On Thu, Nov 24, 2022 at 01:41:23AM +0100, Daniel Borkmann wrote: > On 11/21/22 10:31 PM, Jiri Olsa wrote: > > We hit following issues [1] [2] when we attach bpf program that calls > > bpf_trace_printk helper to the contention_begin tracepoint. > > > > As described in [3] with multiple bpf programs that call bpf_trace_printk > > helper attached to the contention_begin might result in exhaustion of > > printk buffer or cause a deadlock [2]. > > > > There's also another possible deadlock when multiple bpf programs attach > > to bpf_trace_printk tracepoint and call one of the printk bpf helpers. > > > > This change denies the attachment of bpf program to contention_begin > > and bpf_trace_printk tracepoints if the bpf program calls one of the > > printk bpf helpers. > > > > Adding also verifier check for tb_btf programs, so this can be cought > > in program loading time with error message like: > > > > Can't attach program with bpf_trace_printk#6 helper to contention_begin tracepoint. > > > > [1] https://lore.kernel.org/bpf/CACkBjsakT_yWxnSWr4r-0TpPvbKm9-OBmVUhJb7hV3hY8fdCkw@xxxxxxxxxxxxxx/ > > [2] https://lore.kernel.org/bpf/CACkBjsaCsTovQHFfkqJKto6S4Z8d02ud1D7MPESrHa1cVNNTrw@xxxxxxxxxxxxxx/ > > [3] https://lore.kernel.org/bpf/Y2j6ivTwFmA0FtvY@krava/ > > > > Reported-by: Hao Sun <sunhao.th@xxxxxxxxx> > > Suggested-by: Alexei Starovoitov <ast@xxxxxxxxxx> > > Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx> > > --- > > include/linux/bpf.h | 1 + > > include/linux/bpf_verifier.h | 2 ++ > > kernel/bpf/syscall.c | 3 +++ > > kernel/bpf/verifier.c | 46 ++++++++++++++++++++++++++++++++++++ > > 4 files changed, 52 insertions(+) > > > > diff --git a/include/linux/bpf.h b/include/linux/bpf.h > > index c9eafa67f2a2..3ccabede0f50 100644 > > --- a/include/linux/bpf.h > > +++ b/include/linux/bpf.h > > @@ -1319,6 +1319,7 @@ struct bpf_prog { > > enforce_expected_attach_type:1, /* Enforce expected_attach_type checking at attach time */ > > call_get_stack:1, /* Do we call bpf_get_stack() or bpf_get_stackid() */ > > call_get_func_ip:1, /* Do we call get_func_ip() */ > > + call_printk:1, /* Do we call trace_printk/trace_vprintk */ > > tstamp_type_access:1; /* Accessed __sk_buff->tstamp_type */ > > enum bpf_prog_type type; /* Type of BPF program */ > > enum bpf_attach_type expected_attach_type; /* For some prog types */ > > diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h > > index 545152ac136c..7118c2fda59d 100644 > > --- a/include/linux/bpf_verifier.h > > +++ b/include/linux/bpf_verifier.h > > @@ -618,6 +618,8 @@ bool is_dynptr_type_expected(struct bpf_verifier_env *env, > > struct bpf_reg_state *reg, > > enum bpf_arg_type arg_type); > > +int bpf_check_tp_printk_denylist(const char *name, struct bpf_prog *prog); > > + > > /* this lives here instead of in bpf.h because it needs to dereference tgt_prog */ > > static inline u64 bpf_trampoline_compute_key(const struct bpf_prog *tgt_prog, > > struct btf *btf, u32 btf_id) > > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c > > index 35972afb6850..9a69bda7d62b 100644 > > --- a/kernel/bpf/syscall.c > > +++ b/kernel/bpf/syscall.c > > @@ -3329,6 +3329,9 @@ static int bpf_raw_tp_link_attach(struct bpf_prog *prog, > > return -EINVAL; > > } > > + if (bpf_check_tp_printk_denylist(tp_name, prog)) > > + return -EACCES; > > + > > btp = bpf_get_raw_tracepoint(tp_name); > > if (!btp) > > return -ENOENT; > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > > index f07bec227fef..b662bc851e1c 100644 > > --- a/kernel/bpf/verifier.c > > +++ b/kernel/bpf/verifier.c > > @@ -7472,6 +7472,47 @@ static void update_loop_inline_state(struct bpf_verifier_env *env, u32 subprogno > > state->callback_subprogno == subprogno); > > } > > +int bpf_check_tp_printk_denylist(const char *name, struct bpf_prog *prog) > > +{ > > + static const char * const denylist[] = { > > + "contention_begin", > > + "bpf_trace_printk", > > + }; > > + int i; > > + > > + /* Do not allow attachment to denylist[] tracepoints, > > + * if the program calls some of the printk helpers, > > + * because there's possibility of deadlock. > > + */ > > What if that prog doesn't but tail calls into another one which calls printk helpers? right, I'll deny that for all BPF_PROG_TYPE_RAW_TRACEPOINT* programs, because I don't see easy way to check on that we can leave printk check for tracing BPF_TRACE_RAW_TP programs, because verifier known the exact tracepoint already thanks, jirka > > > + if (!prog->call_printk) > > + return 0; > > + > > + for (i = 0; i < ARRAY_SIZE(denylist); i++) { > > + if (!strcmp(denylist[i], name)) > > + return 1; > > + } > > + return 0; > > +} > > + > > +static int check_tp_printk_denylist(struct bpf_verifier_env *env, int func_id) > > +{ > > + struct bpf_prog *prog = env->prog; > > + > > + if (prog->type != BPF_PROG_TYPE_TRACING || > > + prog->expected_attach_type != BPF_TRACE_RAW_TP) > > + return 0; > > + > > + if (WARN_ON_ONCE(!prog->aux->attach_func_name)) > > + return -EINVAL; > > + > > + if (!bpf_check_tp_printk_denylist(prog->aux->attach_func_name, prog)) > > + return 0; > > + > > + verbose(env, "Can't attach program with %s#%d helper to %s tracepoint.\n", > > + func_id_name(func_id), func_id, prog->aux->attach_func_name); > > + return -EACCES; > > +} > > + > > static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn, > > int *insn_idx_p) > > { > > @@ -7675,6 +7716,11 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn > > err = __check_func_call(env, insn, insn_idx_p, meta.subprogno, > > set_user_ringbuf_callback_state); > > break; > > + case BPF_FUNC_trace_printk: > > + case BPF_FUNC_trace_vprintk: > > + env->prog->call_printk = 1; > > + err = check_tp_printk_denylist(env, func_id); > > + break; > > } > > if (err) > > >
