Re: [PATCH v2] bpf: Fix KASAN use-after-free Read in compute_effective_progs

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Hi Andrii,
On 4/20/22 10:07, Andrii Nakryiko wrote:
diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index 128028efda64..5a64cece09f3 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -723,10 +723,8 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,
         pl->link = NULL;

         err = update_effective_progs(cgrp, atype);
-       if (err)
-               goto cleanup;

-       /* now can actually delete it from this cgroup list */
+       /* now can delete it from this cgroup list */
         list_del(&pl->node);
         kfree(pl);
         if (list_empty(progs))
@@ -735,12 +733,55 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,
         if (old_prog)
                 bpf_prog_put(old_prog);
         static_branch_dec(&cgroup_bpf_enabled_key[atype]);
-       return 0;
+
+       if (!err)
+               return 0;

  cleanup:
-       /* restore back prog or link */
-       pl->prog = old_prog;
-       pl->link = link;
+       /*
+        * If compute_effective_progs failed with -ENOMEM, i.e. alloc for
+        * cgrp->bpf.inactive table failed, we can recover by removing
+        * the detached prog from effective table and rearranging it.
+        */
+       if (err == -ENOMEM) {
+               struct bpf_prog_array_item *item;
+               struct bpf_prog *prog_tmp, *prog_detach, *prog_last;
+               struct bpf_prog_array *array;
+               int index = 0, index_detach = -1;
+
+               array = cgrp->bpf.effective[atype];
+               item = &array->items[0];
+
+               if (prog)
+                       prog_detach = prog;
+               else
+                       prog_detach = link->link.prog;
+
+               if (!prog_detach)
+                       return -EINVAL;
+
+               while ((prog_tmp = READ_ONCE(item->prog))) {
+                       if (prog_tmp == prog_detach)
+                               index_detach = index;
+                       item++;
+                       index++;
+                       prog_last = prog_tmp;
+               }
+
+               /* Check if we found what's needed for removing the prog */
+               if (index_detach == -1 || index_detach == index-1)
+                       return -EINVAL;
+
+               /* Remove the last program in the array */
+               if (bpf_prog_array_delete_safe_at(array, index-1))
+                       return -EINVAL;
+
+               /* and update the detached with the last just removed */
+               if (bpf_prog_array_update_at(array, index_detach, prog_last))
+                       return -EINVAL;
+
+               err = 0;
+       }

Thanks for feedback, and sorry for delay. I got pulled into something else.

There are a bunch of problems with this implementation.

1. We should do this fallback right after update_effective_progs()
returns error, before we get to list_del(&pl->node) and subsequent
code that does some additional things (like clearing flags and stuff).
This additional code needs to run even if update_effective_progs()
fails. So I suggest to extract the logic of removing program from
effective prog arrays into a helper function and doing

err = update_effective_progs(...);
if (err)
     purge_effective_progs();

where purge_effective_progs() will be the logic you are adding. And it
will be void function because it can't fail.

I have implemented that in v3, will send that out soon.


2. We have to update not just cgrp->bpf.effective array, but all the
descendants' lists as well. See what update_effective_progs() is
doing, it has css_for_each_descendant_pre() iteration. You need to do
it here as well. But instead of doing compute_effective_progs() which
allocates a new copy of an array we'll need to update existing array
in place.

3. Not clear why you need to do both bpf_prog_array_delete_safe_at()
and bpf_prog_array_update_at(), isn't delete_safe_at() enought?

I thought that we need to reshuffle the table and move the progs around,
but your are right, delete_safe_at() is enough.

--
Thanks,
Tadeusz



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