[RFC PATCH v2 2/6] module: preserve Elf information for livepatch modules

[Jessica Yu]

For livepatch modules, copy Elf section, symbol, and string information
from the load_info struct in the module loader.

Livepatch uses special relocation sections in order to be able to patch
modules that are not yet loaded, as well as apply patches to the kernel
when the addresses of symbols cannot be determined at compile time (for
example, when kaslr is enabled). Livepatch modules must preserve Elf
information such as section indices in order to apply the remaining
relocation sections at the appropriate time (i.e. when the target module
loads).

Signed-off-by: Jessica Yu <jeyu@xxxxxxxxxx>
---
 include/linux/module.h |  9 +++++
 kernel/module.c        | 98 ++++++++++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 105 insertions(+), 2 deletions(-)

diff --git a/include/linux/module.h b/include/linux/module.h
index 3a19c79..9b46256 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -425,6 +425,14 @@ struct module {
 
 	/* Notes attributes */
 	struct module_notes_attrs *notes_attrs;
+
+	/* Elf information (optionally saved) */
+	Elf_Ehdr *hdr;
+	Elf_Shdr *sechdrs;
+	char *secstrings;
+	struct {
+		unsigned int sym, str, mod, vers, info, pcpu;
+	} index;
 #endif
 
 	/* The command line arguments (may be mangled).  People like
@@ -461,6 +469,7 @@ struct module {
 #endif
 
 #ifdef CONFIG_LIVEPATCH
+	bool klp; /* Is this a livepatch module? */
 	bool klp_alive;
 #endif
 
diff --git a/kernel/module.c b/kernel/module.c
index 8f051a1..433c2d6 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -1984,6 +1984,13 @@ static void unset_module_core_ro_nx(struct module *mod) { }
 static void unset_module_init_ro_nx(struct module *mod) { }
 #endif
 
+static void free_module_elf(struct module *mod)
+{
+	kfree(mod->hdr);
+	kfree(mod->sechdrs);
+	kfree(mod->secstrings);
+}
+
 void __weak module_memfree(void *module_region)
 {
 	vfree(module_region);
@@ -2022,6 +2029,9 @@ static void free_module(struct module *mod)
 	/* Free any allocated parameters. */
 	destroy_params(mod->kp, mod->num_kp);
 
+	/* Free Elf information if it was saved */
+	free_module_elf(mod);
+
 	/* Now we can delete it from the lists */
 	mutex_lock(&module_mutex);
 	/* Unlink carefully: kallsyms could be walking list. */
@@ -2137,6 +2147,10 @@ static int simplify_symbols(struct module *mod, const struct load_info *info)
 			       (long)sym[i].st_value);
 			break;
 
+		case SHN_LIVEPATCH:
+			/* klp symbols are resolved by livepatch */
+			break;
+
 		case SHN_UNDEF:
 			ksym = resolve_symbol_wait(mod, info, name);
 			/* Ok if resolved.  */
@@ -2185,6 +2199,10 @@ static int apply_relocations(struct module *mod, const struct load_info *info)
 		if (!(info->sechdrs[infosec].sh_flags & SHF_ALLOC))
 			continue;
 
+		/* klp relocation sections are applied by livepatch */
+		if (info->sechdrs[i].sh_flags & SHF_RELA_LIVEPATCH)
+			continue;
+
 		if (info->sechdrs[i].sh_type == SHT_REL)
 			err = apply_relocate(info->sechdrs, info->strtab,
 					     info->index.sym, i, mod);
@@ -2393,6 +2411,11 @@ static char elf_type(const Elf_Sym *sym, const struct load_info *info)
 {
 	const Elf_Shdr *sechdrs = info->sechdrs;
 
+	if (ELF_ST_BIND(sym->st_info) == STB_LIVEPATCH_EXT)
+		return 'K';
+	if (sym->st_shndx == SHN_LIVEPATCH)
+		return 'k';
+
 	if (ELF_ST_BIND(sym->st_info) == STB_WEAK) {
 		if (ELF_ST_TYPE(sym->st_info) == STT_OBJECT)
 			return 'v';
@@ -2475,7 +2498,7 @@ static void layout_symtab(struct module *mod, struct load_info *info)
 
 	/* Compute total space required for the core symbols' strtab. */
 	for (ndst = i = 0; i < nsrc; i++) {
-		if (i == 0 ||
+		if (i == 0 || mod->klp ||
 		    is_core_symbol(src+i, info->sechdrs, info->hdr->e_shnum)) {
 			strtab_size += strlen(&info->strtab[src[i].st_name])+1;
 			ndst++;
@@ -2517,7 +2540,7 @@ static void add_kallsyms(struct module *mod, const struct load_info *info)
 	mod->core_strtab = s = mod->module_core + info->stroffs;
 	src = mod->symtab;
 	for (ndst = i = 0; i < mod->num_symtab; i++) {
-		if (i == 0 ||
+		if (i == 0 || mod->klp ||
 		    is_core_symbol(src+i, info->sechdrs, info->hdr->e_shnum)) {
 			dst[ndst] = src[i];
 			dst[ndst++].st_name = s - mod->core_strtab;
@@ -2638,6 +2661,64 @@ static int elf_header_check(struct load_info *info)
 	return 0;
 }
 
+/*
+ * copy_module_elf - preserve Elf information about a module
+ */
+static int copy_module_elf(struct module *mod, struct load_info *info)
+{
+	unsigned int size;
+	int ret = 0;
+	Elf_Shdr *symsect;
+
+	/* Elf header */
+	size = sizeof(Elf_Ehdr);
+	mod->hdr = kzalloc(size, GFP_KERNEL);
+	if (mod->hdr == NULL) {
+		ret = -ENOMEM;
+		goto out;
+	}
+	memcpy(mod->hdr, info->hdr, size);
+
+	/* Elf section header table */
+	size = sizeof(Elf_Shdr) * info->hdr->e_shnum;
+	mod->sechdrs = kzalloc(size, GFP_KERNEL);
+	if (mod->sechdrs == NULL) {
+		ret = -ENOMEM;
+		goto free_hdr;
+	}
+	memcpy(mod->sechdrs, info->sechdrs, size);
+
+	/* Elf section name string table */
+	size = info->sechdrs[info->hdr->e_shstrndx].sh_size;
+	mod->secstrings = kzalloc(size, GFP_KERNEL);
+	if (mod->secstrings == NULL) {
+		ret = -ENOMEM;
+		goto free_sechdrs;
+	}
+	memcpy(mod->secstrings, info->secstrings, size);
+
+	/* Elf section indices */
+	memcpy(&mod->index, &info->index, sizeof(info->index));
+
+	/*
+	 * Update symtab's sh_addr to point to a valid
+	 * symbol table, as the temporary symtab in module
+	 * init memory will be freed
+	 */
+	symsect = mod->sechdrs + mod->index.sym;
+	symsect->sh_addr = (unsigned long)mod->core_symtab;
+
+	return ret;
+
+free_sechdrs:
+	kfree(mod->sechdrs);
+free_hdr:
+	kfree(mod->hdr);
+out:
+	return ret;
+}
+
+
 #define COPY_CHUNK_SIZE (16*PAGE_SIZE)
 
 static int copy_chunked_from_user(void *dst, const void __user *usrc, unsigned long len)
@@ -2866,6 +2947,9 @@ static int check_modinfo(struct module *mod, struct load_info *info, int flags)
 			"is unknown, you have been warned.\n", mod->name);
 	}
 
+	if (get_modinfo(info, "livepatch"))
+		mod->klp = true;
+
 	/* Set up license info based on the info section */
 	set_license(mod, get_modinfo(info, "license"));
 
@@ -3530,6 +3614,16 @@ static int load_module(struct load_info *info, const char __user *uargs,
 	if (err < 0)
 		goto bug_cleanup;
 
+	/*
+	 * Save sechdrs, indices, and other data from info
+	 * in order to patch to-be-loaded modules.
+	 * Do not call free_copy() for livepatch modules.
+	 */
+	if (mod->klp)
+		err = copy_module_elf(mod, info);
+	if (err < 0)
+		goto bug_cleanup;
+
 	/* Get rid of temporary copy. */
 	free_copy(info);
 
-- 
2.4.3

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