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

On 7/16/21 10:43 PM, Andrii Nakryiko wrote:

On Fri, Jul 16, 2021 at 3:05 AM Lorenz Bauer [off-list ref] wrote:

quoted

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 <redacted>
---

Well, oops. Thanks for the fix!

Acked-by: Andrii Nakryiko <andrii@kernel.org>

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?

quoted

  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)

Lorenz, does this compile when you don't have CONFIG_NET configured? I would assume
this needs to go right below the netns one depending on CONFIG_NET.. at least the
bpf_xdp_link_lops are in net/core/dev.c which is only built under CONFIG_NET.

Thanks,
Daniel