bpf: enforce exact retval range on subprog/callback exit

Instead of relying on potentially imprecise tnum representation of
expected return value range for callbacks and subprogs, validate that
smin/smax range satisfy exact expected range of return values.

E.g., if callback would need to return [0, 2] range, tnum can't
represent this precisely and instead will allow [0, 3] range. By
checking smin/smax range, we can make sure that subprog/callback indeed
returns only valid [0, 2] range.
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231202175705.885270-5-andrii@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/bpf_verifier.h

@@ -275,6 +275,11 @@ struct bpf_reference_state {
 	int callback_ref;
 };
 
+struct bpf_retval_range {
+	s32 minval;
+	s32 maxval;
+};
+
 /* state of the program:
  * type of all registers and stack info
  */
@@ -297,7 +302,7 @@ struct bpf_func_state {
 	 * void foo(void) { bpf_timer_set_callback(,foo); }
 	 */
 	u32 async_entry_cnt;
-	struct tnum callback_ret_range;
+	struct bpf_retval_range callback_ret_range;
 	bool in_callback_fn;
 	bool in_async_callback_fn;
 	bool in_exception_callback_fn;

kernel/bpf/verifier.c

@@ -2305,6 +2305,11 @@ static void init_reg_state(struct bpf_verifier_env *env,
 	regs[BPF_REG_FP].frameno = state->frameno;
 }
 
+static struct bpf_retval_range retval_range(s32 minval, s32 maxval)
+{
+	return (struct bpf_retval_range){ minval, maxval };
+}
+
 #define BPF_MAIN_FUNC (-1)
 static void init_func_state(struct bpf_verifier_env *env,
 			    struct bpf_func_state *state,
@@ -2313,7 +2318,7 @@ static void init_func_state(struct bpf_verifier_env *env,
 	state->callsite = callsite;
 	state->frameno = frameno;
 	state->subprogno = subprogno;
-	state->callback_ret_range = tnum_range(0, 0);
+	state->callback_ret_range = retval_range(0, 0);
 	init_reg_state(env, state);
 	mark_verifier_state_scratched(env);
 }
@@ -9396,7 +9401,7 @@ static int set_map_elem_callback_state(struct bpf_verifier_env *env,
 		return err;
 
 	callee->in_callback_fn = true;
-	callee->callback_ret_range = tnum_range(0, 1);
+	callee->callback_ret_range = retval_range(0, 1);
 	return 0;
 }
 
@@ -9418,7 +9423,7 @@ static int set_loop_callback_state(struct bpf_verifier_env *env,
 	__mark_reg_not_init(env, &callee->regs[BPF_REG_5]);
 
 	callee->in_callback_fn = true;
-	callee->callback_ret_range = tnum_range(0, 1);
+	callee->callback_ret_range = retval_range(0, 1);
 	return 0;
 }
 
@@ -9448,7 +9453,7 @@ static int set_timer_callback_state(struct bpf_verifier_env *env,
 	__mark_reg_not_init(env, &callee->regs[BPF_REG_4]);
 	__mark_reg_not_init(env, &callee->regs[BPF_REG_5]);
 	callee->in_async_callback_fn = true;
-	callee->callback_ret_range = tnum_range(0, 1);
+	callee->callback_ret_range = retval_range(0, 1);
 	return 0;
 }
 
@@ -9476,7 +9481,7 @@ static int set_find_vma_callback_state(struct bpf_verifier_env *env,
 	__mark_reg_not_init(env, &callee->regs[BPF_REG_4]);
 	__mark_reg_not_init(env, &callee->regs[BPF_REG_5]);
 	callee->in_callback_fn = true;
-	callee->callback_ret_range = tnum_range(0, 1);
+	callee->callback_ret_range = retval_range(0, 1);
 	return 0;
 }
 
@@ -9499,7 +9504,7 @@ static int set_user_ringbuf_callback_state(struct bpf_verifier_env *env,
 	__mark_reg_not_init(env, &callee->regs[BPF_REG_5]);
 
 	callee->in_callback_fn = true;
-	callee->callback_ret_range = tnum_range(0, 1);
+	callee->callback_ret_range = retval_range(0, 1);
 	return 0;
 }
 
@@ -9531,7 +9536,7 @@ static int set_rbtree_add_callback_state(struct bpf_verifier_env *env,
 	__mark_reg_not_init(env, &callee->regs[BPF_REG_4]);
 	__mark_reg_not_init(env, &callee->regs[BPF_REG_5]);
 	callee->in_callback_fn = true;
-	callee->callback_ret_range = tnum_range(0, 1);
+	callee->callback_ret_range = retval_range(0, 1);
 	return 0;
 }
 
@@ -9560,6 +9565,11 @@ static bool in_rbtree_lock_required_cb(struct bpf_verifier_env *env)
 	return is_rbtree_lock_required_kfunc(kfunc_btf_id);
 }
 
+static bool retval_range_within(struct bpf_retval_range range, const struct bpf_reg_state *reg)
+{
+	return range.minval <= reg->smin_value && reg->smax_value <= range.maxval;
+}
+
 static int prepare_func_exit(struct bpf_verifier_env *env, int *insn_idx)
 {
 	struct bpf_verifier_state *state = env->cur_state, *prev_st;
@@ -9583,9 +9593,6 @@ static int prepare_func_exit(struct bpf_verifier_env *env, int *insn_idx)
 
 	caller = state->frame[state->curframe - 1];
 	if (callee->in_callback_fn) {
-		/* enforce R0 return value range [0, 1]. */
-		struct tnum range = callee->callback_ret_range;
-
 		if (r0->type != SCALAR_VALUE) {
 			verbose(env, "R0 not a scalar value\n");
 			return -EACCES;
@@ -9597,7 +9604,11 @@ static int prepare_func_exit(struct bpf_verifier_env *env, int *insn_idx)
 		if (err)
 			return err;
 
-		if (!tnum_in(range, r0->var_off)) {
+		/* enforce R0 return value range */
+		if (!retval_range_within(callee->callback_ret_range, r0)) {
+			struct tnum range = tnum_range(callee->callback_ret_range.minval,
+						       callee->callback_ret_range.maxval);
+
 			verbose_invalid_scalar(env, r0, &range, "callback return", "R0");
 			return -EINVAL;
 		}