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

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Message-ID: <20251107114127.4e130fb2@pumpkin>
Date: Fri, 7 Nov 2025 11:41:27 +0000
From: David Laight <david.laight.linux@...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>, Yafang Shao <laoar.shao@...il.com>,
 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,  7 Nov 2025 13:03:05 +0300
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.

That code is insane.
The size being calculated looks like a kernel memory size.
You really don't want to be allocating single structures that exceed 4GB.

	David

> 
> 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>
> ---
> 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);