Improve compartment security

In this example, we tighten security even further. We store the DDC/PCC
of `switch_compartment` within its memory region. Then, for each
compartment, we create a copy of a capability pointing to this DDC/PCC
pair, sealed with `lpb`. This allows compartments to call
`switch_compartment` without having access to any information pertaining
to it.

We provide an additional example (`shared_try_deref.S`), where a
malicious compartment attempts to dereference its local copy, to gain
access to `switch_compartment`.

hybrid/compartment_examples/inter_comp_call/base/Makefile.morello-hybrid

@@ -0,0 +1,10 @@
+# Copyright (c) 2021 The CapableVMs "CHERI Examples" Contributors.
+# SPDX-License-Identifier: MIT OR Apache-2.0
+
+SHARED_SOURCES := shared.s
+CFILES := $(wildcard *.c)
+ROOTDIR=../../../..
+
+include $(ROOTDIR)/build/Makefile.vars.morello-hybrid
+include $(ROOTDIR)/build/Makefile.vars.common
+include $(ROOTDIR)/build/Makefile.simple

hybrid/compartment_examples/inter_comp_call/main.c → hybrid/compartment_examples/inter_comp_call/base/main.c

@@ -19,8 +19,8 @@
  * executable area is approximated (`main.c:96`).
  */
 
-#include "../../../include/common.h"
-#include "../../include/utils.h"
+#include "../../../../include/common.h"
+#include "../../../include/utils.h"
 
 #include <assert.h>
 #include <stddef.h>

hybrid/compartment_examples/inter_comp_call/secure-try_deref/Makefile.morello-hybrid

@@ -0,0 +1,10 @@
+# Copyright (c) 2021 The CapableVMs "CHERI Examples" Contributors.
+# SPDX-License-Identifier: MIT OR Apache-2.0
+
+SHARED_SOURCES := shared_try_deref.s
+CFILES := $(wildcard *.c)
+ROOTDIR=../../../..
+
+include $(ROOTDIR)/build/Makefile.vars.morello-hybrid
+include $(ROOTDIR)/build/Makefile.vars.common
+include $(ROOTDIR)/build/Makefile.simple

hybrid/compartment_examples/inter_comp_call/secure-try_deref/main.c

@@ -0,0 +1,203 @@
+/* In this example, we tighten security even further, building on previous
+ * examples [1]. We store the DDC and the PCC of `switch_compartment`
+ * consecutively within its own bounds. Then, we create a capability pointing
+ * to the DDC, which is sealed such that it can only be used in an `lpb`-type
+ * call, and provide local copies of this capability to each compartment. Thus,
+ * compartments are allowed to call `switch_compartment` via `ldpblr`, without
+ * access to either its PCC or DDC (by nature of capabilities).
+ *
+ * The local copies are stored in the heap space of each compartment (as we do
+ * not implement memory management at this point, this has no bearing at
+ * present, but most likely will in the future). This could essentially be
+ * considered a separate region of memory. Further, when using the local
+ * capabilities to call `switch_compartment` from a compartment, the
+ * destination register *must* be `c29`.
+ *
+ * Note that in the current design, each compartment has a single function it
+ * executes when it is called. Allowing multiple entry points can be modelled
+ * by having more capabilities be saved, in addition to the compartment
+ * switching one, in each compartment's heap. Alternatively, we could, within
+ * the compartment switcher, determine in which compartment we need to switch
+ * to call a given function, based on PCC bounds for each compartment.
+
+ * Additionally, we also changed the expected memory layout in the
+ * compartments. In the absence of a custom memory allocator, we note that the
+ * heap is not used, except for manually storing the local capability copy as
+ * detailed above. Current compartment memory layout:
+ *
+ *       -----------------  < DDC + total_comp_size
+ *      |  switcher_call  |
+ *      |                 |
+ *      |       ^         |
+ *      |      HEAP       |
+ * sp > |-----------------| < DDC + comp_stack_size
+ *      |     STACK       |
+ *      |       v         |
+ *       -----------------  < `DDC value`
+ *
+ * [1]
+ https://github.com/capablevms/cheri-examples/tree/master/hybrid/compartment_examples/inter_comp_call
+ */
+
+#include "../../../../include/common.h"
+#include "../../../include/utils.h"
+
+#include <assert.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/sysctl.h>
+
+#if !defined(__CHERI_CAPABILITY_WIDTH__) || defined(__CHERI_PURE_CAPABILITY__)
+#error "This example only works on CHERI hybrid mode"
+#endif
+
+/*******************************************************************************
+ * Assembly Functions & Consts
+ ******************************************************************************/
+
+extern void executive_switch(void *__capability);
+extern int switch_compartment();
+extern void comp_f_fn();
+extern void comp_g_fn();
+extern void *comp_f_fn_end;
+extern void *comp_g_fn_end;
+
+/*******************************************************************************
+ * Types & Consts
+ ******************************************************************************/
+
+const size_t comp_stack_size = 2000;
+const size_t total_comp_size = 4992;
+size_t id = 0;
+
+/* Capabilities representing DDC and PCC of the switcher, stored in
+ * `switcher_caps`. They are saved successively to facilitate calling via
+ * `ldpblr`, with the first being the DDC, and the second, the PCC.
+ */
+const size_t switcher_caps_count = 2;
+void *__capability switcher_caps[switcher_caps_count];
+
+/* Capability required by compartments to call `switch_compartment` securely.
+ * Duplicated in each compartment. */
+void *__capability switcher_call;
+
+/* Abstract representation of a compartment. Within 80 bytes we represent:
+ * - an id (8B)
+ * - address to the start of the compartment memory area (also lowest address
+ *   of the stack) (8B)
+ * - address to the start (highest address) of the compartment's stack
+ *   (corresponding to the lowest address of the heap) (8B)
+ *   the size of the stack (8B)
+ * - address to the top (highest addressof the compartment's heap (8B)
+ *   the size of the heap (8B)
+ * - alignment padding (8B)
+ * - the ddc corresponding to the compartment (16B)
+ * - the function within the compartment to be executed upon switching (16B)
+ */
+struct comp
+{
+	size_t id;
+	void *compartment_start;
+	void *stack_addr;
+	size_t stack_len;
+	size_t heap_len;
+	void *__capability ddc;
+	void *__capability comp_fn;
+};
+
+// ASM offsets, included here for validation
+#include "main.h"
+
+static_assert(COMP_SIZE == sizeof(struct comp), "Invalid `COMP_SIZE` provided");
+static_assert(COMP_OFFSET_STK_ADDR == offsetof(struct comp, stack_addr),
+			  "Invalid `COMP_OFFSET_STK_ADDR` provided.");
+static_assert(COMP_OFFSET_STK_LEN == offsetof(struct comp, stack_len),
+			  "Invalid `COMP_OFFSET_STK_LEN` provided.");
+static_assert(COMP_OFFSET_DDC == offsetof(struct comp, ddc), "Invalid `COMP_OFFSET_DDC` provided.");
+static_assert(COMP_OFFSET_PCC == offsetof(struct comp, comp_fn),
+			  "Invalid `COMP_OFFSET_PCC` provided.");
+
+struct comp comps[COMP_COUNT];
+
+/*******************************************************************************
+ * Privileged Functions
+ ******************************************************************************/
+
+/* Create and save required capabilities. Currently, this means:
+ * - PCC of `switch_compartment`
+ * - DDC of `switch_compartment`
+ * - capability to allow compartments to call `switch_compartment` via `lpdblr`
+ */
+void init_comps()
+{
+	void *__capability switch_cap = (void *__capability) switch_compartment;
+	switch_cap = cheri_bounds_set(switch_cap, 80 * 4);
+	switcher_caps[1] = switch_cap;
+
+	void *__capability comps_addr = (void *__capability) &comps;
+	comps_addr = cheri_bounds_set(comps_addr, COMP_COUNT * COMP_SIZE);
+	switcher_caps[0] = comps_addr;
+
+	switcher_call = (void *__capability) switcher_caps;
+	// Seal this capability to be only used via a `lpb` type call
+	asm("seal %w0, %w0, lpb" : "+r"(switcher_call) :);
+}
+
+void add_comp(uint8_t *_start_addr, void (*_comp_fn)(), void *_comp_fn_end)
+{
+	assert(id < COMP_COUNT);
+	struct comp new_comp;
+	new_comp.id = id;
+
+	new_comp.compartment_start = (void *) _start_addr;
+	new_comp.stack_addr = (void *) (_start_addr + comp_stack_size);
+	new_comp.stack_len = comp_stack_size;
+	new_comp.heap_len = total_comp_size - comp_stack_size;
+
+	// Ensure 16-byte alignment throught the compartment bounds
+	assert(((uintptr_t) new_comp.compartment_start) % 16 == 0);
+	assert(((uintptr_t) new_comp.stack_addr) % 16 == 0);
+	assert(total_comp_size % 16 == 0);
+
+	// When creating a compartment, store a local copy of the capability which
+	// will allow us to call `switch_compartment` in the heap of the compartment.
+	void *heap_top = (void *) (_start_addr + total_comp_size - sizeof(void *__capability));
+	memcpy(heap_top, &switcher_call, sizeof(void *__capability));
+
+	void *__capability comp_ddc = (void *__capability) _start_addr;
+	comp_ddc = cheri_bounds_set(comp_ddc, total_comp_size);
+	new_comp.ddc = comp_ddc;
+
+	// Set up a capability pointing to the function we want to call within the
+	// compartment. This will be loaded as the PCC when the function is called.
+	void *__capability comp_fn = (void *__capability) _comp_fn;
+	// 40 is arbitary; meant to be the size of the executable function within
+	// compartment
+	size_t comp_fn_size = (uintptr_t) _comp_fn_end - (uintptr_t) _comp_fn;
+	comp_fn = cheri_bounds_set(comp_fn, comp_fn_size);
+	new_comp.comp_fn = comp_fn;
+
+	comps[id] = new_comp;
+	++id;
+}
+
+/*******************************************************************************
+ * Main
+ ******************************************************************************/
+
+int main()
+{
+	init_comps();
+
+	uint8_t *comp_f = malloc(total_comp_size);
+	add_comp(comp_f, comp_f_fn, &comp_f_fn_end);
+	uint8_t *comp_g = malloc(total_comp_size);
+	add_comp(comp_g, comp_g_fn, &comp_g_fn_end);
+
+	executive_switch(switcher_caps[0]);
+
+	// Check compartment did indeed execute
+	assert(comp_g[4000] == 42);
+}

hybrid/compartment_examples/inter_comp_call/secure-try_deref/main.h

@@ -0,0 +1,6 @@
+#define COMP_COUNT 2
+#define COMP_SIZE 80
+#define COMP_OFFSET_STK_ADDR 16
+#define COMP_OFFSET_STK_LEN 24
+#define COMP_OFFSET_DDC 48
+#define COMP_OFFSET_PCC 64