bpf: Fix possible memleak when updating hash maps

[kernel-patches-daemon-bpf]

Pull request for series with
subject: bpf: Fix possible memleak when updating hash maps
version: 1
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=1012411

[kernel-patches-daemon-bpf]

Upstream branch: 5fb750e
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=1012411
version: 1

[kernel-patches-review-bot]

commit c50bee1f62c487dddb09bd7cb2328c082c0c7244
Author: Leon Hwang <leon.hwang@linux.dev>

bpf: Fix possible memleak when updating hash maps with BPF_F_LOCK

This patch fixes a memory leak in hash map updates with BPF_F_LOCK by
adding check_and_free_fields() calls after copy_map_value_locked() to
properly free old kptr field values that were replaced.

> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> index 26308adc9..65009ea3e 100644
> --- a/kernel/bpf/hashtab.c
> +++ b/kernel/bpf/hashtab.c
> @@ -1124,6 +1124,7 @@ static long htab_map_update_elem(struct bpf_map *map, void *key, void *value,
>  			copy_map_value_locked(map,
>  					      htab_elem_value(l_old, key_size),
>  					      value, false);
> +			check_and_free_fields(htab, l_old);
>  			return 0;
>  		}
>  		/* fall through, grab the bucket lock and lookup again.
> @@ -1152,6 +1153,7 @@ static long htab_map_update_elem(struct bpf_map *map, void *key, void *value,
>  		copy_map_value_locked(map,
>  				      htab_elem_value(l_old, key_size),
>  				      value, false);
> +		check_and_free_fields(htab, l_old);
>  		ret = 0;
>  		goto err;
>  	}

The fix looks correct for hash maps. However, does the same issue exist
in other map types that use copy_map_value_locked() with BPF_F_LOCK?

In bpf_local_storage_update() at kernel/bpf/bpf_local_storage.c, there
are two code paths that perform in-place updates with BPF_F_LOCK:

1. Fast path without bucket lock (around line 610):

	if (old_sdata && selem_linked_to_storage_lockless(SELEM(old_sdata))) {
		copy_map_value_locked(&smap->map, old_sdata->data,
				      value, false);
		return old_sdata;
	}

Can the old kptr fields leak here when copy_map_value_locked() replaces
them without calling bpf_obj_free_fields() on old_sdata->data?

2. Slow path with bucket lock held (around line 642):

	if (old_sdata && (map_flags & BPF_F_LOCK)) {
		copy_map_value_locked(&smap->map, old_sdata->data, value,
				      false);
		selem = SELEM(old_sdata);
		goto unlock;
	}

Does this path also leak the old kptr fields in old_sdata->data?

Note that other cleanup paths in bpf_local_storage.c do call
bpf_obj_free_fields() properly (lines 217, 240, 252), which suggests
these in-place update paths should too.

Similarly, in cgroup_storage_update_elem() at kernel/bpf/local_storage.c
around line 163:

	if (flags & BPF_F_LOCK) {
		copy_map_value_locked(map, storage->buf->data, value, false);
		return 0;
	}

Can the old kptr fields in storage->buf->data leak here as well?

For reference, arraymap.c already has the correct pattern at line 391
where it calls bpf_obj_free_fields() after copy_map_value_locked().

---

AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

In-Reply-To-Subject: bpf: Fix possible memleak when updating hash maps with BPF_F_LOCK
CI run summary: https://github.com/kernel-patches/bpf/actions/runs/18565916076

[kernel-patches-daemon-bpf]

Forwarding comment 3411425561 via email
In-Reply-To: 20251016145801.47552-3-leon.hwang@linux.dev
Patch: https://patchwork.kernel.org/project/netdevbpf/patch/20251016145801.47552-3-leon.hwang@linux.dev/

[kernel-patches-daemon-bpf]

Upstream branch: 0c1999e
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=1012411
version: 1

[kernel-patches-daemon-bpf]

Upstream branch: 8adc470
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=1012411
version: 1

[kernel-patches-daemon-bpf]

Upstream branch: f6fddc6
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=1012411
version: 1

[kernel-patches-daemon-bpf]

Upstream branch: f6fddc6
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=1012411
version: 1

[kernel-patches-daemon-bpf]

Upstream branch: a1e83d4
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=1012411
version: 1

[kernel-patches-daemon-bpf]

Upstream branch: a1e83d4
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=1012411
version: 1

[kernel-patches-daemon-bpf]

Upstream branch: 1c64efc
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=1012411
version: 1