[Linux-kernel-mentees] [PATCH v3] bpf: Fix NULL pointer dereference in __btf_resolve_helper_id()

[Daniel Borkmann]

On 7/14/20 8:09 PM, Peilin Ye wrote:
> Prevent __btf_resolve_helper_id() from dereferencing `btf_vmlinux`
> as NULL. This patch fixes the following syzbot bug:
> 
>      https://syzkaller.appspot.com/bug?id=f823224ada908fa5c207902a5a62065e53ca0fcc
> 
> Reported-by: syzbot+ee09bda7017345f1fbe6 at syzkaller.appspotmail.com
> Signed-off-by: Peilin Ye <yepeilin.cs at gmail.com>

Looks good, applied, thanks! As far as I can tell all the other occurrences are
gated behind btf_parse_vmlinux() where we also init struct_opts, etc.

So for bpf-next this would then end up looking like ...

int btf_resolve_helper_id(struct bpf_verifier_log *log,
                           const struct bpf_func_proto *fn, int arg)
{
         int id;

         if (fn->arg_type[arg] != ARG_PTR_TO_BTF_ID)
                 return -EINVAL;
         id = fn->btf_id[arg];
         if (!id || !btf_vmlinux || id > btf_vmlinux->nr_types)
                 return -EINVAL;
         return id;
}

> ---
> Sorry, I got the link wrong. Thank you for pointing that out.
> 
> Change in v3:
>      - Fix incorrect syzbot dashboard link.
> 
> Change in v2:
>      - Split NULL and IS_ERR cases.
> 
>   kernel/bpf/btf.c | 5 +++++
>   1 file changed, 5 insertions(+)
> 
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 30721f2c2d10..092116a311f4 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -4088,6 +4088,11 @@ static int __btf_resolve_helper_id(struct bpf_verifier_log *log, void *fn,
>   	const char *tname, *sym;
>   	u32 btf_id, i;
>   
> +	if (!btf_vmlinux) {
> +		bpf_log(log, "btf_vmlinux doesn't exist\n");
> +		return -EINVAL;
> +	}
> +
>   	if (IS_ERR(btf_vmlinux)) {
>   		bpf_log(log, "btf_vmlinux is malformed\n");
>   		return -EINVAL;
>