This example demonstrates a simple overflow of a

pointer that has had 0x1001 bytes allocated using malloc().
It shows the expected behaviours in three different CHERI environments,
displaying a message to the user immediately before the program is
expected to crash in each respective environment: riscv64-purecap,
morello-purecap and morello-hybrid. The program can also be compiled
and run in a non-CHERI environment, but the behaviour is undefined,
so the point at which the program crashes (or whether it crashes) is
unpredictable.

compare_platforms/Makefile.morello-hybrid

@@ -0,0 +1,6 @@
+# Copyright (c) 2023 The CapableVMs "CHERI Examples" Contributors.
+# SPDX-License-Identifier: MIT OR Apache-2.0
+
+include ../build/Makefile.vars.morello-hybrid
+include ../build/Makefile.vars.common
+include build/Makefile.review

compare_platforms/Makefile.riscv64-purecap

@@ -0,0 +1,6 @@
+# Copyright (c) 2023 The CapableVMs "CHERI Examples" Contributors.
+# SPDX-License-Identifier: MIT OR Apache-2.0
+
+include ../build/Makefile.vars.riscv64-purecap
+include ../build/Makefile.vars.common
+include build/Makefile.review

compare_platforms/build/Makefile.review

@@ -4,7 +4,7 @@
 # Common Makefile for the compare_platforms example.
 # This should not be invoked directly.
 
-RUNDIR := $(RUNDIR)/review
+RUNDIR := $(RUNDIR)/compare_platforms
 HFILES := $(wildcard include/*.h) $(wildcard ../include/*.h)
 export
 

compare_platforms/compare_platforms_overflow.c

@@ -0,0 +1,146 @@
+/*
+ * This example demonstrates a simple overflow of a pointer that has had
+ * 0x1001 bytes allocated using malloc(). It shows the expected behaviours
+ * in three different CHERI environments, displaying a message to the user
+ * immediately before the program is expected to crash in each respective
+ * environment:
+ *
+ * - riscv64-purecap
+ * - morello-purecap
+ * - morello-hybrid
+ *
+ * The program can also be compiled and run in a non-CHERI environment,
+ * but the behaviour is undefined, so the point at which the program crashes
+ * (or whether it crashes) is unpredictable.
+ */
+
+#ifdef __CHERI_PURE_CAPABILITY__
+#include "../include/common.h"
+#endif
+
+#include <assert.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+struct review
+{
+    char *publicreview;
+    char *permissions;
+};
+
+void print_details(struct review *rv)
+{
+    printf("Public Review: %s\n", rv->publicreview);
+    printf("Permissions: %s\n", rv->permissions);
+    fflush(stdout);
+}
+
+void overflow_publicreview(size_t offset, size_t reps, size_t overflow_amt, struct review *rv)
+{
+    printf("\nOverflowing publicreview by %zx\n", overflow_amt);
+    memset(rv->publicreview + offset, 'w', reps);
+    printf("Permissions: %s\n", rv->permissions);
+    fflush(stdout);
+}
+
+char *min(char *a, char *b)
+{
+    return (a > b ? b : a);
+}
+
+char *max(char *a, char *b)
+{
+    return (a > b ? a : b);
+}
+
+void assert_positions(char *publicreview, char *permissions, size_t smsz, size_t bigsz)
+{
+    if (min(publicreview, permissions) == publicreview)
+    {
+        assert((publicreview + bigsz) <= permissions);
+    }
+    else
+    {
+        assert((permissions + smsz) <= publicreview);
+    }
+}
+
+void free_pointers(struct review *rv)
+{
+    free(rv->publicreview);
+    free(rv->permissions);
+}
+
+int main(int argc, char *argv[])
+{
+    // initialization values for struct member sizes
+    const size_t smallsz = 0x20;
+    const size_t biggersz = 0x1001;
+
+    struct review review;
+
+    char *publicreview = malloc(biggersz);
+    review.publicreview = strcpy(
+        publicreview, "I am a little unclear as to the contribution. I think the authors could "
+                      "strengthen their case considerably if they conducted an RCT. Weak reject.");
+
+    char *permissions = malloc(smallsz);
+    review.permissions = strcpy(permissions, "r");
+
+    print_details(&review);
+
+    // Just to prove that the allocated spaces of the pointers are not
+    // overlapping before we attempt to overflow them:
+    assert_positions(publicreview, permissions, smallsz, biggersz);
+
+#if (!defined(__CHERI_CAPABILITY_WIDTH__) && !defined(__CHERI_PURE_CAPABILITY__))
+    printf("\nIn a non-CHERI environment, you should reach this line, after which "
+           "one of the attempts to overflow may or may not cause a crash.\n"
+           "This behaviour is undefined, so the timing of the crash is unpredictable, "
+           "if it happens at all.\n");
+#endif
+
+#if ((defined(__CHERI_PURE_CAPABILITY__)) && (defined(__aarch64__)))
+    printf("\nOn morello-purecap, you should reach this line, then the attempt to overflow by 1 "
+           "should "
+           "cause a SIGPROT.\n");
+    printf("This is because the representable length of 0x%zx bytes is 0x%zx.\n", biggersz,
+           __builtin_cheri_round_representable_length(biggersz));
+#endif
+
+    size_t offset = biggersz + 1;
+    overflow_publicreview(offset, 1, 1, &review);
+
+    offset++;
+    const size_t oversz = review.permissions - review.publicreview + 2 - biggersz;
+
+#if ((defined(__CHERI_PURE_CAPABILITY__)) && (defined(__riscv)))
+    size_t riscv64_padding = (__builtin_cheri_round_representable_length(biggersz) - offset) + 1;
+    size_t overflow_amt = ((offset - biggersz) + riscv64_padding);
+    printf("\nOn riscv64-purecap, you should reach this line, then the attempt to overflow by %zx "
+           "should "
+           "cause a SIGPROT.\n",
+           overflow_amt);
+    printf("You got this far because the representable length of 0x%zx bytes is 0x%zx.\n", biggersz,
+           __builtin_cheri_round_representable_length(biggersz));
+    overflow_publicreview(offset, riscv64_padding, overflow_amt, &review);
+#endif
+
+    overflow_publicreview(offset, oversz, oversz, &review);
+
+    // If we reached this line, we should have acquired write privileges on the review.
+    print_details(&review);
+
+    free_pointers(&review);
+
+#if (defined(__aarch64__) && defined(__CHERI_CAPABILITY_WIDTH__) &&                                \
+     !defined(__CHERI_PURE_CAPABILITY__))
+    printf("\nOn morello-hybrid, you should make it all the way to the end without crashing.\n");
+#endif
+
+    return 0;
+}

tests/run_tests.sh

@@ -70,6 +70,7 @@ if [ "$1" = "riscv64" ] || [ "$1" = "morello-purecap" ]; then
     run to_fail capability_sharing/cap-to-file cap_to_file
     run to_fail capability_sharing/leak-capability leak-capability
     run to_fail capability_sharing/mmap-shared-vs-private shared_file_main
+    run to_fail compare_platforms compare_platforms_overflow
     run to_fail compare_platforms compare_platforms_read_only
 elif [ "$1" = "morello-hybrid" ]; then
     # HYBRID TESTS
@@ -80,6 +81,7 @@ elif [ "$1" = "morello-hybrid" ]; then
     run to_fail hybrid/compartment_examples/inter_comp_call/malicious_compartments inter_comp_call-secure-redirect_clr
     run to_fail hybrid/compartment_examples/inter_comp_call/malicious_compartments inter_comp_call-secure-update_ddc
     # Tests that should pass
+    run OK compare_platforms compare_platforms_overflow
     run OK hybrid/ddc_compartment_switching ddc_compartment_switching
     run OK hybrid basic_ddc
     run OK hybrid/compartment_examples/inter_comp_call/base main