On Wed, Nov 30, 2022 at 03:29:39PM -0800, Andrii Nakryiko wrote: > On Fri, Nov 25, 2022 at 1:35 AM Jiri Olsa <olsajiri@xxxxxxxxx> wrote: > > > > On Thu, Nov 24, 2022 at 09:17:22AM -0800, Alexei Starovoitov wrote: > > > On Thu, Nov 24, 2022 at 1:42 AM Jiri Olsa <olsajiri@xxxxxxxxx> wrote: > > > > > > > > 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 > > > > > > This is all fragile and merely a stop gap. > > > Doesn't sound that the issue is limited to bpf_trace_printk > > > > hm, I don't have a better idea how to fix that.. I can't deny > > contention_begin completely, because we use it in perf via > > tp_btf/contention_begin (perf lock contention) and I don't > > think there's another way for perf to do that > > > > fwiw the last version below denies BPF_PROG_TYPE_RAW_TRACEPOINT > > programs completely and tracing BPF_TRACE_RAW_TP with printks > > > > I think disabling bpf_trace_printk() tracepoint for any BPF program is > totally fine. This tracepoint was never intended to be attached to. > > But as for the general bpf_trace_printk() deadlocking. Should we > discuss how to make it not deadlock instead of starting to denylist > things left and right? > > Do I understand that we take trace_printk_lock only to protect that > static char buf[]? Can we just make this buf per-CPU and do a trylock > instead? We'll only fail to bpf_trace_printk() something if we have > nested BPF programs (rare) or NMI (also rare). > > And it's a printk(), it's never mission-critical, so if we drop some > message in rare case it's totally fine. What about contention_begin? I wonder if we can disallow recursions for those in the deny list like using bpf_prog_active.. Thanks, Namhyung
