Re: [PATCH] bpf: Fix KASAN use-after-free Read in compute_effective_progs

[Tadeusz Struk <tadeusz.struk@linaro.org>]

On 4/5/22 10:03, Tadeusz Struk wrote:
> Syzbot found a Use After Free bug in compute_effective_progs().
> The reproducer creates a number of BPF links, and causes a fault
> injected alloc to fail, while calling bpf_link_detach on them.
> Link detach triggers the link to be freed by bpf_link_free(),
> which calls __cgroup_bpf_detach() and update_effective_progs().
> If the memory allocation in this function fails, the function restores
> the pointer to the bpf_cgroup_link on the cgroup list, but the memory
> gets freed just after it returns. After this, every subsequent call to
> update_effective_progs() causes this already deallocated pointer to be
> dereferenced in prog_list_length(), and triggers KASAN UAF error.
> To fix this don't preserve the pointer to the link on the cgroup list
> in __cgroup_bpf_detach(), but proceed with the cleanup and retry calling
> update_effective_progs() again afterwards.
> 
> 
> Cc: "Alexei Starovoitov" <ast@kernel.org>
> Cc: "Daniel Borkmann" <daniel@iogearbox.net>
> Cc: "Andrii Nakryiko" <andrii@kernel.org>
> Cc: "Martin KaFai Lau" <kafai@fb.com>
> Cc: "Song Liu" <songliubraving@fb.com>
> Cc: "Yonghong Song" <yhs@fb.com>
> Cc: "John Fastabend" <john.fastabend@gmail.com>
> Cc: "KP Singh" <kpsingh@kernel.org>
> Cc: <netdev@vger.kernel.org>
> Cc: <bpf@vger.kernel.org>
> Cc: <stable@vger.kernel.org>
> Cc: <linux-kernel@vger.kernel.org>
> 
> Link: https://syzkaller.appspot.com/bug?id=8ebf179a95c2a2670f7cf1ba62429ec044369db4
> Fixes: af6eea57437a ("bpf: Implement bpf_link-based cgroup BPF program attachment")
> Reported-by: <syzbot+f264bffdfbd5614f3bb2@syzkaller.appspotmail.com>
> Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org>
> ---
>   kernel/bpf/cgroup.c | 25 ++++++++++++++-----------
>   1 file changed, 14 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> index 128028efda64..b6307337a3c7 100644
> --- a/kernel/bpf/cgroup.c
> +++ b/kernel/bpf/cgroup.c
> @@ -723,10 +723,11 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,
>   	pl->link = NULL;
>   
>   	err = update_effective_progs(cgrp, atype);
> -	if (err)
> -		goto cleanup;
> -
> -	/* now can actually delete it from this cgroup list */
> +	/*
> +	 * Proceed regardless of error. The link and/or prog will be freed
> +	 * just after this function returns so just delete it from this
> +	 * cgroup list and retry calling update_effective_progs again later.
> +	 */
>   	list_del(&pl->node);
>   	kfree(pl);
>   	if (list_empty(progs))
> @@ -735,12 +736,11 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,
>   	if (old_prog)
>   		bpf_prog_put(old_prog);
>   	static_branch_dec(&cgroup_bpf_enabled_key[atype]);
> -	return 0;
>   
> -cleanup:
> -	/* restore back prog or link */
> -	pl->prog = old_prog;
> -	pl->link = link;
> +	/* In case of error call update_effective_progs again */
> +	if (err)
> +		err = update_effective_progs(cgrp, atype);
> +
>   	return err;
>   }
>   
> @@ -881,6 +881,7 @@ static void bpf_cgroup_link_release(struct bpf_link *link)
>   	struct bpf_cgroup_link *cg_link =
>   		container_of(link, struct bpf_cgroup_link, link);
>   	struct cgroup *cg;
> +	int err;
>   
>   	/* link might have been auto-detached by dying cgroup already,
>   	 * in that case our work is done here
> @@ -896,8 +897,10 @@ static void bpf_cgroup_link_release(struct bpf_link *link)
>   		return;
>   	}
>   
> -	WARN_ON(__cgroup_bpf_detach(cg_link->cgroup, NULL, cg_link,
> -				    cg_link->type));
> +	err = __cgroup_bpf_detach(cg_link->cgroup, NULL, cg_link,
> +				  cg_link->type);
> +	if (err)
> +		pr_warn("cgroup_bpf_detach() failed, err %d\n", err);
>   
>   	cg = cg_link->cgroup;
>   	cg_link->cgroup = NULL;

Hi,
Any feedback/comments on this one?

-- 
Thanks,
Tadeusz