind;
-       bool lsm_link_found = false;
        int err, num_lsm_progs = 0;

        kind = bpf_attach_type_to_tramp(link->link.prog);
@@ -595,18 +612,14 @@ static int __bpf_trampoline_unlink_prog(struct
bpf_tramp_link *link, struct bpf_
                                     tramp_hlist) {
                        if (link_exiting->link.prog->type == BPF_PROG_TYPE_LSM)
                                num_lsm_progs++;
-
-                       if (link_exiting->link.prog == link->link.prog)
-                               lsm_link_found = true;
                }
        }

        hlist_del_init(&link->tramp_hlist);
        tr->progs_cnt[kind]--;

-       if (lsm_link_found && num_lsm_progs == 1)
-               bpf_lsm_toggle_hook(tr->func.addr, false);
-
+       if (link->link.prog->type == BPF_PROG_TYPE_LSM)
+               bpf_trampoline_toggle_lsm(tr, kind);
        return bpf_trampoline_update(tr, true /* lock_direct_mutex */);
 }


- KP



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