linux-kernel - Re: [PATCH v2] bpf: hashtab: fix 32-bit overflow in memory usage calculation

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Message-ID: <CALOAHbDGw_kfz-Vv7BRwY0Rk3SofMQ4rzvvQph2wBT=_9ZK0HQ@mail.gmail.com>
Date: Fri, 7 Nov 2025 19:35:02 +0800
From: Yafang Shao <laoar.shao@...il.com>
To: Alexei Safin <a.safin@...a.ru>
Cc: Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, 
	Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, bpf@...r.kernel.org, linux-kernel@...r.kernel.org, 
	lvc-patches@...uxtesting.org, stable@...r.kernel.org
Subject: Re: [PATCH v2] bpf: hashtab: fix 32-bit overflow in memory usage calculation

On Fri, Nov 7, 2025 at 6:03 PM Alexei Safin <a.safin@...a.ru> wrote:
>
> The intermediate product value_size * num_possible_cpus() is evaluated
> in 32-bit arithmetic and only then promoted to 64 bits. On systems with
> large value_size and many possible CPUs this can overflow and lead to
> an underestimated memory usage.
>
> Found by Linux Verification Center (linuxtesting.org) with SVACE.
>
> Fixes: 304849a27b34 ("bpf: hashtab memory usage")
> Cc: stable@...r.kernel.org
> Suggested-by: Yafang Shao <laoar.shao@...il.com>
> Signed-off-by: Alexei Safin <a.safin@...a.ru>

Acked-by: Yafang Shao <laoar.shao@...il.com>

> ---
> v2: Promote value_size to u64 at declaration to avoid 32-bit overflow
> in all arithmetic using this variable (suggested by Yafang Shao)
>  kernel/bpf/hashtab.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> index 570e2f723144..1f0add26ba3f 100644
> --- a/kernel/bpf/hashtab.c
> +++ b/kernel/bpf/hashtab.c
> @@ -2252,7 +2252,7 @@ static long bpf_for_each_hash_elem(struct bpf_map *map, bpf_callback_t callback_
>  static u64 htab_map_mem_usage(const struct bpf_map *map)
>  {
>         struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
> -       u32 value_size = round_up(htab->map.value_size, 8);
> +       u64 value_size = round_up(htab->map.value_size, 8);
>         bool prealloc = htab_is_prealloc(htab);
>         bool percpu = htab_is_percpu(htab);
>         bool lru = htab_is_lru(htab);
> --
> 2.50.1 (Apple Git-155)
>


-- 
Regards
Yafang