bpf: Implement accurate raw_tp context access via BTF

libbpf analyzes bpf C program, searches in-kernel BTF for given type name
and stores it into expected_attach_type.
The kernel verifier expects this btf_id to point to something like:
typedef void (*btf_trace_kfree_skb)(void *, struct sk_buff *skb, void *loc);
which represents signature of raw_tracepoint "kfree_skb".

Then btf_ctx_access() matches ctx+0 access in bpf program with 'skb'
and 'ctx+8' access with 'loc' arguments of "kfree_skb" tracepoint.
In first case it passes btf_id of 'struct sk_buff *' back to the verifier core
and 'void *' in second case.

Then the verifier tracks PTR_TO_BTF_ID as any other pointer type.
Like PTR_TO_SOCKET points to 'struct bpf_sock',
PTR_TO_TCP_SOCK points to 'struct bpf_tcp_sock', and so on.
PTR_TO_BTF_ID points to in-kernel structs.
If 1234 is btf_id of 'struct sk_buff' in vmlinux's BTF
then PTR_TO_BTF_ID#1234 points to one of in kernel skbs.

When PTR_TO_BTF_ID#1234 is dereferenced (like r2 = *(u64 *)r1 + 32)
the btf_struct_access() checks which field of 'struct sk_buff' is
at offset 32. Checks that size of access matches type definition
of the field and continues to track the dereferenced type.
If that field was a pointer to 'struct net_device' the r2's type
will be PTR_TO_BTF_ID#456. Where 456 is btf_id of 'struct net_device'
in vmlinux's BTF.

Such verifier analysis prevents "cheating" in BPF C program.
The program cannot cast arbitrary pointer to 'struct sk_buff *'
and access it. C compiler would allow type cast, of course,
but the verifier will notice type mismatch based on BPF assembly
and in-kernel BTF.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20191016032505.2089704-7-ast@kernel.org

2 files changed, 20 insertions, 1 deletions

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index f916380675dd..028555fcd10d 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -16,6 +16,7 @@
 #include <linux/u64_stats_sync.h>
 
 struct bpf_verifier_env;
+struct bpf_verifier_log;
 struct perf_event;
 struct bpf_prog;
 struct bpf_map;
@@ -281,6 +282,7 @@ enum bpf_reg_type {
 	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 */
 	PTR_TO_XDP_SOCK,	 /* reg points to struct xdp_sock */
+	PTR_TO_BTF_ID,		 /* reg points to kernel struct */
 };
 
 /* The information passed from prog-specific *_is_valid_access
@@ -288,7 +290,11 @@ enum bpf_reg_type {
  */
 struct bpf_insn_access_aux {
 	enum bpf_reg_type reg_type;
-	int ctx_field_size;
+	union {
+		int ctx_field_size;
+		u32 btf_id;
+	};
+	struct bpf_verifier_log *log; /* for verbose logs */
 };
 
 static inline void
@@ -483,6 +489,7 @@ struct bpf_event_entry {
 
 bool bpf_prog_array_compatible(struct bpf_array *array, const struct bpf_prog *fp);
 int bpf_prog_calc_tag(struct bpf_prog *fp);
+const char *kernel_type_name(u32 btf_type_id);
 
 const struct bpf_func_proto *bpf_get_trace_printk_proto(void);
 
@@ -748,6 +755,14 @@ int bpf_prog_test_run_skb(struct bpf_prog *prog, const union bpf_attr *kattr,
 int bpf_prog_test_run_flow_dissector(struct bpf_prog *prog,
 				     const union bpf_attr *kattr,
 				     union bpf_attr __user *uattr);
+bool btf_ctx_access(int off, int size, enum bpf_access_type type,
+		    const struct bpf_prog *prog,
+		    struct bpf_insn_access_aux *info);
+int btf_struct_access(struct bpf_verifier_log *log,
+		      const struct btf_type *t, int off, int size,
+		      enum bpf_access_type atype,
+		      u32 *next_btf_id);
+
 #else /* !CONFIG_BPF_SYSCALL */
 static inline struct bpf_prog *bpf_prog_get(u32 ufd)
 {
diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 713efae62e96..6e7284ea1468 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -52,6 +52,8 @@ struct bpf_reg_state {
 		 */
 		struct bpf_map *map_ptr;
 
+		u32 btf_id; /* for PTR_TO_BTF_ID */
+
 		/* Max size from any of the above. */
 		unsigned long raw;
 	};
@@ -399,6 +401,8 @@ __printf(2, 0) void bpf_verifier_vlog(struct bpf_verifier_log *log,
 				      const char *fmt, va_list args);
 __printf(2, 3) void bpf_verifier_log_write(struct bpf_verifier_env *env,
 					   const char *fmt, ...);
+__printf(2, 3) void bpf_log(struct bpf_verifier_log *log,
+			    const char *fmt, ...);
 
 static inline struct bpf_func_state *cur_func(struct bpf_verifier_env *env)
 {