From patchwork Thu Mar 24 20:00:35 2016 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Konrad Rzeszutek Wilk X-Patchwork-Id: 8664331 Return-Path: X-Original-To: patchwork-xen-devel@patchwork.kernel.org Delivered-To: patchwork-parsemail@patchwork1.web.kernel.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.136]) by patchwork1.web.kernel.org (Postfix) with ESMTP id 2D1199F326 for ; Thu, 24 Mar 2016 20:03:53 +0000 (UTC) Received: from mail.kernel.org (localhost [127.0.0.1]) by mail.kernel.org (Postfix) with ESMTP id AD17020395 for ; Thu, 24 Mar 2016 20:03:47 +0000 (UTC) Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (using TLSv1.2 with cipher AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id AAEC720380 for ; Thu, 24 Mar 2016 20:03:45 +0000 (UTC) Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.84_2) (envelope-from ) id 1ajBRb-0007W0-RF; Thu, 24 Mar 2016 20:01:23 +0000 Received: from mail6.bemta6.messagelabs.com ([85.158.143.247]) by lists.xenproject.org with esmtp (Exim 4.84_2) (envelope-from ) id 1ajBRZ-0007Qi-HW for xen-devel@lists.xenproject.org; Thu, 24 Mar 2016 20:01:21 +0000 Received: from [85.158.143.35] by server-2.bemta-6.messagelabs.com id 9D/26-09532-09744F65; Thu, 24 Mar 2016 20:01:20 +0000 X-Env-Sender: konrad@char.us.oracle.com X-Msg-Ref: server-5.tower-21.messagelabs.com!1458849678!5809985!1 X-Originating-IP: [141.146.126.69] X-SpamReason: No, hits=0.0 required=7.0 tests=sa_preprocessor: VHJ1c3RlZCBJUDogMTQxLjE0Ni4xMjYuNjkgPT4gMjc3MjE4\n X-StarScan-Received: X-StarScan-Version: 8.11; banners=-,-,- X-VirusChecked: Checked Received: (qmail 5596 invoked from network); 24 Mar 2016 20:01:19 -0000 Received: from aserp1040.oracle.com (HELO aserp1040.oracle.com) (141.146.126.69) by server-5.tower-21.messagelabs.com with DHE-RSA-AES256-GCM-SHA384 encrypted SMTP; 24 Mar 2016 20:01:19 -0000 Received: from userv0022.oracle.com (userv0022.oracle.com [156.151.31.74]) by aserp1040.oracle.com (Sentrion-MTA-4.3.2/Sentrion-MTA-4.3.2) with ESMTP id u2OK18EZ015326 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 24 Mar 2016 20:01:09 GMT Received: from aserv0121.oracle.com (aserv0121.oracle.com [141.146.126.235]) by userv0022.oracle.com (8.14.4/8.13.8) with ESMTP id u2OK17MW006946 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK); Thu, 24 Mar 2016 20:01:07 GMT Received: from abhmp0013.oracle.com (abhmp0013.oracle.com [141.146.116.19]) by aserv0121.oracle.com (8.13.8/8.13.8) with ESMTP id u2OK177e014989; Thu, 24 Mar 2016 20:01:07 GMT Received: from char.us.oracle.com (/10.137.176.158) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Thu, 24 Mar 2016 13:01:06 -0700 Received: by char.us.oracle.com (Postfix, from userid 1000) id 2C0F26A00EB; Thu, 24 Mar 2016 16:01:04 -0400 (EDT) From: Konrad Rzeszutek Wilk To: xen-devel@lists.xenproject.org, ross.lagerwall@citrix.com, konrad@kernel.org, andrew.cooper3@citrix.com, mpohlack@amazon.de, sasha.levin@oracle.com Date: Thu, 24 Mar 2016 16:00:35 -0400 Message-Id: <1458849640-22588-24-git-send-email-konrad.wilk@oracle.com> X-Mailer: git-send-email 2.5.0 In-Reply-To: <1458849640-22588-1-git-send-email-konrad.wilk@oracle.com> References: <1458849640-22588-1-git-send-email-konrad.wilk@oracle.com> X-Source-IP: userv0022.oracle.com [156.151.31.74] Cc: Keir Fraser , Jan Beulich , Konrad Rzeszutek Wilk Subject: [Xen-devel] [PATCH v5 23/28] xsplice: Stacking build-id dependency checking. X-BeenThere: xen-devel@lists.xen.org X-Mailman-Version: 2.1.18 Precedence: list List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , MIME-Version: 1.0 Errors-To: xen-devel-bounces@lists.xen.org Sender: "Xen-devel" X-Spam-Status: No, score=-4.2 required=5.0 tests=BAYES_00, RCVD_IN_DNSWL_MED, UNPARSEABLE_RELAY autolearn=unavailable version=3.3.1 X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on mail.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP We now expect that the ELF payloads be built with the --build-id. Also the .xsplice.deps section has to have the contents of the hypervisor (or a preceding payload) build-id. We already have the code to verify the Elf_Note build-id so export parts of it. This dependency means the hypervisor MUST be compiled with --build-id - so we gate the build of xSplice on the availability of said functionality. This does not impact the ordering of how the payloads can be loaded, but it does enforce an STRICT ordering when the payloads are applied. Also the REPLACE is special - we need to check that its dependency against the hypervisor - not the last applied patch. To make this easier to test we also add an extra test-case to be used - which can only be applied on top of the xen_hello_world payload. As in, one can apply xen_hello_world and then xen_bye_world on top of that. Not the other way. Signed-off-by: Konrad Rzeszutek Wilk --- Cc: Keir Fraser Cc: Jan Beulich Cc: Andrew Cooper v3: First time included. v4: Andrew fix against the build_id.o mutilations. Andrew fix to not include extra symbols in binary.id v5: s/ssize_t/unsigned int/ --- .gitignore | 1 + Config.mk | 1 + docs/misc/xsplice.markdown | 84 ++++++++++++++++++--------- xen/arch/x86/test/Makefile | 41 +++++++++++-- xen/arch/x86/test/xen_bye_world.c | 34 +++++++++++ xen/arch/x86/test/xen_bye_world_func.c | 25 ++++++++ xen/common/Kconfig | 6 +- xen/common/version.c | 40 +++++++++---- xen/common/xsplice.c | 103 ++++++++++++++++++++++++++++++++- xen/include/xen/version.h | 3 + xen/include/xen/xsplice.h | 5 ++ 11 files changed, 297 insertions(+), 46 deletions(-) create mode 100644 xen/arch/x86/test/xen_bye_world.c create mode 100644 xen/arch/x86/test/xen_bye_world_func.c diff --git a/.gitignore b/.gitignore index 8dc76b5..f1cadcf 100644 --- a/.gitignore +++ b/.gitignore @@ -247,6 +247,7 @@ xen/arch/x86/efi/disabled xen/arch/x86/efi/mkreloc xen/arch/x86/test/config.h xen/arch/x86/test/xen_hello_world.xsplice +xen/arch/x86/test/xen_bye_world.xsplice xen/arch/*/efi/boot.c xen/arch/*/efi/compat.c xen/arch/*/efi/efi.h diff --git a/Config.mk b/Config.mk index c8e89fe..80629e3 100644 --- a/Config.mk +++ b/Config.mk @@ -134,6 +134,7 @@ ifeq ($(call ld-ver-build-id,$(LD)),n) build_id_linker := else CFLAGS += -DBUILD_ID +export XEN_HAS_BUILD_ID=y build_id_linker := --build-id=sha1 endif diff --git a/docs/misc/xsplice.markdown b/docs/misc/xsplice.markdown index be9cd71..8b9ce9c 100644 --- a/docs/misc/xsplice.markdown +++ b/docs/misc/xsplice.markdown @@ -283,8 +283,7 @@ The xSplice core code loads the payload as a standard ELF binary, relocates it and handles the architecture-specifc sections as needed. This process is much like what the Linux kernel module loader does. -The payload contains a section (xsplice_patch_func) with an array of structures -describing the functions to be patched: +The payload contains at least three sections: It optionally may contain the address of functions to be called right before being applied and after being reverted: @@ -292,6 +291,15 @@ being applied and after being reverted: * `.xsplice.hooks.unload` - an array of function pointers. + * `.xsplice.funcs` - which is an array of xsplice_patch_func structures. + * `.xsplice.depends` - which is an ELF Note that describes what the payload + depends on. + * `.note.gnu.build-id` - the build-id of this payload. + +### .xsplice.funcs + +The `.xsplice.funcs` contains an array of xsplice_patch_func structures +which describe the functions to be patched: 
 struct xsplice_patch_func {  
     const char *name;  
@@ -333,7 +341,7 @@ When reverting a patch, the hypervisor iterates over each `xsplice_patch_func`
 and the core code copies the data from the undo buffer (private internal copy)
 to `old_addr`.
 
-### Example
+### Example of .xsplice.funcs
 
 A simple example of what a payload file can be:
 
@@ -379,10 +387,29 @@ Each entry in this array is eight bytes.
 
 The type definition of the function are as follow:
 
+
 
 typedef void (*xsplice_loadcall_t)(void);  
 typedef void (*xsplice_unloadcall_t)(void);   
 

+
+### .xsplice.depends and .note.gnu.build-id
+
+To support dependencies checking and safe loading (to load the
+appropiate payload against the right hypervisor) there is a need
+to embbed an build-id dependency.
+
+This is done by the payload containing an section `.xsplice.depends`
+which follows the format of an ELF Note. The contents of this
+(name, and description) are specific to the linker utilized to
+build the hypevisor and payload.
+
+If GNU linker is used then the name is `GNU` and the description
+is an NT_GNU_BUILD_ID type ID. The description can be an SHA1
+checksum, MD5 checksum or any unique value.
+
+The size of these structures varies with the --build-id linker option.
+
 ## Hypercalls
 
 We will employ the sub operations of the system management hypercall (sysctl).
@@ -879,30 +906,6 @@ This is implemented in the Xen Project hypervisor.
 
 Only the privileged domain should be allowed to do this operation.
 
-
-# Not Yet Done
-
-This is for further development of xSplice.
-
-## Goals
-
-The implementation must also have a mechanism for:
-
- *  An dependency mechanism for the payloads. To use that information to load:
-    - The appropiate payload. To verify that payload is built against the
-      hypervisor. This can be done via the `build-id`
-      or via providing an copy of the old code - so that the hypervisor can
-       verify it against the code in memory.
-    - To construct an appropiate order of payloads to load in case they
-      depend on each other.
- * Be able to lookup in the Xen hypervisor the symbol names of functions from the ELF payload.
- * Be able to patch .rodata, .bss, and .data sections.
- * Further safety checks (blacklist of which functions cannot be patched, check
-   the stack, make sure the payload is built with same compiler as hypervisor,
-   and NMI/MCE handlers and do_nmi for right now - until an safe solution is found).
- * NOP out the code sequence if `new_size` is zero.
- * Deal with other relocation types:  R_X86_64_[8,16,32,32S], R_X86_64_PC[8,16,64] in payload file.
-
 ### xSplice interdependencies
 
 xSplice patches interdependencies are tricky.
@@ -929,6 +932,33 @@ being loaded and requires an hypervisor build-id to match against.
 The old code allows much more flexibility and an additional guard,
 but is more complex to implement.
 
+The second option which requires an build-id of the hypervisor
+is implemented in the Xen Project hypervisor.
+
+Specifically each payload has two build-id ELF notes:
+ * The build-id of the payload itself (generated via --build-id).
+ * The build-id of the payload it depends on (extracted from the
+   the previous payload or hypervisor during build time).
+
+This means that the very first payload depends on the hypervisor
+build-id.
+
+# Not Yet Done
+
+This is for further development of xSplice.
+
+## Goals
+
+The implementation must also have a mechanism for:
+
+ * Be able to lookup in the Xen hypervisor the symbol names of functions from the ELF payload.
+ * Be able to patch .rodata, .bss, and .data sections.
+ * Further safety checks (blacklist of which functions cannot be patched, check
+   the stack, make sure the payload is built with same compiler as hypervisor,
+   and NMI/MCE handlers and do_nmi for right now - until an safe solution is found).
+ * NOP out the code sequence if `new_size` is zero.
+ * Deal with other relocation types:  R_X86_64_[8,16,32,32S], R_X86_64_PC[8,16,64] in payload file.
+
 ### Handle inlined __LINE__
 
 This problem is related to hotpatch construction
diff --git a/xen/arch/x86/test/Makefile b/xen/arch/x86/test/Makefile
index 22fcb14..8d355fb 100644
--- a/xen/arch/x86/test/Makefile
+++ b/xen/arch/x86/test/Makefile
@@ -7,13 +7,16 @@ CODE_SZ=$(shell nm --defined -S $(1) | grep $(2) | awk '{ print "0x"$$2}')
 ifdef CONFIG_XSPLICE
 
 XSPLICE := xen_hello_world.xsplice
+XSPLICE_BYE := xen_bye_world.xsplice
 
 default: xsplice
 
 install: xsplice
 	$(INSTALL_DATA) $(XSPLICE) $(DESTDIR)$(DEBUG_DIR)/$(XSPLICE)
+	$(INSTALL_DATA) $(XSPLICE_BYE) $(DESTDIR)$(DEBUG_DIR)/$(XSPLICE_BYE)
 uninstall:
 	rm -f $(DESTDIR)$(DEBUG_DIR)/$(XSPLICE)
+	rm -f $(DESTDIR)$(DEBUG_DIR)/$(XSPLICE_BYE)
 
 else
 
@@ -25,7 +28,7 @@ endif
 
 .PHONY: clean
 clean::
-	rm -f *.o .*.o.d $(XSPLICE) config.h
+	rm -f *.o .*.o.d $(XSPLICE) config.h build_id.o
 
 #
 # To compute these values we need the binary files: xen-syms
@@ -37,15 +40,43 @@ clean::
 .PHONY: config.h
 config.h: OLD_CODE_SZ=$(call CODE_SZ,$(BASEDIR)/xen-syms,xen_extra_version)
 config.h: NEW_CODE_SZ=$(call CODE_SZ,$<,xen_hello_world)
-config.h: xen_hello_world_func.o
+config.h: xen_hello_world_func.o xen_bye_world_func.o
 	(set -e; \
 	 echo "#define NEW_CODE_SZ $(NEW_CODE_SZ)"; \
 	 echo "#define OLD_CODE_SZ $(OLD_CODE_SZ)") > $@
 
+#
+# This target is only accessible if CONFIG_XSPLICE is defined, which
+# depends on $(build_id_linker) being available. Hence we do not
+# need any checks.
+#
+.PHONY: build_id.o
+build_id.o:
+	$(OBJCOPY) -O binary --only-section=.note $(BASEDIR)/xen-syms $@.bin
+	$(OBJCOPY) -I binary -O elf64-x86-64 -B i386:x86-64 \
+		   --rename-section=.data=.xsplice.depends -S $@.bin $@
+	rm -f $@.bin
+
+#
+# Extract the build-id of the xen_hello_world.xsplice
+# (which xen_bye_world will depend on).
+#
+.PHONY: hello_world_build_id.o
+hello_world_build_id.o:
+	$(OBJCOPY) -O binary --only-section=.note.gnu.build-id $(XSPLICE) $@.bin
+	$(OBJCOPY)  -I binary -O elf64-x86-64 -B i386:x86-64 \
+		   --rename-section=.data=.xsplice.depends -S $@.bin $@
+	rm -f $@.bin
+
 .PHONY: xsplice
-xsplice: config.h
+xsplice: config.h build_id.o
 	# Need to have these done in sequential order
 	$(MAKE) -f $(BASEDIR)/Rules.mk xen_hello_world_func.o
 	$(MAKE) -f $(BASEDIR)/Rules.mk xen_hello_world.o
-	$(LD) $(LDFLAGS) -r -o $(XSPLICE) xen_hello_world_func.o \
-		xen_hello_world.o
+	$(LD) $(LDFLAGS) $(build_id_linker) -r -o $(XSPLICE) \
+		xen_hello_world_func.o xen_hello_world.o build_id.o
+	$(MAKE) -f $(BASEDIR)/Rules.mk xen_bye_world_func.o
+	$(MAKE) -f $(BASEDIR)/Rules.mk xen_bye_world.o
+	$(MAKE) -f $(BASEDIR)/Rules.mk hello_world_build_id.o
+	$(LD) $(LDFLAGS) $(build_id_linker) -r -o $(XSPLICE_BYE) \
+		xen_bye_world_func.o xen_bye_world.o hello_world_build_id.o
diff --git a/xen/arch/x86/test/xen_bye_world.c b/xen/arch/x86/test/xen_bye_world.c
new file mode 100644
index 0000000..f9e3b8a
--- /dev/null
+++ b/xen/arch/x86/test/xen_bye_world.c
@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) 2016 Oracle and/or its affiliates. All rights reserved.
+ *
+ */
+
+#include 
+#include 
+#include 
+#include "config.h"
+#include 
+
+static char xen_bye_world_name[] = "xen_bye_world";
+extern const char *xen_bye_world(void);
+
+/* External symbol. */
+extern const char *xen_extra_version(void);
+
+struct xsplice_patch_func __section(".xsplice.funcs") xsplice_xen_bye_world = {
+    .name = xen_bye_world_name,
+    .new_addr = (unsigned long)(xen_bye_world),
+    .old_addr = (unsigned long)(xen_extra_version),
+    .new_size = NEW_CODE_SZ,
+    .old_size = OLD_CODE_SZ,
+};
+
+/*
+ * Local variables:
+ * mode: C
+ * c-file-style: "BSD"
+ * c-basic-offset: 4
+ * tab-width: 4
+ * indent-tabs-mode: nil
+ * End:
+ */
diff --git a/xen/arch/x86/test/xen_bye_world_func.c b/xen/arch/x86/test/xen_bye_world_func.c
new file mode 100644
index 0000000..574268c
--- /dev/null
+++ b/xen/arch/x86/test/xen_bye_world_func.c
@@ -0,0 +1,25 @@
+/*
+ * Copyright (c) 2016 Oracle and/or its affiliates. All rights reserved.
+ *
+ */
+
+#include 
+#include 
+#include 
+#include 
+
+/* Our replacement function for xen_hello_world. */
+const char *xen_bye_world(void)
+{
+    return "Bye World!";
+}
+
+/*
+ * Local variables:
+ * mode: C
+ * c-file-style: "BSD"
+ * c-basic-offset: 4
+ * tab-width: 4
+ * indent-tabs-mode: nil
+ * End:
+ */
diff --git a/xen/common/Kconfig b/xen/common/Kconfig
index d80dddb..0b8917c 100644
--- a/xen/common/Kconfig
+++ b/xen/common/Kconfig
@@ -54,6 +54,10 @@ config HAS_GDBSX
 config HAS_IOPORTS
 	bool
 
+config HAS_BUILD_ID
+	string
+	option env="XEN_HAS_BUILD_ID"
+
 # Enable/Disable kexec support
 config KEXEC
 	bool "kexec support"
@@ -186,7 +190,7 @@ endmenu
 config XSPLICE
 	bool "xSplice live patching support"
 	default n
-	depends on X86
+	depends on X86 && HAS_BUILD_ID = "y"
 	---help---
 	  Allows a running Xen hypervisor to be dynamically patched using
 	  binary patches without rebooting. This is primarily used to binarily
diff --git a/xen/common/version.c b/xen/common/version.c
index 35078c7..b722a38 100644
--- a/xen/common/version.c
+++ b/xen/common/version.c
@@ -70,10 +70,29 @@ const char *xen_deny(void)
 /* Defined in linker script. */
 extern const Elf_Note __note_gnu_build_id_start[], __note_gnu_build_id_end[];
 
+int xen_build_id_check(const Elf_Note *n, const void **p, unsigned int *len)
+{
+    /* Check if we really have a build-id. */
+    if ( NT_GNU_BUILD_ID != n->type )
+        return -ENODATA;
+
+    /* Sanity check, name should be "GNU" for ld-generated build-id. */
+    if ( strncmp(ELFNOTE_NAME(n), "GNU", n->namesz) != 0 )
+        return -ENODATA;
+
+    if ( len )
+        *len = n->descsz;
+    if ( p )
+        *p = ELFNOTE_DESC(n);
+
+    return 0;
+}
+
 int xen_build_id(const void **p, unsigned int *len)
 {
     const Elf_Note *n = __note_gnu_build_id_start;
     static bool_t checked = 0;
+    int rc;
 
     if ( checked )
     {
@@ -89,23 +108,20 @@ int xen_build_id(const void **p, unsigned int *len)
     if ( &n[1] > __note_gnu_build_id_end )
         return -ENODATA;
 
-    /* Check if we really have a build-id. */
-    if ( NT_GNU_BUILD_ID != n->type )
-        return -ENODATA;
-
-    /* Sanity check, name should be "GNU" for ld-generated build-id. */
-    if ( strncmp(ELFNOTE_NAME(n), "GNU", n->namesz) != 0 )
-        return -ENODATA;
-
-    *len = n->descsz;
-    *p = ELFNOTE_DESC(n);
+    rc = xen_build_id_check(n, p, len);
+    if ( !rc )
+        checked = 1;
 
-    checked = 1;
-    return 0;
+    return rc;
 }
 
 #else
 
+int xen_build_id_check(const Elf_Note *n, const void **p, unsigned int *len)
+{
+    return -ENODATA;
+}
+
 int xen_build_id(const void **p, unsigned int *len)
 {
     return -ENODATA;
diff --git a/xen/common/xsplice.c b/xen/common/xsplice.c
index ec47f31..d7a65fe 100644
--- a/xen/common/xsplice.c
+++ b/xen/common/xsplice.c
@@ -4,6 +4,7 @@
  */
 
 #include 
+#include 
 #include 
 #include 
 #include 
@@ -68,6 +69,8 @@ struct payload {
     xsplice_unloadcall_t *unload_funcs;  /* load and unload of the payload. */
     unsigned int n_load_funcs;           /* Nr of the funcs to load and execute. */
     unsigned int n_unload_funcs;         /* Nr of funcs to call durung unload. */
+    struct xsplice_build_id id;          /* ELFNOTE_DESC(.note.gnu.build-id) of the payload. */
+    struct xsplice_build_id dep;         /* ELFNOTE_DESC(.xsplice.depends). */
     char name[XEN_XSPLICE_NAME_SIZE];    /* Name of it. */
 };
 
@@ -440,7 +443,9 @@ static int check_special_sections(struct payload *payload,
                                   struct xsplice_elf *elf)
 {
     unsigned int i;
-    static const char *const names[] = { ".xsplice.funcs" };
+    static const char *const names[] = { ".xsplice.funcs" ,
+                                         ".xsplice.depends",
+                                         ".note.gnu.build-id"};
 
     for ( i = 0; i < ARRAY_SIZE(names); i++ )
     {
@@ -461,6 +466,8 @@ static int check_special_sections(struct payload *payload,
     return 0;
 }
 
+#define NT_GNU_BUILD_ID 3
+
 static int prepare_payload(struct payload *payload,
                            struct xsplice_elf *elf)
 {
@@ -468,6 +475,7 @@ static int prepare_payload(struct payload *payload,
     unsigned int i;
     struct xsplice_patch_func *f;
     struct virtual_region *region;
+    Elf_Note *n;
 
     sec = xsplice_elf_sec_by_name(elf, ".xsplice.funcs");
     if ( sec )
@@ -545,6 +553,33 @@ static int prepare_payload(struct payload *payload,
         payload->n_unload_funcs = sec->sec->sh_size / (sizeof *payload->unload_funcs);
     }
 
+    sec = xsplice_elf_sec_by_name(elf, ".note.gnu.build-id");
+    if ( sec )
+    {
+        n = (Elf_Note *)sec->load_addr;
+        if ( sec->sec->sh_size <= sizeof *n )
+            return -EINVAL;
+
+        if ( xen_build_id_check(n, &payload->id.p, &payload->id.len) )
+            return -EINVAL;
+
+        if ( !payload->id.len || !payload->id.p )
+            return -EINVAL;
+    }
+
+    sec = xsplice_elf_sec_by_name(elf, ".xsplice.depends");
+    {
+        n = (Elf_Note *)sec->load_addr;
+        if ( sec->sec->sh_size <= sizeof *n )
+            return -EINVAL;
+
+        if ( xen_build_id_check(n, &payload->dep.p, &payload->dep.len) )
+            return -EINVAL;
+
+        if ( !payload->dep.len || !payload->dep.p )
+            return -EINVAL;
+    }
+
     /* Setup the virtual region with proper data. */
     region = &payload->region;
 
@@ -1261,6 +1296,54 @@ void check_for_xsplice_work(void)
     }
 }
 
+/*
+ * Only allow dependent payload is applied on top of the correct
+ * build-id.
+ *
+ * This enforces an stacking order - the first payload MUST be against the
+ * hypervisor. The second against the first payload, and so on.
+ *
+ * Unless the 'ignore' parameter is used - in which case we only
+ * check against the hypervisor.
+ */
+static int build_id_dep(struct payload *payload, bool_t ignore)
+{
+    const void *id = NULL;
+    unsigned int len = 0;
+    int rc;
+    const char *name = "hypervisor";
+
+    ASSERT(payload->dep.len && payload->dep.p);
+
+    /* First time user is against hypervisor. */
+    if ( ignore || list_empty(&applied_list) )
+    {
+        rc = xen_build_id(&id, &len);
+        if ( rc )
+            return rc;
+    }
+    else
+    {
+        /* We should be against the last applied one. */
+        struct payload *data = list_last_entry(&applied_list, struct payload,
+                                               applied_list);
+
+        id = data->id.p;
+        len = data->id.len;
+        name = data->name;
+    }
+
+    if ( payload->dep.len != len ||
+         memcmp(id, payload->dep.p, len) )
+    {
+        dprintk(XENLOG_DEBUG, "%s%s: check against %s build-id failed!\n",
+                XSPLICE, payload->name, name);
+        return -EINVAL;
+    }
+
+    return 0;
+}
+
 static int xsplice_action(xen_sysctl_xsplice_action_t *action)
 {
     struct payload *data;
@@ -1304,6 +1387,18 @@ static int xsplice_action(xen_sysctl_xsplice_action_t *action)
     case XSPLICE_ACTION_REVERT:
         if ( data->state == XSPLICE_STATE_APPLIED )
         {
+            struct payload *p = list_last_entry(&applied_list, struct payload,
+                                                   applied_list);
+
+            ASSERT(p);
+            /* We should be the last applied one. */
+            if ( p != data )
+            {
+                dprintk(XENLOG_DEBUG, "%s%s: can't unload. Top is %s!\n",
+                        XSPLICE, data->name, p->name);
+                rc = -EBUSY;
+                break;
+            }
             data->rc = -EAGAIN;
             rc = schedule_work(data, action->cmd, action->timeout);
         }
@@ -1312,6 +1407,9 @@ static int xsplice_action(xen_sysctl_xsplice_action_t *action)
     case XSPLICE_ACTION_APPLY:
         if ( data->state == XSPLICE_STATE_CHECKED )
         {
+            rc = build_id_dep(data, 0 /* against top one. */);
+            if ( rc )
+                break;
             data->rc = -EAGAIN;
             rc = schedule_work(data, action->cmd, action->timeout);
         }
@@ -1320,6 +1418,9 @@ static int xsplice_action(xen_sysctl_xsplice_action_t *action)
     case XSPLICE_ACTION_REPLACE:
         if ( data->state == XSPLICE_STATE_CHECKED )
         {
+            rc = build_id_dep(data, 1 /* against hypervisor. */);
+            if ( rc )
+                break;
             data->rc = -EAGAIN;
             rc = schedule_work(data, action->cmd, action->timeout);
         }
diff --git a/xen/include/xen/version.h b/xen/include/xen/version.h
index 4b57e18..a06bb61 100644
--- a/xen/include/xen/version.h
+++ b/xen/include/xen/version.h
@@ -17,4 +17,7 @@ const char *xen_deny(void);
 #include 
 int xen_build_id(const void **p, unsigned int *len);
 
+#include 
+int xen_build_id_check(const Elf_Note *n, const void **p, unsigned int *len);
+
 #endif /* __XEN_VERSION_H__ */
diff --git a/xen/include/xen/xsplice.h b/xen/include/xen/xsplice.h
index 47ef1c2..fa50fb5 100644
--- a/xen/include/xen/xsplice.h
+++ b/xen/include/xen/xsplice.h
@@ -40,6 +40,11 @@ struct xsplice_symbol {
     bool_t new_symbol;
 };
 
+struct xsplice_build_id {
+   const void *p;
+   unsigned int len;
+};
+
 int xsplice_op(struct xen_sysctl_xsplice_op *);
 void check_for_xsplice_work(void);
 bool_t is_patch(const void *addr);