Re: [PATCH bpf v2 1/2] bpf: don't call bpf_get_current_[ancestor_]cgroup_id() in sleepable progs

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Sun, Aug 8, 2021 at 11:03 PM Yonghong Song <yhs@fb.com> wrote:
>
> Currently, if bpf_get_current_cgroup_id() or
> bpf_get_current_ancestor_cgroup_id() helper is
> called with sleepable programs e.g., sleepable
> fentry/fmod_ret/fexit/lsm programs, a rcu warning
> may appear. For example, if I added the following
> hack to test_progs/test_lsm sleepable fentry program
> test_sys_setdomainname:
>
>   --- a/tools/testing/selftests/bpf/progs/lsm.c
>   +++ b/tools/testing/selftests/bpf/progs/lsm.c
>   @@ -168,6 +168,10 @@ int BPF_PROG(test_sys_setdomainname, struct pt_regs *regs)
>           int buf = 0;
>           long ret;
>
>   +       __u64 cg_id = bpf_get_current_cgroup_id();
>   +       if (cg_id == 1000)
>   +               copy_test++;
>   +
>           ret = bpf_copy_from_user(&buf, sizeof(buf), ptr);
>           if (len == -2 && ret == 0 && buf == 1234)
>                   copy_test++;
>
> I will hit the following rcu warning:
>
>   include/linux/cgroup.h:481 suspicious rcu_dereference_check() usage!
>   other info that might help us debug this:
>     rcu_scheduler_active = 2, debug_locks = 1
>     1 lock held by test_progs/260:
>       #0: ffffffffa5173360 (rcu_read_lock_trace){....}-{0:0}, at: __bpf_prog_enter_sleepable+0x0/0xa0
>     stack backtrace:
>     CPU: 1 PID: 260 Comm: test_progs Tainted: G           O      5.14.0-rc2+ #176
>     Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
>     Call Trace:
>       dump_stack_lvl+0x56/0x7b
>       bpf_get_current_cgroup_id+0x9c/0xb1
>       bpf_prog_a29888d1c6706e09_test_sys_setdomainname+0x3e/0x89c
>       bpf_trampoline_6442469132_0+0x2d/0x1000
>       __x64_sys_setdomainname+0x5/0x110
>       do_syscall_64+0x3a/0x80
>       entry_SYSCALL_64_after_hwframe+0x44/0xae
>
> I can get similar warning using bpf_get_current_ancestor_cgroup_id() helper.
> syzbot reported a similar issue in [1] for syscall program. Helper
> bpf_get_current_cgroup_id() or bpf_get_current_ancestor_cgroup_id()
> has the following callchain:
>    task_dfl_cgroup
>      task_css_set
>        task_css_set_check
> and we have
>    #define task_css_set_check(task, __c)                                   \
>            rcu_dereference_check((task)->cgroups,                          \
>                    lockdep_is_held(&cgroup_mutex) ||                       \
>                    lockdep_is_held(&css_set_lock) ||                       \
>                    ((task)->flags & PF_EXITING) || (__c))
> Since cgroup_mutex/css_set_lock is not held and the task
> is not existing and rcu read_lock is not held, a warning
> will be issued. Note that bpf sleepable program is protected by
> rcu_read_lock_trace().
>
> To fix the issue, let us make these two helpers not available
> to sleepable program. I marked the patch fixing 95b861a7935b
> ("bpf: Allow bpf_get_current_ancestor_cgroup_id for tracing")
> which added bpf_get_current_ancestor_cgroup_id() to
> 5.14. I think backporting 5.14 is probably good enough as sleepable
> progrems are not widely used.
>
> This patch should fix [1] as well since syscall program is a sleepable
> program and bpf_get_current_cgroup_id() is not available to
> syscall program any more.
>
>  [1] https://lore.kernel.org/bpf/0000000000006d5cab05c7d9bb87@google.com/
>
> Reported-by: syzbot+7ee5c2c09c284495371f@syzkaller.appspotmail.com
> Fixes: 95b861a7935b ("bpf: Allow bpf_get_current_ancestor_cgroup_id for tracing")
> Signed-off-by: Yonghong Song <yhs@fb.com>
> ---
>  kernel/trace/bpf_trace.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index b4916ef388ad..eaa8a8ffbe46 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -1016,9 +1016,11 @@ bpf_tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>  #endif
>  #ifdef CONFIG_CGROUPS
>         case BPF_FUNC_get_current_cgroup_id:
> -               return &bpf_get_current_cgroup_id_proto;
> +               return prog->aux->sleepable ?
> +                      NULL : &bpf_get_current_cgroup_id_proto;
>         case BPF_FUNC_get_current_ancestor_cgroup_id:
> -               return &bpf_get_current_ancestor_cgroup_id_proto;
> +               return prog->aux->sleepable ?
> +                      NULL : &bpf_get_current_ancestor_cgroup_id_proto;

This feels too extreme. I bet these helpers are as useful in sleepable
BPF progs as they are in non-sleepable ones.

Why don't we just implement a variant of get_current_cgroup_id (and
the ancestor variant as well) which takes that cgroup_mutex lock, and
just pick the appropriate implementation. Wouldn't that work?

>  #endif
>         case BPF_FUNC_send_signal:
>                 return &bpf_send_signal_proto;
> --
> 2.30.2
>