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?
+ 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)
