linux-kernel - Re: [PATCH] bpf: dereference of null in __cgroup_bpf

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Date: Mon, 8 Apr 2024 17:10:48 -0700
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: Mikhail Lobanov <m.lobanov@...alinux.ru>
Cc: Alexei Starovoitov <ast@...nel.org>, Andrii Nakryiko <andrii@...nel.org>,
 Song Liu <song@...nel.org>, Roman Gushchin <roman.gushchin@...ux.dev>,
 bpf@...r.kernel.org, linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] bpf: dereference of null in __cgroup_bpf_query() function

On 4/6/24 8:14 AM, Mikhail Lobanov wrote:
> In the __cgroup_bpf_query() function, it is possible to dereference
> the null pointer in the line id = prog->aux->id; since there is no
> check for a non-zero value of the variable prog.
> 
> Found by Linux Verification Center (linuxtesting.org) with SVACE.
> 
> Fixes: af6eea57437a ("bpf: Implement bpf_link-based cgroup BPF program attachment")
> Cc: stable@...r.kernel.org
> Signed-off-by: Mikhail Lobanov <m.lobanov@...alinux.ru>
> ---
>   kernel/bpf/cgroup.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> index 491d20038cbe..7f2db96f0c6a 100644
> --- a/kernel/bpf/cgroup.c
> +++ b/kernel/bpf/cgroup.c
> @@ -1092,6 +1092,8 @@ static int __cgroup_bpf_query(struct cgroup *cgrp, const union bpf_attr *attr,
>   			i = 0;
>   			hlist_for_each_entry(pl, progs, node) {
>   				prog = prog_list_prog(pl);
> +               	       	if (!prog_list_prog(pl))

prog cannot be null. It is under cgroup_lock().

> +				continue;
>   				id = prog->aux->id;
>   				if (copy_to_user(prog_ids + i, &id, sizeof(id)))
>   					return -EFAULT;