bpf: Combine dynptr_get_spi and is_spi_bounds_valid

Currently, a check on spi resides in dynptr_get_spi, while others
checking its validity for being within the allocated stack slots happens
in is_spi_bounds_valid. Almost always barring a couple of cases (where
being beyond allocated stack slots is not an error as stack slots need
to be populated), both are used together to make checks. Hence, subsume
the is_spi_bounds_valid check in dynptr_get_spi, and return -ERANGE to
specially distinguish the case where spi is valid but not within
allocated slots in the stack state.

The is_spi_bounds_valid function is still kept around as it is a generic
helper that will be useful for other objects on stack similar to dynptr
in the future.
Suggested-by: Joanne Koong <joannelkoong@gmail.com>
Acked-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20230121002241.2113993-7-memxor@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -644,6 +644,28 @@ static int __get_spi(s32 off)
 	return (-off - 1) / BPF_REG_SIZE;
 }
 
+static struct bpf_func_state *func(struct bpf_verifier_env *env,
+				   const struct bpf_reg_state *reg)
+{
+	struct bpf_verifier_state *cur = env->cur_state;
+
+	return cur->frame[reg->frameno];
+}
+
+static bool is_spi_bounds_valid(struct bpf_func_state *state, int spi, int nr_slots)
+{
+       int allocated_slots = state->allocated_stack / BPF_REG_SIZE;
+
+       /* We need to check that slots between [spi - nr_slots + 1, spi] are
+	* within [0, allocated_stack).
+	*
+	* Please note that the spi grows downwards. For example, a dynptr
+	* takes the size of two stack slots; the first slot will be at
+	* spi and the second slot will be at spi - 1.
+	*/
+       return spi - nr_slots + 1 >= 0 && spi < allocated_slots;
+}
+
 static int dynptr_get_spi(struct bpf_verifier_env *env, struct bpf_reg_state *reg)
 {
 	int off, spi;
@@ -664,29 +686,10 @@ static int dynptr_get_spi(struct bpf_verifier_env *env, struct bpf_reg_state *re
 		verbose(env, "cannot pass in dynptr at an offset=%d\n", off);
 		return -EINVAL;
 	}
-	return spi;
-}
 
-static bool is_spi_bounds_valid(struct bpf_func_state *state, int spi, int nr_slots)
-{
-	int allocated_slots = state->allocated_stack / BPF_REG_SIZE;
-
-	/* We need to check that slots between [spi - nr_slots + 1, spi] are
-	 * within [0, allocated_stack).
-	 *
-	 * Please note that the spi grows downwards. For example, a dynptr
-	 * takes the size of two stack slots; the first slot will be at
-	 * spi and the second slot will be at spi - 1.
-	 */
-	return spi - nr_slots + 1 >= 0 && spi < allocated_slots;
-}
-
-static struct bpf_func_state *func(struct bpf_verifier_env *env,
-				   const struct bpf_reg_state *reg)
-{
-	struct bpf_verifier_state *cur = env->cur_state;
-
-	return cur->frame[reg->frameno];
+	if (!is_spi_bounds_valid(func(env, reg), spi, BPF_DYNPTR_NR_SLOTS))
+		return -ERANGE;
+	return spi;
 }
 
 static const char *kernel_type_name(const struct btf* btf, u32 id)
@@ -788,9 +791,6 @@ static int mark_stack_slots_dynptr(struct bpf_verifier_env *env, struct bpf_reg_
 	if (spi < 0)
 		return spi;
 
-	if (!is_spi_bounds_valid(state, spi, BPF_DYNPTR_NR_SLOTS))
-		return -EINVAL;
-
 	/* We cannot assume both spi and spi - 1 belong to the same dynptr,
 	 * hence we need to call destroy_if_dynptr_stack_slot twice for both,
 	 * to ensure that for the following example:
@@ -844,9 +844,6 @@ static int unmark_stack_slots_dynptr(struct bpf_verifier_env *env, struct bpf_re
 	if (spi < 0)
 		return spi;
 
-	if (!is_spi_bounds_valid(state, spi, BPF_DYNPTR_NR_SLOTS))
-		return -EINVAL;
-
 	for (i = 0; i < BPF_REG_SIZE; i++) {
 		state->stack[spi].slot_type[i] = STACK_INVALID;
 		state->stack[spi - 1].slot_type[i] = STACK_INVALID;
@@ -951,20 +948,18 @@ static int destroy_if_dynptr_stack_slot(struct bpf_verifier_env *env,
 
 static bool is_dynptr_reg_valid_uninit(struct bpf_verifier_env *env, struct bpf_reg_state *reg)
 {
-	struct bpf_func_state *state = func(env, reg);
 	int spi;
 
 	if (reg->type == CONST_PTR_TO_DYNPTR)
 		return false;
 
 	spi = dynptr_get_spi(env, reg);
+	/* For -ERANGE (i.e. spi not falling into allocated stack slots), we
+	 * will do check_mem_access to check and update stack bounds later, so
+	 * return true for that case.
+	 */
 	if (spi < 0)
-		return false;
-
-	/* We will do check_mem_access to check and update stack bounds later */
-	if (!is_spi_bounds_valid(state, spi, BPF_DYNPTR_NR_SLOTS))
-		return true;
-
+		return spi == -ERANGE;
 	/* We allow overwriting existing unreferenced STACK_DYNPTR slots, see
 	 * mark_stack_slots_dynptr which calls destroy_if_dynptr_stack_slot to
 	 * ensure dynptr objects at the slots we are touching are completely
@@ -988,8 +983,7 @@ static bool is_dynptr_reg_valid_init(struct bpf_verifier_env *env, struct bpf_re
 	spi = dynptr_get_spi(env, reg);
 	if (spi < 0)
 		return false;
-	if (!is_spi_bounds_valid(state, spi, BPF_DYNPTR_NR_SLOTS) ||
-	    !state->stack[spi].spilled_ptr.dynptr.first_slot)
+	if (!state->stack[spi].spilled_ptr.dynptr.first_slot)
 		return false;
 
 	for (i = 0; i < BPF_REG_SIZE; i++) {
@@ -6160,7 +6154,7 @@ int process_dynptr_func(struct bpf_verifier_env *env, int regno,
 	if (reg->type == PTR_TO_STACK) {
 		int err = dynptr_get_spi(env, reg);
 
-		if (err < 0)
+		if (err < 0 && err != -ERANGE)
 			return err;
 	}
 
@@ -6668,10 +6662,7 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 			 */
 			if (reg->type == PTR_TO_STACK) {
 				spi = dynptr_get_spi(env, reg);
-				if (spi < 0)
-					return spi;
-				if (!is_spi_bounds_valid(state, spi, BPF_DYNPTR_NR_SLOTS) ||
-				    !state->stack[spi].spilled_ptr.ref_obj_id) {
+				if (spi < 0 || !state->stack[spi].spilled_ptr.ref_obj_id) {
 					verbose(env, "arg %d is an unacquired reference\n", regno);
 					return -EINVAL;
 				}