malloc(9): Zero padding space in realloc with CHERI.

When growing an existing allocation with an unrepresentable length,
we need to initialize any padding space to ensure there are no data leaks.
When copying data to a new allocation, we must ensure that no data
past the original capability length is copied.

sys/kern/kern_malloc.c

@@ -1126,8 +1126,7 @@ realloc(void *addr, size_t size, struct malloc_type *mtp, int flags)
 	unsigned long alloc;
 	void *newaddr;
 #ifdef __CHERI_PURE_CAPABILITY__
-	uma_keg_t keg;
-	int i;
+	size_t olength;
 #endif
 
 	KASSERT(mtp->ks_version == M_VERSION,
@@ -1143,6 +1142,7 @@ realloc(void *addr, size_t size, struct malloc_type *mtp, int flags)
 		panic("Expect valid capability");
 	if (__predict_false(cheri_getsealed(addr)))
 		panic("Expect unsealed capability");
+	olength = cheri_getlen(addr);
 #endif
 
 	/*
@@ -1167,18 +1167,26 @@ realloc(void *addr, size_t size, struct malloc_type *mtp, int flags)
 	/* Get the size of the original block */
 	switch (GET_SLAB_COOKIE(slab)) {
 	case __predict_true(SLAB_COOKIE_SLAB_PTR):
-#ifdef __CHERI_PURE_CAPABILITY__
-		alloc = zone->uz_size;
-		keg = zone->uz_keg;
-		i = slab_item_index(slab, keg, addr);
-		addr = slab_item(slab, keg, i);
-		addr = cheri_setboundsexact(addr, alloc);
-#else
 		alloc = zone->uz_size;
+#ifdef __CHERI_PURE_CAPABILITY__
+		if (size > olength) {
+			addr = uma_zgrow_bounds(zone, addr);
+			KASSERT(cheri_getlen(addr) == alloc,
+			    ("realloc mismatch uma_zgrow_bounds: %zx != %zx",
+			        cheri_getlen(addr), alloc));
+		}
 #endif
 		break;
 	case SLAB_COOKIE_MALLOC_LARGE:
 		alloc = malloc_large_size(slab);
+#ifdef __CHERI_PURE_CAPABILITY__
+		if (size > olength) {
+			addr = malloc_large_grow_bounds(zone, addr);
+			KASSERT(cheri_getlen(addr) == size,
+			    ("realloc large object bounds mismatch: %zx != %zx",
+			        cheri_getlen(addr), size));
+		}
+#endif
 		break;
 	default:
 #ifdef INVARIANTS
@@ -1192,7 +1200,19 @@ realloc(void *addr, size_t size, struct malloc_type *mtp, int flags)
 	if (size <= alloc &&
 	    (size > (alloc >> REALLOC_FRACTION) || alloc == MINALLOCSIZE)) {
 		kasan_mark((void *)addr, size, alloc, KASAN_MALLOC_REDZONE);
+#ifdef __CHERI_PURE_CAPABILITY__
+		addr = cheri_setbounds(addr, size);
+		/*
+		 * Zero only the non-representable portion of allocation
+		 * that was not reachable from the original capability.
+		 */
+		if (size > olength) {
+			bzero((void *)((uintptr_t)addr + olength),
+			    cheri_getlen(addr) - olength);
+		}
+#else
 		return (addr);
+#endif
 	}
 #endif /* !DEBUG_REDZONE */
 
@@ -1205,7 +1225,16 @@ realloc(void *addr, size_t size, struct malloc_type *mtp, int flags)
 	 * valid before performing the copy.
 	 */
 	kasan_mark(addr, alloc, alloc, 0);
+#ifdef __CHERI_PURE_CAPABILITY__
+	/*
+	 * We have unbounded addr here, need to avoid copying
+	 * past the original length.
+	 * XXX-AM: is it worth it re-setting bounds on addr?
+	 */
+	bcopy(addr, newaddr, min(size, olength));
+#else
 	bcopy(addr, newaddr, min(size, alloc));
+#endif
 	free(addr, mtp);
 	return (newaddr);
 }