[PATCH bpf-next v3 1/5] bpf: add writable context for raw tracepoints

Matt Mullins
2019-04-19T21:05:22Z

This is an opt-in interface that allows a tracepoint to provide a safe
buffer that can be written from a BPF_PROG_TYPE_RAW_TRACEPOINT program.
The size of the buffer must be a compile-time constant, and is checked
before allowing a BPF program to attach to a tracepoint that uses this
feature.

The pointer to this buffer will be the first argument of tracepoints
that opt in; the buffer is readable by both BPF_PROG_TYPE_RAW_TRACEPOINT
and BPF_PROG_TYPE_RAW_TRACEPOINT_WRITABLE programs that attach to such a
tracepoint, but the buffer to which it points may only be written by the
latter.

Signed-off-by: Matt Mullins <mmullins@fb.com>
---
include/linux/bpf.h | 2 ++
include/linux/bpf_types.h | 1 +
include/linux/tracepoint-defs.h | 1 +
include/trace/bpf_probe.h | 27 +++++++++++++++++++++++++--
include/uapi/linux/bpf.h | 1 +
kernel/bpf/syscall.c | 8 ++++++--
kernel/bpf/verifier.c | 31 +++++++++++++++++++++++++++++++
kernel/trace/bpf_trace.c | 21 +++++++++++++++++++++
8 files changed, 88 insertions(+), 4 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index a2132e09dc1c..d3c71fd67476 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -263,6 +263,7 @@ enum bpf_reg_type {
PTR_TO_SOCK_COMMON_OR_NULL, /* reg points to sock_common or NULL */
PTR_TO_TCP_SOCK, /* reg points to struct tcp_sock */
PTR_TO_TCP_SOCK_OR_NULL, /* reg points to struct tcp_sock or NULL */
+ PTR_TO_TP_BUFFER, /* reg points to a writable raw tp's buffer */
};

/* The information passed from prog-specific *_is_valid_access
@@ -352,6 +353,7 @@ struct bpf_prog_aux {
u32 used_map_cnt;
u32 max_ctx_offset;
u32 max_pkt_offset;
+ u32 max_tp_access;
u32 stack_depth;
u32 id;
u32 func_cnt; /* used by non-func prog as the number of func progs */
diff --git a/include/linux/bpf_types.h b/include/linux/bpf_types.h
index 08bf2f1fe553..c766108608cb 100644
--- a/include/linux/bpf_types.h
+++ b/include/linux/bpf_types.h
@@ -25,6 +25,7 @@ BPF_PROG_TYPE(BPF_PROG_TYPE_KPROBE, kprobe)
BPF_PROG_TYPE(BPF_PROG_TYPE_TRACEPOINT, tracepoint)
BPF_PROG_TYPE(BPF_PROG_TYPE_PERF_EVENT, perf_event)
BPF_PROG_TYPE(BPF_PROG_TYPE_RAW_TRACEPOINT, raw_tracepoint)
+BPF_PROG_TYPE(BPF_PROG_TYPE_RAW_TRACEPOINT_WRITABLE, raw_tracepoint_writable)
#endif
#ifdef CONFIG_CGROUP_BPF
BPF_PROG_TYPE(BPF_PROG_TYPE_CGROUP_DEVICE, cg_dev)
diff --git a/include/linux/tracepoint-defs.h b/include/linux/tracepoint-defs.h
index 49ba9cde7e4b..b29950a19205 100644
--- a/include/linux/tracepoint-defs.h
+++ b/include/linux/tracepoint-defs.h
@@ -45,6 +45,7 @@ struct bpf_raw_event_map {
struct tracepoint *tp;
void *bpf_func;
u32 num_args;
+ u32 writable_size;
} __aligned(32);

#endif
diff --git a/include/trace/bpf_probe.h b/include/trace/bpf_probe.h
index 505dae0bed80..d6e556c0a085 100644
--- a/include/trace/bpf_probe.h
+++ b/include/trace/bpf_probe.h
@@ -69,8 +69,7 @@ __bpf_trace_##call(void *__data, proto) \
* to make sure that if the tracepoint handling changes, the
* bpf probe will fail to compile unless it too is updated.
*/
-#undef DEFINE_EVENT
-#define DEFINE_EVENT(template, call, proto, args) \
+#define __DEFINE_EVENT(template, call, proto, args, size) \
static inline void bpf_test_probe_##call(void) \
{ \
check_trace_callback_type_##call(__bpf_trace_##template); \
@@ -81,12 +80,36 @@ __bpf_trace_tp_map_##call = { \
.tp = &__tracepoint_##call, \
.bpf_func = (void *)__bpf_trace_##template, \
.num_args = COUNT_ARGS(args), \
+ .writable_size = size, \
};

+#define FIRST(x, ...) x
+
+#undef DEFINE_EVENT_WRITABLE
+#define DEFINE_EVENT_WRITABLE(template, call, proto, args, size) \
+static inline void bpf_test_buffer_##call(void) \
+{ \
+ /* BUILD_BUG_ON() is ignored if the code is completely eliminated, but \
+ * BUILD_BUG_ON_ZERO() uses a different mechanism that is not \
+ * dead-code-eliminated. \
+ */ \
+ FIRST(proto); \
+ (void)BUILD_BUG_ON_ZERO(size != sizeof(*FIRST(args))); \
+} \
+__DEFINE_EVENT(template, call, PARAMS(proto), PARAMS(args), size)
+
+#undef DEFINE_EVENT
+#define DEFINE_EVENT(template, call, proto, args) \
+ __DEFINE_EVENT(template, call, PARAMS(proto), PARAMS(args), 0)

#undef DEFINE_EVENT_PRINT
#define DEFINE_EVENT_PRINT(template, name, proto, args, print) \
DEFINE_EVENT(template, name, PARAMS(proto), PARAMS(args))

#include TRACE_INCLUDE(TRACE_INCLUDE_FILE)
+
+#undef DEFINE_EVENT_WRITABLE
+#undef __DEFINE_EVENT
+#undef FIRST
+
#endif /* CONFIG_BPF_EVENTS */
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 3c38ac9a92a7..c5335d53ce82 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -166,6 +166,7 @@ enum bpf_prog_type {
BPF_PROG_TYPE_LIRC_MODE2,
BPF_PROG_TYPE_SK_REUSEPORT,
BPF_PROG_TYPE_FLOW_DISSECTOR,
+ BPF_PROG_TYPE_RAW_TRACEPOINT_WRITABLE,
};

enum bpf_attach_type {
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 62f6bced3a3c..27e2f22879a4 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -1720,12 +1720,16 @@ static int bpf_raw_tracepoint_open(const union bpf_attr *attr)
}
raw_tp->btp = btp;

- prog = bpf_prog_get_type(attr->raw_tracepoint.prog_fd,
- BPF_PROG_TYPE_RAW_TRACEPOINT);
+ prog = bpf_prog_get(attr->raw_tracepoint.prog_fd);
if (IS_ERR(prog)) {
err = PTR_ERR(prog);
goto out_free_tp;
}
+ if (prog->type != BPF_PROG_TYPE_RAW_TRACEPOINT &&
+ prog->type != BPF_PROG_TYPE_RAW_TRACEPOINT_WRITABLE) {
+ err = -EINVAL;
+ goto out_put_prog;
+ }

err = bpf_probe_register(raw_tp->btp, prog);
if (err)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index ce166a002d16..32f706e99319 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -411,6 +411,7 @@ static const char * const reg_type_str[] = {
[PTR_TO_SOCK_COMMON_OR_NULL] = "sock_common_or_null",
[PTR_TO_TCP_SOCK] = "tcp_sock",
[PTR_TO_TCP_SOCK_OR_NULL] = "tcp_sock_or_null",
+ [PTR_TO_TP_BUFFER] = "tp_buffer",
};

static char slot_type_char[] = {
@@ -1958,6 +1959,32 @@ static int check_ctx_reg(struct bpf_verifier_env *env,
return 0;
}

+static int check_tp_buffer_access(struct bpf_verifier_env *env,
+ const struct bpf_reg_state *reg,
+ int regno, int off, int size)
+{
+ if (off < 0) {
+ verbose(env,
+ "R%d invalid tracepoint buffer access: off=%d, size=%d",
+ regno, off, size);
+ return -EACCES;
+ }
+ if (!tnum_is_const(reg->var_off) || reg->var_off.value) {
+ char tn_buf[48];
+
+ tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off);
+ verbose(env,
+ "R%d invalid variable buffer offset: off=%d, var_off=%s",
+ regno, off, tn_buf);
+ return -EACCES;
+ }
+ if (off + size > env->prog->aux->max_tp_access)
+ env->prog->aux->max_tp_access = off + size;
+
+ return 0;
+}
+
+
/* truncate register to smaller size (in bytes)
* must be called with size < BPF_REG_SIZE
*/
@@ -2100,6 +2127,10 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
err = check_sock_access(env, insn_idx, regno, off, size, t);
if (!err && value_regno >= 0)
mark_reg_unknown(env, regs, value_regno);
+ } else if (reg->type == PTR_TO_TP_BUFFER) {
+ err = check_tp_buffer_access(env, reg, regno, off, size);
+ if (!err && t == BPF_READ && value_regno >= 0)
+ mark_reg_unknown(env, regs, value_regno);
} else {
verbose(env, "R%d invalid mem access '%s'\n", regno,
reg_type_str[reg->type]);
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index d64c00afceb5..a2dd79dc6871 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -909,6 +909,24 @@ const struct bpf_verifier_ops raw_tracepoint_verifier_ops = {
const struct bpf_prog_ops raw_tracepoint_prog_ops = {
};

+static bool raw_tp_writable_prog_is_valid_access(int off, int size,
+ enum bpf_access_type type,
+ const struct bpf_prog *prog,
+ struct bpf_insn_access_aux *info)
+{
+ if (off == 0 && size == sizeof(u64))
+ info->reg_type = PTR_TO_TP_BUFFER;
+ return raw_tp_prog_is_valid_access(off, size, type, prog, info);
+}
+
+const struct bpf_verifier_ops raw_tracepoint_writable_verifier_ops = {
+ .get_func_proto = raw_tp_prog_func_proto,
+ .is_valid_access = raw_tp_writable_prog_is_valid_access,
+};
+
+const struct bpf_prog_ops raw_tracepoint_writable_prog_ops = {
+};
+
static bool pe_prog_is_valid_access(int off, int size, enum bpf_access_type type,
const struct bpf_prog *prog,
struct bpf_insn_access_aux *info)
@@ -1198,6 +1216,9 @@ static int __bpf_probe_register(struct bpf_raw_event_map *btp, struct bpf_prog *
if (prog->aux->max_ctx_offset > btp->num_args * sizeof(u64))
return -EINVAL;

+ if (prog->aux->max_tp_access > btp->writable_size)
+ return -EINVAL;
+
return tracepoint_probe_register(tp, (void *)btp->bpf_func, prog);
}

--
2.17.1