On 2021/06/27 0:13, Tetsuo Handa wrote: > On 2021/06/26 23:18, Steven Rostedt wrote: >> On Sat, 26 Jun 2021 22:58:45 +0900 >> Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote: >> >>> syzbot is hitting WARN_ON_ONCE() at tracepoint_add_func() [1], but >>> func_add() returning -EEXIST and func_remove() returning -ENOENT are >>> not kernel bugs that can justify crashing the system. >> >> There should be no path that registers a tracepoint twice. That's a bug >> in the kernel. Looking at the link below, I see the backtrace: >> >> Call Trace: >> tracepoint_probe_register_prio kernel/tracepoint.c:369 [inline] >> tracepoint_probe_register+0x9c/0xe0 kernel/tracepoint.c:389 >> __bpf_probe_register kernel/trace/bpf_trace.c:2154 [inline] >> bpf_probe_register+0x15a/0x1c0 kernel/trace/bpf_trace.c:2159 >> bpf_raw_tracepoint_open+0x34a/0x720 kernel/bpf/syscall.c:2878 >> __do_sys_bpf+0x2586/0x4f40 kernel/bpf/syscall.c:4435 >> do_syscall_64+0x3a/0xb0 arch/x86/entry/common.c:47 >> >> So BPF is allowing the user to register the same tracepoint more than >> once? That looks to be a bug in the BPF code where it shouldn't be >> allowing user space to register the same tracepoint multiple times. > > I didn't catch your question. > > (1) func_add() can reject an attempt to add same tracepoint multiple times > by returning -EINVAL to the caller. Sorry, s/EINVAL/EEXIST/g on (1) (2) (6). > (2) But tracepoint_add_func() (the caller of func_add()) is calling WARN_ON_ONCE() > if func_add() returned -EINVAL. > (3) And tracepoint_add_func() is triggerable via request from userspace. > (4) tracepoint_probe_register_prio() serializes tracepoint_add_func() call > triggered by concurrent request from userspace using tracepoints_mutex mutex. > (5) But tracepoint_add_func() does not check whether same tracepoint multiple > is already registered before calling func_add(). > (6) As a result, tracepoint_add_func() receives -EINVAL from func_add(), and > calls WARN_ON_ONCE() and the system crashes due to panic_on_warn == 1. > > Why this is a bug in the BPF code? The BPF code is not allowing userspace to > register the same tracepoint multiple times. I think that tracepoint_add_func() > is stupid enough to crash the kernel instead of rejecting when an attempt to > register the same tracepoint multiple times is made. >
