On Mon, Aug 9, 2021 at 10:41 AM Yonghong Song <yhs@xxxxxx> wrote:
>
>
>
> On 8/9/21 10:18 AM, Andrii Nakryiko wrote:
> > On Sun, Aug 8, 2021 at 11:03 PM Yonghong Song <yhs@xxxxxx> 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@xxxxxxxxxx/
> >>
> >> Reported-by: syzbot+7ee5c2c09c284495371f@xxxxxxxxxxxxxxxxxxxxxxxxx
> >> Fixes: 95b861a7935b ("bpf: Allow bpf_get_current_ancestor_cgroup_id for tracing")
> >> Signed-off-by: Yonghong Song <yhs@xxxxxx>
> >> ---
> >> 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?
>
> This may not work. e.g., for sleepable fentry program,
> if the to-be-traced function is inside in cgroup_mutex, we will
> have a deadlock.
We can also do preempty_disable() + rcu_read_lock() inside the helper
itself, no? I mean in the new "sleepable" variant.
>
> Currently, affected program types are tracing/fentry.s,
> tracing/fexit.s, tracing/fmod_ret.s, lsm.s and syscall.
> For fmod_ret.s, lsm.s, they all have
> some kind of predefined attachment/context, we might
> be able to check all potential attachment points and
> allow these two helpers when attachment point is not
> surrounded by cgroup_mutex.
I don't think it's feasible to know if any given attached kernel
function can be called with cgroup_mutex taken. Static analysis will
be too complicated and too restrictive. Runtime checks might be too
expensive and/or not generic enough. But see above, we can do
rcu_read_lock() inside the helper while preventing preemption, and it
will behave the same way as if it was called from non-sleepable BPF
prog.
> For syscall program, we should be okay as it is
> called with bpf_prog_test_run interface but I am
> not sure why user wants a cgroup_id for that.
>
> >
> >> #endif
> >> case BPF_FUNC_send_signal:
> >> return &bpf_send_signal_proto;
> >> --
> >> 2.30.2
> >>
