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;
