bpf: Fix KASAN use-after-free Read in compute_effective_progs

Syzbot found a Use After Free bug in compute_effective_progs().
The reproducer creates a number of BPF links, and causes a fault
injected alloc to fail, while calling bpf_link_detach on them.
Link detach triggers the link to be freed by bpf_link_free(),
which calls __cgroup_bpf_detach() and update_effective_progs().
If the memory allocation in this function fails, the function restores
the pointer to the bpf_cgroup_link on the cgroup list, but the memory
gets freed just after it returns. After this, every subsequent call to
update_effective_progs() causes this already deallocated pointer to be
dereferenced in prog_list_length(), and triggers KASAN UAF error.

To fix this issue don't preserve the pointer to the prog or link in the
list, but remove it and replace it with a dummy prog without shrinking
the table. The subsequent call to __cgroup_bpf_detach() or
__cgroup_bpf_detach() will correct it.

Fixes: af6eea57437a ("bpf: Implement bpf_link-based cgroup BPF program attachment")
Reported-by: <syzbot+f264bffdfbd5614f3bb2@syzkaller.appspotmail.com>
Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Cc: <stable@vger.kernel.org>
Link: https://syzkaller.appspot.com/bug?id=8ebf179a95c2a2670f7cf1ba62429ec044369db4
Link: https://lore.kernel.org/bpf/20220517180420.87954-1-tadeusz.struk@linaro.org

diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index afb414b..7a394f7 100644 (file)
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -721,6 +721,60 @@ static struct bpf_prog_list *find_detach_entry(struct list_head *progs,
 }
 
 /**
+ * purge_effective_progs() - After compute_effective_progs fails to alloc new
+ *                           cgrp->bpf.inactive table we can recover by
+ *                           recomputing the array in place.
+ *
+ * @cgrp: The cgroup which descendants to travers
+ * @prog: A program to detach or NULL
+ * @link: A link to detach or NULL
+ * @atype: Type of detach operation
+ */
+static void purge_effective_progs(struct cgroup *cgrp, struct bpf_prog *prog,
+                                 struct bpf_cgroup_link *link,
+                                 enum cgroup_bpf_attach_type atype)
+{
+       struct cgroup_subsys_state *css;
+       struct bpf_prog_array *progs;
+       struct bpf_prog_list *pl;
+       struct list_head *head;
+       struct cgroup *cg;
+       int pos;
+
+       /* recompute effective prog array in place */
+       css_for_each_descendant_pre(css, &cgrp->self) {
+               struct cgroup *desc = container_of(css, struct cgroup, self);
+
+               if (percpu_ref_is_zero(&desc->bpf.refcnt))
+                       continue;
+
+               /* find position of link or prog in effective progs array */
+               for (pos = 0, cg = desc; cg; cg = cgroup_parent(cg)) {
+                       if (pos && !(cg->bpf.flags[atype] & BPF_F_ALLOW_MULTI))
+                               continue;
+
+                       head = &cg->bpf.progs[atype];
+                       list_for_each_entry(pl, head, node) {
+                               if (!prog_list_prog(pl))
+                                       continue;
+                               if (pl->prog == prog && pl->link == link)
+                                       goto found;
+                               pos++;
+                       }
+               }
+found:
+               BUG_ON(!cg);
+               progs = rcu_dereference_protected(
+                               desc->bpf.effective[atype],
+                               lockdep_is_held(&cgroup_mutex));
+
+               /* Remove the program from the array */
+               WARN_ONCE(bpf_prog_array_delete_safe_at(progs, pos),
+                         "Failed to purge a prog from array at index %d", pos);
+       }
+}
+
+/**
  * __cgroup_bpf_detach() - Detach the program or link from a cgroup, and
  *                         propagate the change to descendants
  * @cgrp: The cgroup which descendants to traverse
@@ -739,7 +793,6 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,
        struct bpf_prog_list *pl;
        struct list_head *progs;
        u32 flags;
-       int err;
 
        atype = to_cgroup_bpf_attach_type(type);
        if (atype < 0)
@@ -761,9 +814,12 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,
        pl->prog = NULL;
        pl->link = NULL;
 
-       err = update_effective_progs(cgrp, atype);
-       if (err)
-               goto cleanup;
+       if (update_effective_progs(cgrp, atype)) {
+               /* if update effective array failed replace the prog with a dummy prog*/
+               pl->prog = old_prog;
+               pl->link = link;
+               purge_effective_progs(cgrp, old_prog, link, atype);
+       }
 
        /* now can actually delete it from this cgroup list */
        list_del(&pl->node);
@@ -775,12 +831,6 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,
                bpf_prog_put(old_prog);
        static_branch_dec(&cgroup_bpf_enabled_key[atype]);
        return 0;
-
-cleanup:
-       /* restore back prog or link */
-       pl->prog = old_prog;
-       pl->link = link;
-       return err;
 }
 
 static int cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,