rtld: Use compartment IDs from sub-object compartments for policy enf…

…orcement - Add a helper function to lookup the relevant compartment ID for a given virtual address and shared object. - Save a compartment ID for each PLT (based on the address of the associated PLT GOT) and use this as the "subject" (caller) for policy enforcement when handling PLT GOT relocations. - Use the target address of a function call to determine the "object" (callee). Note: rtld currently does not enforce any policy for access to data via the normal GOT. Co-authored-by: Dapeng Gao <[email protected]>
CTSRD-CHERI · Jan 29, 2025 · ac754ad · ac754ad
1 parent 58e4e87
commit ac754ad
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Showing 8 changed files with 96 additions and 47 deletions.
diff --git a/libexec/rtld-elf/aarch64/reloc.c b/libexec/rtld-elf/aarch64/reloc.c
@@ -492,7 +492,8 @@ reloc_plt(Plt_Entry *plt, int flags, RtldLockState *lockstate)
 				target = (uintptr_t)make_function_pointer(def,
 				    defobj);
 #ifdef CHERI_LIB_C18N
-				target = (uintptr_t)tramp_intern(obj,
+				target = (uintptr_t)tramp_intern(plt,
+				    plt->compart_id,
 				    &(struct tramp_data) {
 					.target = (void *)target,
 					.defobj = defobj,
@@ -571,7 +572,8 @@ reloc_jmpslots(Plt_Entry *plt, int flags, RtldLockState *lockstate)
 			}
 			target = (uintptr_t)make_function_pointer(def, defobj);
 #ifdef CHERI_LIB_C18N
-			target = (uintptr_t)tramp_intern(obj, &(struct tramp_data) {
+			target = (uintptr_t)tramp_intern(plt, plt->compart_id,
+			    &(struct tramp_data) {
 				.target = (void *)target,
 				.defobj = defobj,
 				.def = def,
@@ -632,7 +634,8 @@ reloc_iresolve_one(Obj_Entry *obj, const Elf_Rela *rela,
 #endif
 	lock_release(rtld_bind_lock, lockstate);
 #ifdef CHERI_LIB_C18N
-	ptr = (uintptr_t)tramp_intern(NULL, &(struct tramp_data) {
+	ptr = (uintptr_t)tramp_intern(NULL, RTLD_COMPART_ID,
+	    &(struct tramp_data) {
 		.target = (void *)ptr,
 		.defobj = obj,
 		.sig = (struct func_sig) { .valid = true,
@@ -729,7 +732,8 @@ reloc_gnu_ifunc_plt(Plt_Entry *plt, int flags, RtldLockState *lockstate)
 			lock_release(rtld_bind_lock, lockstate);
 			target = (uintptr_t)rtld_resolve_ifunc(defobj, def);
 #ifdef CHERI_LIB_C18N
-			target = (uintptr_t)tramp_intern(obj, &(struct tramp_data) {
+			target = (uintptr_t)tramp_intern(plt, plt->compart_id,
+			    &(struct tramp_data) {
 				.target = (void *)target,
 				.defobj = defobj,
 				.def = def,

diff --git a/libexec/rtld-elf/aarch64/rtld_c18n_machdep.c b/libexec/rtld-elf/aarch64/rtld_c18n_machdep.c
@@ -67,6 +67,7 @@ tramp_compile(char **entry, const struct tramp_data *data)
 	char *buf = *entry;
 	size_t hook_off, count_off;
 	size_t header_off, target_off, landing_off, unused_regs;
+	compart_id_t callee;
 	bool executive = cheri_getperm(data->target) & CHERI_PERM_EXECUTIVE;
 	bool count = ld_compartment_switch_count != NULL;
 	bool hook = ld_compartment_utrace != NULL ||
@@ -143,9 +144,10 @@ tramp_compile(char **entry, const struct tramp_data *data)
 	target_off = size + offsetof(struct tramp_header, target);
 	*entry = buf + size;
 	size += offsetof(struct tramp_header, entry);
+	callee = compart_id_for_address(data->defobj, (ptraddr_t)data->target);
 
 	COPY(push_frame);
-	PATCH_MOV(push_frame, cid, cid_to_index(data->defobj->compart_id).val);
+	PATCH_MOV(push_frame, cid, cid_to_index(callee).val);
 	landing_off = PATCH_OFF(push_frame, landing);
 	/*
 	 * The trampoline computes the number of return value registers and
@@ -299,7 +301,7 @@ _rtld_sandbox_code(void *target, struct func_sig sig)
 		target = cheri_sealentry(target_unsealed);
 	}
 
-	target = tramp_intern(NULL, &(struct tramp_data) {
+	target = tramp_intern(NULL, RTLD_COMPART_ID, &(struct tramp_data) {
 		.target = target,
 		.defobj = obj,
 		.sig = sig

diff --git a/libexec/rtld-elf/aarch64/rtld_machdep.h b/libexec/rtld-elf/aarch64/rtld_machdep.h
@@ -86,15 +86,17 @@ uintptr_t reloc_jmpslot(uintptr_t *where, uintptr_t target,
 /* TODO: Per-function captable/PLT/FNDESC support */
 #ifdef CHERI_LIB_C18N
 #define call_init_array_pointer(_obj, _target)				\
-	(((InitArrFunc)tramp_intern(NULL, &(struct tramp_data) {	\
+	(((InitArrFunc)tramp_intern(NULL, RTLD_COMPART_ID,		\
+	    &(struct tramp_data) {					\
 		.target = (void *)(_target).value,			\
 		.defobj = _obj,						\
 		.sig = (struct func_sig) { .valid = true,		\
 		    .reg_args = 3, .mem_args = false, .ret_args = NONE }\
 	}))(main_argc, main_argv, environ))
 
 #define call_fini_array_pointer(_obj, _target)				\
-	(((InitFunc)tramp_intern(NULL, &(struct tramp_data) {		\
+	(((InitFunc)tramp_intern(NULL, RTLD_COMPART_ID,			\
+	    &(struct tramp_data) {					\
 		.target = (void *)(_target).value,			\
 		.defobj = _obj,						\
 		.sig = (struct func_sig) { .valid = true,		\

diff --git a/libexec/rtld-elf/rtld.c b/libexec/rtld-elf/rtld.c
@@ -1152,7 +1152,8 @@ _rtld(Elf_Addr *sp, func_ptr_type *exit_proc, Obj_Entry **objp)
     if (rtld_exit_ptr == NULL) {
 	rtld_exit_ptr = make_rtld_function_pointer(rtld_exit);
 #ifdef CHERI_LIB_C18N
-	rtld_exit_ptr = tramp_intern(NULL, &(struct tramp_data) {
+	rtld_exit_ptr = tramp_intern(NULL, RTLD_COMPART_ID,
+	    &(struct tramp_data) {
 		.target = rtld_exit_ptr,
 		.defobj = &obj_rtld,
 		.sig = (struct func_sig) {
@@ -1166,7 +1167,8 @@ _rtld(Elf_Addr *sp, func_ptr_type *exit_proc, Obj_Entry **objp)
     *objp = obj_main;
 
 #ifdef CHERI_LIB_C18N
-    return ((func_ptr_type)tramp_intern(NULL, &(struct tramp_data) {
+    return ((func_ptr_type)tramp_intern(NULL, RTLD_COMPART_ID,
+	&(struct tramp_data) {
 	    .target = cheri_sealentry(obj_main->entry),
 	    .defobj = obj_main,
 	    .sig = (struct func_sig) {
@@ -1187,7 +1189,7 @@ rtld_resolve_ifunc(const Obj_Entry *obj, const Elf_Sym *def)
 
 	ptr = (void *)make_function_pointer(def, obj);
 #ifdef CHERI_LIB_C18N
-	ptr = tramp_intern(NULL, &(struct tramp_data) {
+	ptr = tramp_intern(NULL, RTLD_COMPART_ID, &(struct tramp_data) {
 		.target = ptr,
 		.defobj = obj,
 		.def = def,
@@ -1238,7 +1240,8 @@ _rtld_bind(Plt_Entry *plt, Elf_Size reloff)
 #ifdef __CHERI_PURE_CAPABILITY__
 	target = (uintptr_t)make_function_pointer(def, defobj);
 #ifdef CHERI_LIB_C18N
-	target = (uintptr_t)tramp_intern(obj, &(struct tramp_data) {
+	target = (uintptr_t)tramp_intern(plt, plt->compart_id,
+	    &(struct tramp_data) {
 	    .target = (void *)target,
 	    .defobj = defobj,
 	    .def = def,
@@ -3644,7 +3647,8 @@ objlist_call_init(Objlist *list, RtldLockState *lockstate)
 	if (reg != NULL) {
 		func_ptr_type exit_ptr = make_rtld_function_pointer(rtld_exit);
 #ifdef CHERI_LIB_C18N
-		exit_ptr = tramp_intern(NULL, &(struct tramp_data) {
+		exit_ptr = tramp_intern(NULL, RTLD_COMPART_ID,
+		    &(struct tramp_data) {
 			.target = exit_ptr,
 			.defobj = &obj_rtld,
 			.sig = (struct func_sig) {
@@ -3657,7 +3661,8 @@ objlist_call_init(Objlist *list, RtldLockState *lockstate)
 		reg(exit_ptr);
 		rtld_exit_ptr = make_rtld_function_pointer(rtld_nop_exit);
 #ifdef CHERI_LIB_C18N
-		rtld_exit_ptr = tramp_intern(NULL, &(struct tramp_data) {
+		rtld_exit_ptr = tramp_intern(NULL, RTLD_COMPART_ID,
+		    &(struct tramp_data) {
 			.target = rtld_exit_ptr,
 			.defobj = &obj_rtld,
 			.sig = (struct func_sig) {
@@ -4830,7 +4835,7 @@ dl_iterate_phdr(__dl_iterate_hdr_callback callback, void *param)
 	error = 0;
 
 #ifdef CHERI_LIB_C18N
-	callback = tramp_intern(NULL, &(struct tramp_data) {
+	callback = tramp_intern(NULL, RTLD_COMPART_ID, &(struct tramp_data) {
 		.target = callback,
 		.defobj = obj_from_addr(callback),
 		.sig = (struct func_sig) {
@@ -5141,7 +5146,7 @@ get_program_var_addr(const char *name, RtldLockState *lockstate)
     if (ELF_ST_TYPE(req.sym_out->st_info) == STT_FUNC) {
 	void *target = make_function_pointer(req.sym_out, req.defobj_out);
 #ifdef CHERI_LIB_C18N
-	target = tramp_intern(NULL, &(struct tramp_data) {
+	target = tramp_intern(NULL, RTLD_COMPART_ID, &(struct tramp_data) {
 		.target = target,
 		.defobj = req.defobj_out,
 		.def = req.sym_out
@@ -5151,7 +5156,7 @@ get_program_var_addr(const char *name, RtldLockState *lockstate)
     } else if (ELF_ST_TYPE(req.sym_out->st_info) == STT_GNU_IFUNC) {
 	void *target = rtld_resolve_ifunc(req.defobj_out, req.sym_out);
 #ifdef CHERI_LIB_C18N
-	target = tramp_intern(NULL, &(struct tramp_data) {
+	target = tramp_intern(NULL, RTLD_COMPART_ID, &(struct tramp_data) {
 		.target = target,
 		.defobj = req.defobj_out,
 		.def = req.sym_out
@@ -6319,8 +6324,8 @@ c18n_setup_compartments(Obj_Entry *obj, const char *name)
 	const Elf_Phdr *ph;
 	size_t len;
 
-	assert(obj->compart_id == 0);
-	obj->compart_id = compart_id_allocate(name);
+	assert(obj->default_compart_id == 0);
+	obj->default_compart_id = compart_id_allocate(name);
 
 	for (ph = obj->phdr;  (const char *)ph < (const char *)obj->phdr +
 	    obj->phsize; ph++) {
@@ -6358,6 +6363,28 @@ c18n_setup_compartments(Obj_Entry *obj, const char *name)
 	}
 }
 
+compart_id_t
+compart_id_for_address(const Obj_Entry *obj, Elf_Addr addr)
+{
+	assert(cheri_is_address_inbounds(obj->relocbase, addr));
+
+	for (unsigned long i = 0; i < obj->ncomparts; i++) {
+		if (addr >= obj->comparts[i].start &&
+		    addr < obj->comparts[i].end)
+			return (obj->comparts[i].compart_id);
+	}
+	return (obj->default_compart_id);
+}
+
+static void
+c18n_assign_plt_compartments(Obj_Entry *obj)
+{
+	for (unsigned long i = 0; i < obj->nplts; i++) {
+		obj->plts[i].compart_id = compart_id_for_address(obj,
+		    (ptraddr_t)obj->plts[i].pltgot);
+	}
+}
+
 static bool
 c18n_add_obj(Obj_Entry *obj, const char *name)
 {
@@ -6382,6 +6409,7 @@ c18n_add_obj(Obj_Entry *obj, const char *name)
 	}
 
 	c18n_setup_compartments(obj, name);
+	c18n_assign_plt_compartments(obj);
 	return (true);
 }
 #endif

diff --git a/libexec/rtld-elf/rtld.h b/libexec/rtld-elf/rtld.h
@@ -188,6 +188,9 @@ typedef struct Struct_Plt_Entry {
     const Elf_Rela *rela;	/* PLT relocation entries with addend */
     unsigned long relasize;	/* Size in bytes of PLT addend reloc info */
     bool jmpslots_done : 1;	/* Already have relocated the jump slots */
+#ifdef CHERI_LIB_C18N
+    uint16_t compart_id;
+#endif
     MD_PLT_ENTRY;
 } Plt_Entry;
 
@@ -320,7 +323,7 @@ typedef struct Struct_Obj_Entry {
 
 #ifdef CHERI_LIB_C18N
     const char *soname;
-    uint16_t compart_id;
+    uint16_t default_compart_id;
     const struct func_sig *sigtab;
 #endif
 

diff --git a/libexec/rtld-elf/rtld_c18n.c b/libexec/rtld-elf/rtld_c18n.c
@@ -561,8 +561,10 @@ evaluate_rules(compart_id_t caller, compart_id_t callee, const char *sym)
 }
 
 static bool
-tramp_should_include(const Obj_Entry *reqobj, const struct tramp_data *data)
+tramp_should_include(const Plt_Entry *plt, compart_id_t caller,
+    const struct tramp_data *data)
 {
+	compart_id_t callee;
 	const char *sym;
 
 	/* XXX: This not be needed once function pointers are wrapped. */
@@ -580,23 +582,26 @@ tramp_should_include(const Obj_Entry *reqobj, const struct tramp_data *data)
 	if (string_base_search(&uni_compart.trusts, sym) != -1)
 		return (false);
 
-	if (reqobj == NULL)
-		return (true);
+	callee = compart_id_for_address(data->defobj, (ptraddr_t)data->target);
 
-	if (reqobj->compart_id == data->defobj->compart_id)
+	/*
+	 * Jump slots within the same compartment do not require a
+	 * trampoline.
+	 */
+	if (plt != NULL && caller == callee)
 		return (false);
 
-	if (string_base_search(&comparts.data[reqobj->compart_id].trusts, sym)
+	if (string_base_search(&comparts.data[caller].trusts, sym)
 	    != -1)
 		return (false);
 
-	if (evaluate_rules(reqobj->compart_id, data->defobj->compart_id, sym))
+	if (evaluate_rules(caller, callee, sym))
 		return (true);
 
 	rtld_fatal("c18n: Policy violation: %s is not allowed to access symbol "
 	    "%s defined by %s",
-	    comparts.data[reqobj->compart_id].name, sym,
-	    comparts.data[data->defobj->compart_id].name);
+	    comparts.data[caller].name, sym,
+	    comparts.data[callee].name);
 }
 
 /*
@@ -1363,7 +1368,8 @@ tramp_check_sig(const struct tramp_header *found, const Obj_Entry *reqobj,
 		    "%s requests " C18N_SIG_FORMAT_STRING " but "
 		    "%s provides " C18N_SIG_FORMAT_STRING,
 		    symname(found->defobj, found->symnum),
-		    reqobj->path, C18N_SIG_FORMAT(data->sig),
+		    reqobj != NULL ? reqobj->path : "",
+		    C18N_SIG_FORMAT(data->sig),
 		    found->defobj->path, C18N_SIG_FORMAT(sig));
 	}
 }
@@ -1394,7 +1400,8 @@ tramp_make_entry(const struct tramp_header *header)
 }
 
 void *
-tramp_intern(const Obj_Entry *reqobj, const struct tramp_data *data)
+tramp_intern(const Plt_Entry *plt, compart_id_t caller,
+    const struct tramp_data *data)
 {
 	RtldLockState lockstate;
 	const struct tramp_header *header;
@@ -1413,15 +1420,11 @@ tramp_intern(const Obj_Entry *reqobj, const struct tramp_data *data)
 	assert(cheri_gettag(data->defobj));
 	if (data->def == NULL)
 		/*
-		 * XXX-DG: reqobj != NULL causes policies to be evaluated which
-		 * might result in a trampoline being elided. This is only safe
-		 * to do for jump slot relocations.
-		 *
 		 * Currently, the decision to elide the trampoline or not is
 		 * coupled with the decision of whether the symbol should be
-		 * made accesible to the requesting object. This is insecure.
+		 * made accessible to the requesting object. This is insecure.
 		 */
-		assert(reqobj == NULL);
+		assert(plt == NULL);
 	else if (data->def == &sym_zero)
 		assert(data->target == NULL);
 	else
@@ -1430,7 +1433,7 @@ tramp_intern(const Obj_Entry *reqobj, const struct tramp_data *data)
 		    data->defobj->dynsymcount);
 	assert(func_sig_legal(data->sig));
 
-	if (!tramp_should_include(reqobj, data))
+	if (!tramp_should_include(plt, caller, data))
 		return (data->target);
 
 start:
@@ -1531,7 +1534,7 @@ tramp_intern(const Obj_Entry *reqobj, const struct tramp_data *data)
 end:
 	lock_release(rtld_tramp_lock, &lockstate);
 
-	tramp_check_sig(header, reqobj, data);
+	tramp_check_sig(header, plt != NULL ? plt->obj : NULL, data);
 
 	/*
 	 * Most consumers use type (void *) for function pointers.
@@ -1739,7 +1742,8 @@ c18n_init2(Obj_Entry *obj_rtld)
 	 * XXX: Manually wrap _rtld_unw_setcontext_impl in a trampoline for now
 	 * because it is called via a function pointer.
 	*/
-	_rtld_unw_setcontext_ptr = tramp_intern(NULL, &(struct tramp_data) {
+	_rtld_unw_setcontext_ptr = tramp_intern(NULL, RTLD_COMPART_ID,
+	    &(struct tramp_data) {
 		.target = &_rtld_unw_setcontext_impl,
 		.defobj = obj_rtld,
 		.sig = (struct func_sig) {
@@ -1763,7 +1767,8 @@ _rtld_thread_start_init(void (*p)(struct pthread *))
 {
 	assert((cheri_getperm(p) & CHERI_PERM_EXECUTIVE) == 0);
 	assert(thr_thread_start == NULL);
-	thr_thread_start = tramp_intern(NULL, &(struct tramp_data) {
+	thr_thread_start = tramp_intern(NULL, RTLD_COMPART_ID,
+	    &(struct tramp_data) {
 		.target = p,
 		.defobj = obj_from_addr(p),
 		.sig = (struct func_sig) {
@@ -1907,7 +1912,8 @@ _rtld_sighandler_init(__siginfohandler_t *handler)
 {
 	assert((cheri_getperm(handler) & CHERI_PERM_EXECUTIVE) == 0);
 	assert(signal_dispatcher == sigdispatch);
-	signal_dispatcher = tramp_intern(NULL, &(struct tramp_data) {
+	signal_dispatcher = tramp_intern(NULL, RTLD_COMPART_ID,
+	    &(struct tramp_data) {
 		.target = handler,
 		.defobj = obj_from_addr(handler),
 		.sig = (struct func_sig) {
@@ -2145,13 +2151,16 @@ _rtld_siginvoke(int sig, siginfo_t *info, ucontext_t *ucp,
 	header = tramp_reflect(sigfunc);
 	if (header == NULL) {
 		defobj = obj_from_addr(sigfunc);
-		sigfunc = tramp_intern(NULL, &(struct tramp_data) {
+		callee = compart_id_for_address(defobj, (ptraddr_t)sigfunc);
+		sigfunc = tramp_intern(NULL, RTLD_COMPART_ID,
+		    &(struct tramp_data) {
 		    .target = sigfunc,
 		    .defobj = defobj
 		});
-	} else
-		defobj = header->defobj;
-	callee = defobj->compart_id;
+	} else {
+		callee = compart_id_for_address(header->defobj,
+		    (ptraddr_t)header->target);
+	}
 	callee_idx = cid_to_index(callee);
 
 	/*