bpf: Add reference tracking to verifier

Allow helper functions to acquire a reference and return it into a
register. Specific pointer types such as the PTR_TO_SOCKET will
implicitly represent such a reference. The verifier must ensure that
these references are released exactly once in each path through the
program.

To achieve this, this commit assigns an id to the pointer and tracks it
in the 'bpf_func_state', then when the function or program exits,
verifies that all of the acquired references have been freed. When the
pointer is passed to a function that frees the reference, it is removed
from the 'bpf_func_state` and all existing copies of the pointer in
registers are marked invalid.

Signed-off-by: Joe Stringer <joe@wand.net.nz>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

include/linux/bpf_verifier.h

@@ -104,6 +104,17 @@ struct bpf_stack_state {
 	u8 slot_type[BPF_REG_SIZE];
 };
 
+struct bpf_reference_state {
+	/* Track each reference created with a unique id, even if the same
+	 * instruction creates the reference multiple times (eg, via CALL).
+	 */
+	int id;
+	/* Instruction where the allocation of this reference occurred. This
+	 * is used purely to inform the user of a reference leak.
+	 */
+	int insn_idx;
+};
+
 /* state of the program:
  * type of all registers and stack info
  */
@@ -121,7 +132,9 @@ struct bpf_func_state {
 	 */
 	u32 subprogno;
 
-	/* should be second to last. See copy_func_state() */
+	/* The following fields should be last. See copy_func_state() */
+	int acquired_refs;
+	struct bpf_reference_state *refs;
 	int allocated_stack;
 	struct bpf_stack_state *stack;
 };
@@ -217,11 +230,16 @@ __printf(2, 0) void bpf_verifier_vlog(struct bpf_verifier_log *log,
 __printf(2, 3) void bpf_verifier_log_write(struct bpf_verifier_env *env,
 					   const char *fmt, ...);
 
-static inline struct bpf_reg_state *cur_regs(struct bpf_verifier_env *env)
+static inline struct bpf_func_state *cur_func(struct bpf_verifier_env *env)
 {
 	struct bpf_verifier_state *cur = env->cur_state;
 
-	return cur->frame[cur->curframe]->regs;
+	return cur->frame[cur->curframe];
+}
+
+static inline struct bpf_reg_state *cur_regs(struct bpf_verifier_env *env)
+{
+	return cur_func(env)->regs;
 }
 
 int bpf_prog_offload_verifier_prep(struct bpf_verifier_env *env);