[lts92] bpf: Fix partial dynptr stack slot reads/writes

kernel/bpf/verifier.c

@@ -695,6 +695,12 @@ static bool dynptr_type_refcounted(enum bpf_dynptr_type type)
 	return type == BPF_DYNPTR_TYPE_RINGBUF;
 }
 
+static void __mark_reg_not_init(const struct bpf_verifier_env *env,
+				struct bpf_reg_state *reg);
+
+static int destroy_if_dynptr_stack_slot(struct bpf_verifier_env *env,
+				        struct bpf_func_state *state, int spi);
+
 static int mark_stack_slots_dynptr(struct bpf_verifier_env *env, struct bpf_reg_state *reg,
 				   enum bpf_arg_type arg_type, int insn_idx)
 {
@@ -762,6 +768,55 @@ static int unmark_stack_slots_dynptr(struct bpf_verifier_env *env, struct bpf_re
 	return 0;
 }
 
+static void __mark_reg_unknown(const struct bpf_verifier_env *env,
+			       struct bpf_reg_state *reg);
+
+static int destroy_if_dynptr_stack_slot(struct bpf_verifier_env *env,
+				        struct bpf_func_state *state, int spi)
+{
+	int i;
+
+	/* We always ensure that STACK_DYNPTR is never set partially,
+	 * hence just checking for slot_type[0] is enough. This is
+	 * different for STACK_SPILL, where it may be only set for
+	 * 1 byte, so code has to use is_spilled_reg.
+	 */
+	if (state->stack[spi].slot_type[0] != STACK_DYNPTR)
+		return 0;
+
+	/* Reposition spi to first slot */
+	if (!state->stack[spi].spilled_ptr.dynptr.first_slot)
+		spi = spi + 1;
+
+	if (dynptr_type_refcounted(state->stack[spi].spilled_ptr.dynptr.type)) {
+		verbose(env, "cannot overwrite referenced dynptr\n");
+		return -EINVAL;
+	}
+
+	mark_stack_slot_scratched(env, spi);
+	mark_stack_slot_scratched(env, spi - 1);
+
+	/* Writing partially to one dynptr stack slot destroys both. */
+	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;
+	}
+
+	/* TODO: Invalidate any slices associated with this dynptr */
+
+	/* Do not release reference state, we are destroying dynptr on stack,
+	 * not using some helper to release it. Just reset register.
+	 */
+	__mark_reg_not_init(env, &state->stack[spi].spilled_ptr);
+	__mark_reg_not_init(env, &state->stack[spi - 1].spilled_ptr);
+
+	/* Same reason as unmark_stack_slots_dynptr above */
+	state->stack[spi].spilled_ptr.live |= REG_LIVE_WRITTEN;
+	state->stack[spi - 1].spilled_ptr.live |= REG_LIVE_WRITTEN;
+
+	return 0;
+}
+
 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);
@@ -1300,9 +1355,6 @@ static const int caller_saved[CALLER_SAVED_REGS] = {
 	BPF_REG_0, BPF_REG_1, BPF_REG_2, BPF_REG_3, BPF_REG_4, BPF_REG_5
 };
 
-static void __mark_reg_not_init(const struct bpf_verifier_env *env,
-				struct bpf_reg_state *reg);
-
 /* This helper doesn't clear reg->id */
 static void ___mark_reg_known(struct bpf_reg_state *reg, u64 imm)
 {
@@ -3052,6 +3104,10 @@ static int check_stack_write_fixed_off(struct bpf_verifier_env *env,
 			env->insn_aux_data[insn_idx].sanitize_stack_spill = true;
 	}
 
+	err = destroy_if_dynptr_stack_slot(env, state, spi);
+	if (err)
+		return err;
+
 	mark_stack_slot_scratched(env, spi);
 	if (reg && !(off % BPF_REG_SIZE) && register_is_bounded(reg) &&
 	    !register_is_null(reg) && env->bpf_capable) {
@@ -3165,6 +3221,14 @@ static int check_stack_write_var_off(struct bpf_verifier_env *env,
 	if (err)
 		return err;
 
+	for (i = min_off; i < max_off; i++) {
+		int spi;
+
+		spi = get_spi(i);
+		err = destroy_if_dynptr_stack_slot(env, state, spi);
+		if (err)
+			return err;
+	}
 
 	/* Variable offset writes destroy any spilled pointers in range. */
 	for (i = min_off; i < max_off; i++) {
@@ -5126,6 +5190,31 @@ static int check_stack_range_initialized(
 	}
 
 	if (meta && meta->raw_mode) {
+		/* Ensure we won't be overwriting dynptrs when simulating byte
+		 * by byte access in check_helper_call using meta.access_size.
+		 * This would be a problem if we have a helper in the future
+		 * which takes:
+		 *
+		 *	helper(uninit_mem, len, dynptr)
+		 *
+		 * Now, uninint_mem may overlap with dynptr pointer. Hence, it
+		 * may end up writing to dynptr itself when touching memory from
+		 * arg 1. This can be relaxed on a case by case basis for known
+		 * safe cases, but reject due to the possibilitiy of aliasing by
+		 * default.
+		 */
+		for (i = min_off; i < max_off + access_size; i++) {
+			int stack_off = -i - 1;
+
+			spi = get_spi(i);
+			/* raw_mode may write past allocated_stack */
+			if (state->allocated_stack <= stack_off)
+				continue;
+			if (state->stack[spi].slot_type[stack_off % BPF_REG_SIZE] == STACK_DYNPTR) {
+				verbose(env, "potential write to dynptr at off=%d disallowed\n", i);
+				return -EACCES;
+			}
+		}
 		meta->access_size = access_size;
 		meta->regno = regno;
 		return 0;