Re: [PATCH bpf v6] bpf, cgroup: Fix storage null-ptr-deref after replacing prog

> Syzkaller reported a storage null-ptr-deref issue after replacing prog.
> This occurs in the following scenario:
> 1. prog A, an empty prog, is attached to a cgrp.
> 2. prog B uses BPF_MAP_TYPE_PERCPU_CGROUP_STORAGE and calls the
>    bpf_get_local_storage helper.
> 3. link_update is called to replace prog A with prog B.
>
> The reason is that __cgroup_bpf_replace fails to alloc and assign the
> required cgrp storage for the incoming replacement prog. Consequently,
> the new prog inherits an uninit storage, leading to null-ptr-deref panic
> when kick the new prog.

Is the description "inherits an uninit storage" precise?

Looking at the code, the storage array is not uninitialized in the C
sense. When prog A (which uses no cgroup storage) is attached, both
pl->storage[] and item->cgroup_storage[] are explicitly initialized to
NULL. When compute_effective_progs() copies pl->storage into
item->cgroup_storage via bpf_cgroup_storages_assign(), it copies the
all-NULL array.

So prog B inherits the previous prog's storage array, which is NULL for
the map prog B needs. The distinction also explains the broader case:
if prog A used a different storage map, the inherited pointer would be
non-NULL but belong to the wrong map, leading to a cross-map
out-of-bounds read rather than just a NULL dereference.

Would "the new prog inherits the previous prog's storage array (NULL for
the map the new prog needs)" be more accurate?


> Fix this by rejecting a link update if new_prog's cgroup storage is
> incompatible with link->prog.

Does the commit message capture the full scope of what this fixes?

The message describes only the NULL case from the syzkaller reproducer.
However, cgroup_bpf_storages_compatible() compares map pointers per
stype, so it also rejects the case where prog A uses map M1 and prog B
uses map M2 for the same storage type.

In that case, item->cgroup_storage[stype] would point to M1's live
storage, but prog B's bpf_get_local_storage() would return M1's buffer
while the verifier sized accesses against M2->value_size. Since
bpf_cgroup_storage_calculate_size() sizes the buffer from the owning
map, this would be a cross-map data disclosure or out-of-bounds read,
not just a NULL dereference.

Would it be clearer to mention that the fix also blocks the wrong-map
case to prevent cross-map access?


The commit message opens with "Syzkaller reported" but includes no
Reported-by or Closes tags. Two public syzbot reports match this issue:

  [email protected]
  [email protected]

Both are titled "general protection fault in bpf_get_local_storage" with
KASAN null-ptr-deref at the same location. The reproducers match the
3-step scenario call-for-call.

Should these include the standard syzbot attribution tags so the reports
get auto-closed?

Also, BPF_LINK_UPDATE calls that previously returned 0 now return
-EINVAL. This is an intentional trade-off for a less risky fix, but the
commit message does not mention the uAPI behaviour change. Might that be
worth noting?


> diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> index 4355ccb78a9c..5cc9b2d92fd5 100644
> --- a/kernel/bpf/cgroup.c
> +++ b/kernel/bpf/cgroup.c
> @@ -1018,6 +1018,20 @@ static void replace_effective_prog(struct cgroup *cgrp,
>  	}
>  }
>
> +static bool cgroup_bpf_storages_compatible(struct bpf_prog *old_prog,
> +					   struct bpf_prog *new_prog)
> +{
> +	enum bpf_cgroup_storage_type stype;
> +
> +	for_each_cgroup_storage_type(stype) {
> +		if (old_prog->aux->cgroup_storage[stype] !=
> +		    new_prog->aux->cgroup_storage[stype])
> +			return false;
> +	}
> +
> +	return true;
> +}
> +
>  /**
>   * __cgroup_bpf_replace() - Replace link's program and propagate the change
>   *                          to descendants
> @@ -1056,6 +1070,9 @@ static int __cgroup_bpf_replace(struct cgroup *cgrp,
>  	if (!found)
>  		return -ENOENT;
>
> +	if (!cgroup_bpf_storages_compatible(link->link.prog, new_prog))
> +		return -EINVAL;
> +
>  	cgrp->bpf.revisions[atype] += 1;
>  	old_prog = xchg(&link->link.prog, new_prog);
>  	replace_effective_prog(cgrp, atype, pl);


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