Re: [PATCH bpf] bpf: fix OOB read when printing XDP link fdinfo

[Andrii Nakryiko]

On Fri, Jul 16, 2021 at 3:05 AM Lorenz Bauer <lmb@xxxxxxxxxxxxxx> wrote:
>
> We got the following UBSAN report on one of our testing machines:
>
>     ================================================================================
>     UBSAN: array-index-out-of-bounds in kernel/bpf/syscall.c:2389:24
>     index 6 is out of range for type 'char *[6]'
>     CPU: 43 PID: 930921 Comm: systemd-coredum Tainted: G           O      5.10.48-cloudflare-kasan-2021.7.0 #1
>     Hardware name: <snip>
>     Call Trace:
>      dump_stack+0x7d/0xa3
>      ubsan_epilogue+0x5/0x40
>      __ubsan_handle_out_of_bounds.cold+0x43/0x48
>      ? seq_printf+0x17d/0x250
>      bpf_link_show_fdinfo+0x329/0x380
>      ? bpf_map_value_size+0xe0/0xe0
>      ? put_files_struct+0x20/0x2d0
>      ? __kasan_kmalloc.constprop.0+0xc2/0xd0
>      seq_show+0x3f7/0x540
>      seq_read_iter+0x3f8/0x1040
>      seq_read+0x329/0x500
>      ? seq_read_iter+0x1040/0x1040
>      ? __fsnotify_parent+0x80/0x820
>      ? __fsnotify_update_child_dentry_flags+0x380/0x380
>      vfs_read+0x123/0x460
>      ksys_read+0xed/0x1c0
>      ? __x64_sys_pwrite64+0x1f0/0x1f0
>      do_syscall_64+0x33/0x40
>      entry_SYSCALL_64_after_hwframe+0x44/0xa9
>     <snip>
>     ================================================================================
>     ================================================================================
>     UBSAN: object-size-mismatch in kernel/bpf/syscall.c:2384:2
>
> From the report, we can infer that some array access in bpf_link_show_fdinfo at index 6
> is out of bounds. The obvious candidate is bpf_link_type_strs[BPF_LINK_TYPE_XDP] with
> BPF_LINK_TYPE_XDP == 6. It turns out that BPF_LINK_TYPE_XDP is missing from bpf_types.h
> and therefore doesn't have an entry in bpf_link_type_strs:
>
>     pos:        0
>     flags:      02000000
>     mnt_id:     13
>     link_type:  (null)
>     link_id:    4
>     prog_tag:   bcf7977d3b93787c
>     prog_id:    4
>     ifindex:    1
>
> Fixes: aa8d3a716b59 ("bpf, xdp: Add bpf_link-based XDP attachment API")
> Signed-off-by: Lorenz Bauer <lmb@xxxxxxxxxxxxxx>
> ---

Well, oops. Thanks for the fix!

Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>

It would be great to have a compilation error for something like this.
I wonder if we can do something to detect this going forward?

>  include/linux/bpf_types.h | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/include/linux/bpf_types.h b/include/linux/bpf_types.h
> index a9db1eae6796..be95f2722ad9 100644
> --- a/include/linux/bpf_types.h
> +++ b/include/linux/bpf_types.h
> @@ -135,3 +135,4 @@ BPF_LINK_TYPE(BPF_LINK_TYPE_ITER, iter)
>  #ifdef CONFIG_NET
>  BPF_LINK_TYPE(BPF_LINK_TYPE_NETNS, netns)
>  #endif
> +BPF_LINK_TYPE(BPF_LINK_TYPE_XDP, xdp)
> --
> 2.30.2
>