add support for Arm MTE memory tagging

- tag slab allocations with [1..14] tags
- tag freed slab allocations with the "15" tag value to detect accesses to freed slab memory
- when generating tag value for a slab slot, always exclude most recent tag value for that slot
(to make use-after-free detection more reliable) and most recent tag values of its immediate
neighbors (to detect linear overflows and underflows)

arm_mte.h

@@ -0,0 +1,86 @@
+#ifndef ARM_MTE_H
+#define ARM_MTE_H
+
+#include <arm_acle.h>
+#include <util.h>
+
+// Returns a tagged pointer.
+// See https://developer.arm.com/documentation/ddi0602/2023-09/Base-Instructions/IRG--Insert-Random-Tag-
+static inline void *arm_mte_create_random_tag(void *p, u64 exclusion_mask) {
+    return __arm_mte_create_random_tag(p, exclusion_mask);
+}
+
+// Tag the memory region with the tag specified in tag bits of tagged_ptr. Memory region itself is
+// zeroed.
+// Arm's software optimization guide says:
+// "it is recommended to use STZGM (or DCZGVA) to set tag if data is not a concern." (STZGM and
+// DCGZVA are zeroing variants of tagging instructions).
+//
+// Contents of this function were copied from scudo:
+// https://android.googlesource.com/platform/external/scudo/+/refs/tags/android-14.0.0_r1/standalone/memtag.h#167
+static inline void arm_mte_store_tags_and_clear(void *tagged_ptr, size_t len) {
+    uintptr_t Begin = (uintptr_t) tagged_ptr;
+    uintptr_t End = Begin + len;
+    uintptr_t LineSize, Next, Tmp;
+    __asm__ __volatile__(
+        ".arch_extension memtag \n\t"
+
+        // Compute the cache line size in bytes (DCZID_EL0 stores it as the log2
+        // of the number of 4-byte words) and bail out to the slow path if DCZID_EL0
+        // indicates that the DC instructions are unavailable.
+        "DCZID .req %[Tmp] \n\t"
+        "mrs DCZID, dczid_el0 \n\t"
+        "tbnz DCZID, #4, 3f \n\t"
+        "and DCZID, DCZID, #15 \n\t"
+        "mov %[LineSize], #4 \n\t"
+        "lsl %[LineSize], %[LineSize], DCZID \n\t"
+        ".unreq DCZID \n\t"
+
+        // Our main loop doesn't handle the case where we don't need to perform any
+        // DC GZVA operations. If the size of our tagged region is less than
+        // twice the cache line size, bail out to the slow path since it's not
+        // guaranteed that we'll be able to do a DC GZVA.
+        "Size .req %[Tmp] \n\t"
+        "sub Size, %[End], %[Cur] \n\t"
+        "cmp Size, %[LineSize], lsl #1 \n\t"
+        "b.lt 3f \n\t"
+        ".unreq Size \n\t"
+
+        "LineMask .req %[Tmp] \n\t"
+        "sub LineMask, %[LineSize], #1 \n\t"
+
+        // STZG until the start of the next cache line.
+        "orr %[Next], %[Cur], LineMask \n\t"
+
+        "1:\n\t"
+        "stzg %[Cur], [%[Cur]], #16 \n\t"
+        "cmp %[Cur], %[Next] \n\t"
+        "b.lt 1b \n\t"
+
+        // DC GZVA cache lines until we have no more full cache lines.
+        "bic %[Next], %[End], LineMask \n\t"
+        ".unreq LineMask \n\t"
+
+        "2: \n\t"
+        "dc gzva, %[Cur] \n\t"
+        "add %[Cur], %[Cur], %[LineSize] \n\t"
+        "cmp %[Cur], %[Next] \n\t"
+        "b.lt 2b \n\t"
+
+        // STZG until the end of the tagged region. This loop is also used to handle
+        // slow path cases.
+
+        "3: \n\t"
+        "cmp %[Cur], %[End] \n\t"
+        "b.ge 4f \n\t"
+        "stzg %[Cur], [%[Cur]], #16 \n\t"
+        "b 3b \n\t"
+
+        "4: \n\t"
+
+        : [Cur] "+&r"(Begin), [LineSize] "=&r"(LineSize), [Next] "=&r"(Next), [Tmp] "=&r"(Tmp)
+        : [End] "r"(End)
+        : "memory"
+    );
+}
+#endif

h_malloc.c

@@ -14,6 +14,7 @@
 
 #include "h_malloc.h"
 #include "memory.h"
+#include "memtag.h"
 #include "mutex.h"
 #include "pages.h"
 #include "random.h"
@@ -66,6 +67,10 @@ static atomic_uint thread_arena_counter = 0;
 static const unsigned thread_arena = 0;
 #endif
 
+#ifdef MEMTAG
+bool __is_memtag_enabled = true;
+#endif
+
 static union {
     struct {
         void *slab_region_start;
@@ -99,6 +104,18 @@ struct slab_metadata {
 #if SLAB_QUARANTINE
     u64 quarantine_bitmap[4];
 #endif
+#ifdef HAS_ARM_MTE
+    // arm_mte_tags is used as a u4 array (MTE tags are 4-bit wide)
+    //
+    // Its size is calculated by the following formula:
+    // (MAX_SLAB_SLOT_COUNT + 2) / 2
+    // MAX_SLAB_SLOT_COUNT is currently 256, 2 extra slots are needed for branchless handling of
+    // edge slots in tag_and_clear_slab_slot()
+    //
+    // It's intentionally placed at the end of struct to improve locality: for most size classes,
+    // slot count is far lower than MAX_SLAB_SLOT_COUNT.
+    u8 arm_mte_tags[129];
+#endif
 };
 
 static const size_t min_align = 16;
@@ -506,6 +523,47 @@ static inline void stats_slab_deallocate(UNUSED struct size_class *c, UNUSED siz
 #endif
 }
 
+static void *tag_and_clear_slab_slot(struct slab_metadata *metadata, void *slot_ptr, size_t slot_idx, size_t slot_size) {
+#ifdef HAS_ARM_MTE
+    if (unlikely(!is_memtag_enabled())) {
+        return slot_ptr;
+    }
+
+    // arm_mte_tags is an array of 4-bit unsigned integers stored as u8 array (MTE tags are 4-bit wide)
+    //
+    // It stores the most recent tag for each slab slot, or 0 if the slot was never used.
+    // Slab indices in arm_mte_tags array are shifted to the right by 1, and size of this array
+    // is (MAX_SLAB_SLOT_COUNT + 2). This means that first and last values of arm_mte_tags array
+    // are always 0, which allows to handle edge slots in a branchless way when tag exclusion mask
+    // is constructed.
+    u8 *slot_tags = metadata->arm_mte_tags;
+
+    // Tag exclusion mask
+    u64 tem = (1 << 0) | (1 << RESERVED_TAG);
+
+    // current or previous tag of left neighbor or 0 if there's no left neighbor or if it was never used
+    tem |= (1 << u4_arr_get(slot_tags, slot_idx));
+    // previous tag of this slot or 0 if it was never used
+    tem |= (1 << u4_arr_get(slot_tags, slot_idx + 1));
+    // current or previous tag of right neighbor or 0 if there's no right neighbor or if it was never used
+    tem |= (1 << u4_arr_get(slot_tags, slot_idx + 2));
+
+    void *tagged_ptr = arm_mte_create_random_tag(slot_ptr, tem);
+    // slot addresses and sizes are always aligned by 16
+    arm_mte_store_tags_and_clear(tagged_ptr, slot_size);
+
+    // store new tag of this slot
+    u4_arr_set(slot_tags, slot_idx + 1, get_pointer_tag(tagged_ptr));
+
+    return tagged_ptr;
+#else
+    (void) metadata;
+    (void) slot_idx;
+    (void) slot_size;
+    return slot_ptr;
+#endif
+}
+
 static inline void *allocate_small(unsigned arena, size_t requested_size) {
     struct size_info info = get_size_info(requested_size);
     size_t size = likely(info.size) ? info.size : 16;
@@ -534,6 +592,7 @@ static inline void *allocate_small(unsigned arena, size_t requested_size) {
             if (requested_size) {
                 write_after_free_check(p, size - canary_size);
                 set_canary(metadata, p, size);
+                p = tag_and_clear_slab_slot(metadata, p, slot, size);
             }
             stats_small_allocate(c, size);
 
@@ -566,6 +625,7 @@ static inline void *allocate_small(unsigned arena, size_t requested_size) {
             void *p = slot_pointer(size, slab, slot);
             if (requested_size) {
                 set_canary(metadata, p, size);
+                p = tag_and_clear_slab_slot(metadata, p, slot, size);
             }
             stats_slab_allocate(c, slab_size);
             stats_small_allocate(c, size);
@@ -588,6 +648,7 @@ static inline void *allocate_small(unsigned arena, size_t requested_size) {
         void *p = slot_pointer(size, slab, slot);
         if (requested_size) {
             set_canary(metadata, p, size);
+            p = tag_and_clear_slab_slot(metadata, p, slot, size);
         }
         stats_slab_allocate(c, slab_size);
         stats_small_allocate(c, size);
@@ -612,6 +673,7 @@ static inline void *allocate_small(unsigned arena, size_t requested_size) {
     if (requested_size) {
         write_after_free_check(p, size - canary_size);
         set_canary(metadata, p, size);
+        p = tag_and_clear_slab_slot(metadata, p, slot, size);
     }
     stats_small_allocate(c, size);
 
@@ -694,7 +756,17 @@ static inline void deallocate_small(void *p, const size_t *expected_size) {
     if (likely(!is_zero_size)) {
         check_canary(metadata, p, size);
 
-        if (ZERO_ON_FREE) {
+        bool skip_zero = false;
+#ifdef HAS_ARM_MTE
+        if (likely(is_memtag_enabled())) {
+            arm_mte_store_tags_and_clear(set_pointer_tag(p, RESERVED_TAG), size);
+            // metadata->arm_mte_tags is intentionally not updated, it should keep the previous slot
+            // tag after slot is freed
+            skip_zero = true;
+        }
+#endif
+
+        if (ZERO_ON_FREE && !skip_zero) {
             memset(p, 0, size - canary_size);
         }
     }
@@ -1123,8 +1195,15 @@ COLD static void init_slow_path(void) {
     if (unlikely(memory_protect_rw_metadata(ra->regions, ra->total * sizeof(struct region_metadata)))) {
         fatal_error("failed to unprotect memory for regions table");
     }
-
+#ifdef HAS_ARM_MTE
+    if (likely(is_memtag_enabled())) {
+        ro.slab_region_start = memory_map_mte(slab_region_size);
+    } else {
+        ro.slab_region_start = memory_map(slab_region_size);
+    }
+#else
     ro.slab_region_start = memory_map(slab_region_size);
+#endif
     if (unlikely(ro.slab_region_start == NULL)) {
         fatal_error("failed to allocate slab region");
     }
@@ -1368,6 +1447,11 @@ EXPORT void *h_calloc(size_t nmemb, size_t size) {
     if (!ZERO_ON_FREE && likely(p != NULL) && total_size && total_size <= max_slab_size_class) {
         memset(p, 0, total_size - canary_size);
     }
+#ifdef HAS_ARM_MTE
+    // use an assert instead of adding a conditional to memset() above (freed memory is always
+    // zeroed when MTE is enabled)
+    static_assert(ZERO_ON_FREE, "disabling ZERO_ON_FREE reduces performance when ARM MTE is enabled");
+#endif
     return p;
 }
 
@@ -1385,11 +1469,14 @@ EXPORT void *h_realloc(void *old, size_t size) {
         }
     }
 
+    void *old_orig = old;
+    old = untag_pointer(old);
+
     size_t old_size;
     if (old < get_slab_region_end() && old >= ro.slab_region_start) {
         old_size = slab_usable_size(old);
         if (size <= max_slab_size_class && get_size_info(size).size == old_size) {
-            return old;
+            return old_orig;
         }
         thread_unseal_metadata();
     } else {
@@ -1502,7 +1589,7 @@ EXPORT void *h_realloc(void *old, size_t size) {
     if (copy_size > 0 && copy_size <= max_slab_size_class) {
         copy_size -= canary_size;
     }
-    memcpy(new, old, copy_size);
+    memcpy(new, old_orig, copy_size);
     if (old_size <= max_slab_size_class) {
         deallocate_small(old, NULL);
     } else {
@@ -1543,6 +1630,8 @@ EXPORT void h_free(void *p) {
         return;
     }
 
+    p = untag_pointer(p);
+
     if (p < get_slab_region_end() && p >= ro.slab_region_start) {
         thread_unseal_metadata();
         deallocate_small(p, NULL);
@@ -1566,6 +1655,8 @@ EXPORT void h_free_sized(void *p, size_t expected_size) {
         return;
     }
 
+    p = untag_pointer(p);
+
     expected_size = adjust_size_for_canary(expected_size);
 
     if (p < get_slab_region_end() && p >= ro.slab_region_start) {
@@ -1619,11 +1710,13 @@ static inline void memory_corruption_check_small(const void *p) {
     mutex_unlock(&c->lock);
 }
 
-EXPORT size_t h_malloc_usable_size(H_MALLOC_USABLE_SIZE_CONST void *p) {
-    if (p == NULL) {
+EXPORT size_t h_malloc_usable_size(H_MALLOC_USABLE_SIZE_CONST void *arg) {
+    if (arg == NULL) {
         return 0;
     }
 
+    void *p = untag_pointer((void *) (uintptr_t) arg);
+
     if (p < get_slab_region_end() && p >= ro.slab_region_start) {
         thread_unseal_metadata();
         memory_corruption_check_small(p);

memory.c

@@ -28,6 +28,20 @@ void *memory_map(size_t size) {
     return p;
 }
 
+#ifdef HAS_ARM_MTE
+// Note that PROT_MTE can't be cleared via mprotect
+void *memory_map_mte(size_t size) {
+    void *p = mmap(NULL, size, PROT_MTE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
+    if (unlikely(p == MAP_FAILED)) {
+        if (errno != ENOMEM) {
+            fatal_error("non-ENOMEM MTE mmap failure");
+        }
+        return NULL;
+    }
+    return p;
+}
+#endif
+
 bool memory_map_fixed(void *ptr, size_t size) {
     void *p = mmap(ptr, size, PROT_NONE, MAP_ANONYMOUS|MAP_PRIVATE|MAP_FIXED, -1, 0);
     bool ret = p == MAP_FAILED;

memory.h

@@ -11,6 +11,9 @@
 int get_metadata_key(void);
 
 void *memory_map(size_t size);
+#ifdef HAS_ARM_MTE
+void *memory_map_mte(size_t size);
+#endif
 bool memory_map_fixed(void *ptr, size_t size);
 bool memory_unmap(void *ptr, size_t size);
 bool memory_protect_ro(void *ptr, size_t size);

memtag.h

@@ -0,0 +1,52 @@
+#ifndef MEMTAG_H
+#define MEMTAG_H
+
+#include "util.h"
+
+#ifdef HAS_ARM_MTE
+#include "arm_mte.h"
+#define MEMTAG 1
+#define RESERVED_TAG 15
+#define TAG_WIDTH 4
+#endif
+
+#ifdef MEMTAG
+extern bool __is_memtag_enabled;
+#endif
+
+static inline bool is_memtag_enabled(void) {
+#ifdef MEMTAG
+    return __is_memtag_enabled;
+#else
+    return false;
+#endif
+}
+
+static inline void *untag_pointer(void *ptr) {
+#ifdef HAS_ARM_MTE
+    const uintptr_t mask = UINTPTR_MAX >> 8;
+    return (void *) ((uintptr_t) ptr & mask);
+#else
+    return ptr;
+#endif
+}
+
+static inline void *set_pointer_tag(void *ptr, u8 tag) {
+#ifdef HAS_ARM_MTE
+    return (void *) (((uintptr_t) tag << 56) | (uintptr_t) untag_pointer(ptr));
+#else
+    (void) tag;
+    return ptr;
+#endif
+}
+
+static inline u8 get_pointer_tag(void *ptr) {
+#ifdef HAS_ARM_MTE
+    return (((uintptr_t) ptr) >> 56) & 0xf;
+#else
+    (void) ptr;
+    return 0;
+#endif
+}
+
+#endif