livepatch: Do not enforce ELF_LIVEPATCH_FUNC section presence

With default implementation the ELF_LIVEPATCH_FUNC section containing
all functions to be replaced or added must be part of the livepatch
payload, otherwise the payload is rejected (with -EINVAL).

However, with the extended hooks implementation, a livepatch may be
constructed of only hooks to perform certain actions without any code
to be added or replaced.
Therefore, do not always expect the functions section and allow it to
be missing, provided there is at least one section containing hooks
present. The functions section, when present in a payload, must be a
single, non-empty section.

Check also all extended hooks sections if they are a single, non-empty
sections each.

At least one of the functions or hooks section must be present in a
valid payload.

Signed-off-by: Pawel Wieczorkiewicz <wipawel@amazon.de>
Reviewed-by: Andra-Irina Paraschiv <andraprs@amazon.com>
Reviewed-by: Bjoern Doebel <doebel@amazon.de>
Reviewed-by: Martin Pohlack <mpohlack@amazon.de>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Ross Lagerwall <ross.lagerwall@citrix.com>

xen/common/livepatch.c

@@ -467,8 +467,7 @@ static int xen_build_id_dep(const struct payload *payload)
 static int check_special_sections(const struct livepatch_elf *elf)
 {
     unsigned int i;
-    static const char *const names[] = { ELF_LIVEPATCH_FUNC,
-                                         ELF_LIVEPATCH_DEPENDS,
+    static const char *const names[] = { ELF_LIVEPATCH_DEPENDS,
                                          ELF_LIVEPATCH_XEN_DEPENDS,
                                          ELF_BUILD_ID_NOTE};
     DECLARE_BITMAP(found, ARRAY_SIZE(names)) = { 0 };
@@ -503,6 +502,64 @@ static int check_special_sections(const struct livepatch_elf *elf)
     return 0;
 }
 
+static int check_patching_sections(const struct livepatch_elf *elf)
+{
+    unsigned int i;
+    static const char *const names[] = { ELF_LIVEPATCH_FUNC,
+                                         ELF_LIVEPATCH_LOAD_HOOKS,
+                                         ELF_LIVEPATCH_UNLOAD_HOOKS,
+                                         ELF_LIVEPATCH_PREAPPLY_HOOK,
+                                         ELF_LIVEPATCH_APPLY_HOOK,
+                                         ELF_LIVEPATCH_POSTAPPLY_HOOK,
+                                         ELF_LIVEPATCH_PREREVERT_HOOK,
+                                         ELF_LIVEPATCH_REVERT_HOOK,
+                                         ELF_LIVEPATCH_POSTREVERT_HOOK};
+    DECLARE_BITMAP(found, ARRAY_SIZE(names)) = { 0 };
+
+    /*
+     * The patching sections are optional, but at least one
+     * must be present. Otherwise, there is nothing to do.
+     * All the existing sections must not be empty and must
+     * be present at most once.
+     */
+    for ( i = 0; i < ARRAY_SIZE(names); i++ )
+    {
+        const struct livepatch_elf_sec *sec;
+
+        sec = livepatch_elf_sec_by_name(elf, names[i]);
+        if ( !sec )
+        {
+            dprintk(XENLOG_DEBUG, LIVEPATCH "%s: %s is missing\n",
+                    elf->name, names[i]);
+            continue; /* This section is optional */
+        }
+
+        if ( !sec->sec->sh_size )
+        {
+            printk(XENLOG_ERR LIVEPATCH "%s: %s is empty\n",
+                   elf->name, names[i]);
+            return -EINVAL;
+        }
+
+        if ( test_and_set_bit(i, found) )
+        {
+            printk(XENLOG_ERR LIVEPATCH "%s: %s was seen more than once\n",
+                   elf->name, names[i]);
+            return -EINVAL;
+        }
+    }
+
+    /* Checking if at least one section is present. */
+    if ( bitmap_empty(found, ARRAY_SIZE(names)) )
+    {
+        printk(XENLOG_ERR LIVEPATCH "%s: Nothing to patch. Aborting...\n",
+               elf->name);
+        return -EINVAL;
+    }
+
+    return 0;
+}
+
 /*
  * Lookup specified section and when exists assign its address to a specified hook.
  * Perform section pointer and size validation: single hook sections must contain a
@@ -542,57 +599,59 @@ static int prepare_payload(struct payload *payload,
     const Elf_Note *n;
 
     sec = livepatch_elf_sec_by_name(elf, ELF_LIVEPATCH_FUNC);
-    ASSERT(sec);
-    if ( !section_ok(elf, sec, sizeof(*payload->funcs)) )
-        return -EINVAL;
-
-    payload->funcs = sec->load_addr;
-    payload->nfuncs = sec->sec->sh_size / sizeof(*payload->funcs);
-
-    for ( i = 0; i < payload->nfuncs; i++ )
+    if ( sec )
     {
-        int rc;
-
-        f = &(payload->funcs[i]);
-
-        if ( f->version != LIVEPATCH_PAYLOAD_VERSION )
-        {
-            printk(XENLOG_ERR LIVEPATCH "%s: Wrong version (%u). Expected %d\n",
-                   elf->name, f->version, LIVEPATCH_PAYLOAD_VERSION);
-            return -EOPNOTSUPP;
-        }
-
-        /* 'old_addr', 'new_addr', 'new_size' can all be zero. */
-        if ( !f->old_size )
-        {
-            printk(XENLOG_ERR LIVEPATCH "%s: Address or size fields are zero\n",
-                   elf->name);
+        if ( !section_ok(elf, sec, sizeof(*payload->funcs)) )
             return -EINVAL;
+
+        payload->funcs = sec->load_addr;
+        payload->nfuncs = sec->sec->sh_size / sizeof(*payload->funcs);
+
+        for ( i = 0; i < payload->nfuncs; i++ )
+        {
+            int rc;
+
+            f = &(payload->funcs[i]);
+
+            if ( f->version != LIVEPATCH_PAYLOAD_VERSION )
+            {
+                printk(XENLOG_ERR LIVEPATCH "%s: Wrong version (%u). Expected %d\n",
+                       elf->name, f->version, LIVEPATCH_PAYLOAD_VERSION);
+                return -EOPNOTSUPP;
+            }
+
+            /* 'old_addr', 'new_addr', 'new_size' can all be zero. */
+            if ( !f->old_size )
+            {
+                printk(XENLOG_ERR LIVEPATCH "%s: Address or size fields are zero\n",
+                       elf->name);
+                return -EINVAL;
+            }
+
+            rc = arch_livepatch_verify_func(f);
+            if ( rc )
+                return rc;
+
+            rc = resolve_old_address(f, elf);
+            if ( rc )
+                return rc;
+
+            rc = livepatch_verify_distance(f);
+            if ( rc )
+                return rc;
         }
-
-        rc = arch_livepatch_verify_func(f);
-        if ( rc )
-            return rc;
-
-        rc = resolve_old_address(f, elf);
-        if ( rc )
-            return rc;
-
-        rc = livepatch_verify_distance(f);
-        if ( rc )
-            return rc;
     }
 
-    LIVEPATCH_ASSIGN_MULTI_HOOK(elf, payload->load_funcs, payload->n_load_funcs, ".livepatch.hooks.load");
-    LIVEPATCH_ASSIGN_MULTI_HOOK(elf, payload->unload_funcs, payload->n_unload_funcs, ".livepatch.hooks.unload");
+    LIVEPATCH_ASSIGN_MULTI_HOOK(elf, payload->load_funcs, payload->n_load_funcs, ELF_LIVEPATCH_LOAD_HOOKS);
+    LIVEPATCH_ASSIGN_MULTI_HOOK(elf, payload->unload_funcs, payload->n_unload_funcs, ELF_LIVEPATCH_UNLOAD_HOOKS);
 
-    LIVEPATCH_ASSIGN_SINGLE_HOOK(elf, payload->hooks.apply.pre, ".livepatch.hooks.preapply");
-    LIVEPATCH_ASSIGN_SINGLE_HOOK(elf, payload->hooks.apply.action, ".livepatch.hooks.apply");
-    LIVEPATCH_ASSIGN_SINGLE_HOOK(elf, payload->hooks.apply.post, ".livepatch.hooks.postapply");
+    LIVEPATCH_ASSIGN_SINGLE_HOOK(elf, payload->hooks.apply.pre, ELF_LIVEPATCH_PREAPPLY_HOOK);
+    LIVEPATCH_ASSIGN_SINGLE_HOOK(elf, payload->hooks.apply.action, ELF_LIVEPATCH_APPLY_HOOK);
+    LIVEPATCH_ASSIGN_SINGLE_HOOK(elf, payload->hooks.apply.post, ELF_LIVEPATCH_POSTAPPLY_HOOK);
 
-    LIVEPATCH_ASSIGN_SINGLE_HOOK(elf, payload->hooks.revert.pre, ".livepatch.hooks.prerevert");
-    LIVEPATCH_ASSIGN_SINGLE_HOOK(elf, payload->hooks.revert.action, ".livepatch.hooks.revert");
-    LIVEPATCH_ASSIGN_SINGLE_HOOK(elf, payload->hooks.revert.post, ".livepatch.hooks.postrevert");
+    LIVEPATCH_ASSIGN_SINGLE_HOOK(elf, payload->hooks.revert.pre, ELF_LIVEPATCH_PREREVERT_HOOK);
+    LIVEPATCH_ASSIGN_SINGLE_HOOK(elf, payload->hooks.revert.action, ELF_LIVEPATCH_REVERT_HOOK);
+    LIVEPATCH_ASSIGN_SINGLE_HOOK(elf, payload->hooks.revert.post, ELF_LIVEPATCH_POSTREVERT_HOOK);
 
     sec = livepatch_elf_sec_by_name(elf, ELF_BUILD_ID_NOTE);
     if ( sec )
@@ -786,8 +845,6 @@ static int build_symbol_table(struct payload *payload,
     struct livepatch_symbol *symtab;
     char *strtab;
 
-    ASSERT(payload->nfuncs);
-
     /* Recall that section @0 is always NULL. */
     for ( i = 1; i < elf->nsym; i++ )
     {
@@ -904,6 +961,10 @@ static int load_payload_data(struct payload *payload, void *raw, size_t len)
     if ( rc )
         goto out;
 
+    rc = check_patching_sections(&elf);
+    if ( rc )
+        goto out;
+
     rc = prepare_payload(payload, &elf);
     if ( rc )
         goto out;

xen/include/xen/livepatch.h

@@ -33,6 +33,14 @@ struct xen_sysctl_livepatch_op;
 #define ELF_LIVEPATCH_DEPENDS     ".livepatch.depends"
 #define ELF_LIVEPATCH_XEN_DEPENDS ".livepatch.xen_depends"
 #define ELF_BUILD_ID_NOTE         ".note.gnu.build-id"
+#define ELF_LIVEPATCH_LOAD_HOOKS      ".livepatch.hooks.load"
+#define ELF_LIVEPATCH_UNLOAD_HOOKS    ".livepatch.hooks.unload"
+#define ELF_LIVEPATCH_PREAPPLY_HOOK   ".livepatch.hooks.preapply"
+#define ELF_LIVEPATCH_APPLY_HOOK      ".livepatch.hooks.apply"
+#define ELF_LIVEPATCH_POSTAPPLY_HOOK  ".livepatch.hooks.postapply"
+#define ELF_LIVEPATCH_PREREVERT_HOOK  ".livepatch.hooks.prerevert"
+#define ELF_LIVEPATCH_REVERT_HOOK     ".livepatch.hooks.revert"
+#define ELF_LIVEPATCH_POSTREVERT_HOOK ".livepatch.hooks.postrevert"
 /* Arbitrary limit for payload size and .bss section size. */
 #define LIVEPATCH_MAX_SIZE     MB(2)
 

xen/test/livepatch/Makefile

@@ -23,6 +23,7 @@ LIVEPATCH_NO_XEN_BUILDID := xen_no_xen_buildid.livepatch
 LIVEPATCH_PREPOST_HOOKS := xen_prepost_hooks.livepatch
 LIVEPATCH_PREPOST_HOOKS_FAIL := xen_prepost_hooks_fail.livepatch
 LIVEPATCH_ACTION_HOOKS := xen_action_hooks.livepatch
+LIVEPATCH_ACTION_HOOKS_NOFUNC := xen_action_hooks_nofunc.livepatch
 
 LIVEPATCHES += $(LIVEPATCH)
 LIVEPATCHES += $(LIVEPATCH_BYE)
@@ -32,6 +33,7 @@ LIVEPATCHES += $(LIVEPATCH_NO_XEN_BUILDID)
 LIVEPATCHES += $(LIVEPATCH_PREPOST_HOOKS)
 LIVEPATCHES += $(LIVEPATCH_PREPOST_HOOKS_FAIL)
 LIVEPATCHES += $(LIVEPATCH_ACTION_HOOKS)
+LIVEPATCHES += $(LIVEPATCH_ACTION_HOOKS_NOFUNC)
 
 LIVEPATCH_DEBUG_DIR ?= $(DEBUG_DIR)/xen-livepatch
 
@@ -152,6 +154,11 @@ xen_actions_hooks.o: config.h
 $(LIVEPATCH_ACTION_HOOKS): xen_action_hooks.o xen_hello_world_func.o note.o xen_note.o
 	$(LD) $(LDFLAGS) $(build_id_linker) -r -o $(LIVEPATCH_ACTION_HOOKS) $^
 
+.PHONY: $(LIVEPATCH_ACTION_HOOKS_NOFUNC)
+$(LIVEPATCH_ACTION_HOOKS_NOFUNC): xen_action_hooks_nofunc.o note.o xen_note.o
+	$(LD) $(LDFLAGS) $(build_id_linker) -r -o $(LIVEPATCH_ACTION_HOOKS_NOFUNC) $^
+
 .PHONY: livepatch
 livepatch: $(LIVEPATCH) $(LIVEPATCH_BYE) $(LIVEPATCH_REPLACE) $(LIVEPATCH_NOP) $(LIVEPATCH_NO_XEN_BUILDID) \
-           $(LIVEPATCH_PREPOST_HOOKS) $(LIVEPATCH_PREPOST_HOOKS_FAIL) $(LIVEPATCH_ACTION_HOOKS)
+           $(LIVEPATCH_PREPOST_HOOKS) $(LIVEPATCH_PREPOST_HOOKS_FAIL) $(LIVEPATCH_ACTION_HOOKS) \
+           $(LIVEPATCH_ACTION_HOOKS_NOFUNC)

xen/test/livepatch/xen_action_hooks_nofunc.c

@@ -0,0 +1,86 @@
+/*
+ * Copyright (c) 2019 Amazon.com, Inc. or its affiliates. All rights reserved.
+ *
+ */
+
+#include "config.h"
+#include <xen/lib.h>
+#include <xen/types.h>
+#include <xen/version.h>
+#include <xen/livepatch.h>
+#include <xen/livepatch_payload.h>
+
+#include <public/sysctl.h>
+
+static unsigned int apply_cnt;
+static unsigned int revert_cnt;
+
+static int apply_hook(livepatch_payload_t *payload)
+{
+    int i;
+
+    printk(KERN_DEBUG "%s: Hook starting.\n", __func__);
+
+    for (i = 0; i < payload->nfuncs; i++)
+    {
+        struct livepatch_func *func = &payload->funcs[i];
+
+        apply_cnt++;
+        printk(KERN_DEBUG "%s: applying: %s\n", __func__, func->name);
+    }
+
+    printk(KERN_DEBUG "%s: Hook done.\n", __func__);
+
+    return 0;
+}
+
+static int revert_hook(livepatch_payload_t *payload)
+{
+    int i;
+
+    printk(KERN_DEBUG "%s: Hook starting.\n", __func__);
+
+    for (i = 0; i < payload->nfuncs; i++)
+    {
+        struct livepatch_func *func = &payload->funcs[i];
+
+        revert_cnt++;
+        printk(KERN_DEBUG "%s: reverting: %s\n", __func__, func->name);
+    }
+
+    printk(KERN_DEBUG "%s: Hook done.\n", __func__);
+
+    return 0;
+}
+
+static void post_revert_hook(livepatch_payload_t *payload)
+{
+    int i;
+
+    printk(KERN_DEBUG "%s: Hook starting.\n", __func__);
+
+    for (i = 0; i < payload->nfuncs; i++)
+    {
+        struct livepatch_func *func = &payload->funcs[i];
+
+        printk(KERN_DEBUG "%s: reverted: %s\n", __func__, func->name);
+    }
+
+    BUG_ON(apply_cnt > 0 || revert_cnt > 0);
+    printk(KERN_DEBUG "%s: Hook done.\n", __func__);
+}
+
+LIVEPATCH_APPLY_HOOK(apply_hook);
+LIVEPATCH_REVERT_HOOK(revert_hook);
+
+LIVEPATCH_POSTREVERT_HOOK(post_revert_hook);
+
+/*
+ * Local variables:
+ * mode: C
+ * c-file-style: "BSD"
+ * c-basic-offset: 4
+ * tab-width: 4
+ * indent-tabs-mode: nil
+ * End:
+ */