> On May 15, 2023, at 1:17 PM, Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
>
> On 5/15/23 5:52 PM, Song Liu wrote:
>> On Mon, May 15, 2023 at 6:09 AM Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
>>>
>>> If it fails to attach fentry, the allocated bpf trampoline image will be
>>> left in the system. That can be verified by checking /proc/kallsyms.
>>>
>>> This meamleak can be verified by a simple bpf program as follows,
>>>
>>> SEC("fentry/trap_init")
>>> int fentry_run()
>>> {
>>> return 0;
>>> }
>>>
>>> It will fail to attach trap_init because this function is freed after
>>> kernel init, and then we can find the trampoline image is left in the
>>> system by checking /proc/kallsyms.
>>> $ tail /proc/kallsyms
>>> ffffffffc0613000 t bpf_trampoline_6442453466_1 [bpf]
>>> ffffffffc06c3000 t bpf_trampoline_6442453466_1 [bpf]
>>>
>>> $ bpftool btf dump file /sys/kernel/btf/vmlinux | grep "FUNC 'trap_init'"
>>> [2522] FUNC 'trap_init' type_id=119 linkage=static
>>>
>>> $ echo $((6442453466 & 0x7fffffff))
>>> 2522
>>>
>>> Note that there are two left bpf trampoline images, that is because the
>>> libbpf will fallback to raw tracepoint if -EINVAL is returned.
>>>
>>> Fixes: e21aa341785c ("bpf: Fix fexit trampoline.")
>>> Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
>>> Cc: Song Liu <song@xxxxxxxxxx>
>>> Cc: Jiri Olsa <olsajiri@xxxxxxxxx>
>> Acked-by: Song Liu <song@xxxxxxxxxx>
>
> Won't this trigger a UAF for the case when progs are already running at
> this address aka modify_fentry() fails where you would then also hit the
> goto out_free path? This looks not correct to me.
I am not following. If modify_fentry() fails, we will not use the new
image anywhere, no? Did I miss something?
Thanks,
Song
