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    SubjectRe: [PATCH bpf] bpf: fix OOB read when printing XDP link fdinfo
    On Fri, 16 Jul 2021 at 22:01, Daniel Borkmann <daniel@iogearbox.net> wrote:
    >
    > On 7/16/21 10:43 PM, Andrii Nakryiko wrote:
    > > On Fri, Jul 16, 2021 at 3:05 AM Lorenz Bauer <lmb@cloudflare.com> 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@cloudflare.com>
    > >> ---
    > >
    > > 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?
    > >
    > >> 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.

    It does compile, since the only use of the macro is to stringify a
    link type for fdinfo. I'll move it into CONFIG_NET to be consistent
    with netdev though.

    --
    Lorenz Bauer | Systems Engineer
    6th Floor, County Hall/The Riverside Building, SE1 7PB, UK

    www.cloudflare.com

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