From 7c8e2bdd5f0d990e2398ee3deafc626dd469fc2d Mon Sep 17 00:00:00 2001 From: Josh Poimboeuf Date: Wed, 29 Apr 2020 10:24:44 -0500 Subject: livepatch: Apply vmlinux-specific KLP relocations early KLP relocations are livepatch-specific relocations which are applied to a KLP module's text or data. They exist for two reasons: 1) Unexported symbols: replacement functions often need to access unexported symbols (e.g. static functions), which "normal" relocations don't allow. 2) Late module patching: this is the ability for a KLP module to bypass normal module dependencies, such that the KLP module can be loaded *before* a to-be-patched module. This means that relocations which need to access symbols in the to-be-patched module might need to be applied to the KLP module well after it has been loaded. Non-late-patched KLP relocations are applied from the KLP module's init function. That usually works fine, unless the patched code wants to use alternatives, paravirt patching, jump tables, or some other special section which needs relocations. Then we run into ordering issues and crashes. In order for those special sections to work properly, the KLP relocations should be applied *before* the special section init code runs, such as apply_paravirt(), apply_alternatives(), or jump_label_apply_nops(). You might think the obvious solution would be to move the KLP relocation initialization earlier, but it's not necessarily that simple. The problem is the above-mentioned late module patching, for which KLP relocations can get applied well after the KLP module is loaded. To "fix" this issue in the past, we created .klp.arch sections: .klp.arch.{module}..altinstructions .klp.arch.{module}..parainstructions Those sections allow KLP late module patching code to call apply_paravirt() and apply_alternatives() after the module-specific KLP relocations (.klp.rela.{module}.{section}) have been applied. But that has a lot of drawbacks, including code complexity, the need for arch-specific code, and the (per-arch) danger that we missed some special section -- for example the __jump_table section which is used for jump labels. It turns out there's a simpler and more functional approach. There are two kinds of KLP relocation sections: 1) vmlinux-specific KLP relocation sections .klp.rela.vmlinux.{sec} These are relocations (applied to the KLP module) which reference unexported vmlinux symbols. 2) module-specific KLP relocation sections .klp.rela.{module}.{sec}: These are relocations (applied to the KLP module) which reference unexported or exported module symbols. Up until now, these have been treated the same. However, they're inherently different. Because of late module patching, module-specific KLP relocations can be applied very late, thus they can create the ordering headaches described above. But vmlinux-specific KLP relocations don't have that problem. There's nothing to prevent them from being applied earlier. So apply them at the same time as normal relocations, when the KLP module is being loaded. This means that for vmlinux-specific KLP relocations, we no longer have any ordering issues. vmlinux-referencing jump labels, alternatives, and paravirt patching will work automatically, without the need for the .klp.arch hacks. All that said, for module-specific KLP relocations, the ordering problems still exist and we *do* still need .klp.arch. Or do we? Stay tuned. Suggested-by: Peter Zijlstra Signed-off-by: Josh Poimboeuf Acked-by: Peter Zijlstra (Intel) Acked-by: Joe Lawrence Acked-by: Miroslav Benes Acked-by: Jessica Yu Signed-off-by: Jiri Kosina --- kernel/livepatch/core.c | 137 ++++++++++++++++++++++++++++++------------------ 1 file changed, 86 insertions(+), 51 deletions(-) (limited to 'kernel/livepatch') diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c index 40cfac8156fd..c02791e5c75b 100644 --- a/kernel/livepatch/core.c +++ b/kernel/livepatch/core.c @@ -191,12 +191,12 @@ static int klp_find_object_symbol(const char *objname, const char *name, return -EINVAL; } -static int klp_resolve_symbols(Elf_Shdr *relasec, struct module *pmod) +static int klp_resolve_symbols(Elf64_Shdr *sechdrs, const char *strtab, + unsigned int symndx, Elf_Shdr *relasec) { int i, cnt, vmlinux, ret; char objname[MODULE_NAME_LEN]; char symname[KSYM_NAME_LEN]; - char *strtab = pmod->core_kallsyms.strtab; Elf_Rela *relas; Elf_Sym *sym; unsigned long sympos, addr; @@ -216,7 +216,7 @@ static int klp_resolve_symbols(Elf_Shdr *relasec, struct module *pmod) relas = (Elf_Rela *) relasec->sh_addr; /* For each rela in this klp relocation section */ for (i = 0; i < relasec->sh_size / sizeof(Elf_Rela); i++) { - sym = pmod->core_kallsyms.symtab + ELF_R_SYM(relas[i].r_info); + sym = (Elf64_Sym *)sechdrs[symndx].sh_addr + ELF_R_SYM(relas[i].r_info); if (sym->st_shndx != SHN_LIVEPATCH) { pr_err("symbol %s is not marked as a livepatch symbol\n", strtab + sym->st_name); @@ -246,54 +246,59 @@ static int klp_resolve_symbols(Elf_Shdr *relasec, struct module *pmod) return 0; } -static int klp_write_object_relocations(struct module *pmod, - struct klp_object *obj) +/* + * At a high-level, there are two types of klp relocation sections: those which + * reference symbols which live in vmlinux; and those which reference symbols + * which live in other modules. This function is called for both types: + * + * 1) When a klp module itself loads, the module code calls this function to + * write vmlinux-specific klp relocations (.klp.rela.vmlinux.* sections). + * These relocations are written to the klp module text to allow the patched + * code/data to reference unexported vmlinux symbols. They're written as + * early as possible to ensure that other module init code (.e.g., + * jump_label_apply_nops) can access any unexported vmlinux symbols which + * might be referenced by the klp module's special sections. + * + * 2) When a to-be-patched module loads -- or is already loaded when a + * corresponding klp module loads -- klp code calls this function to write + * module-specific klp relocations (.klp.rela.{module}.* sections). These + * are written to the klp module text to allow the patched code/data to + * reference symbols which live in the to-be-patched module or one of its + * module dependencies. Exported symbols are supported, in addition to + * unexported symbols, in order to enable late module patching, which allows + * the to-be-patched module to be loaded and patched sometime *after* the + * klp module is loaded. + */ +int klp_apply_section_relocs(struct module *pmod, Elf_Shdr *sechdrs, + const char *shstrtab, const char *strtab, + unsigned int symndx, unsigned int secndx, + const char *objname) { - int i, cnt, ret = 0; - const char *objname, *secname; + int cnt, ret; char sec_objname[MODULE_NAME_LEN]; - Elf_Shdr *sec; + Elf_Shdr *sec = sechdrs + secndx; - if (WARN_ON(!klp_is_object_loaded(obj))) + /* + * Format: .klp.rela.sec_objname.section_name + * See comment in klp_resolve_symbols() for an explanation + * of the selected field width value. + */ + cnt = sscanf(shstrtab + sec->sh_name, ".klp.rela.%55[^.]", + sec_objname); + if (cnt != 1) { + pr_err("section %s has an incorrectly formatted name\n", + shstrtab + sec->sh_name); return -EINVAL; + } - objname = klp_is_module(obj) ? obj->name : "vmlinux"; - - /* For each klp relocation section */ - for (i = 1; i < pmod->klp_info->hdr.e_shnum; i++) { - sec = pmod->klp_info->sechdrs + i; - secname = pmod->klp_info->secstrings + sec->sh_name; - if (!(sec->sh_flags & SHF_RELA_LIVEPATCH)) - continue; - - /* - * Format: .klp.rela.sec_objname.section_name - * See comment in klp_resolve_symbols() for an explanation - * of the selected field width value. - */ - cnt = sscanf(secname, ".klp.rela.%55[^.]", sec_objname); - if (cnt != 1) { - pr_err("section %s has an incorrectly formatted name\n", - secname); - ret = -EINVAL; - break; - } - - if (strcmp(objname, sec_objname)) - continue; - - ret = klp_resolve_symbols(sec, pmod); - if (ret) - break; + if (strcmp(objname ? objname : "vmlinux", sec_objname)) + return 0; - ret = apply_relocate_add(pmod->klp_info->sechdrs, - pmod->core_kallsyms.strtab, - pmod->klp_info->symndx, i, pmod); - if (ret) - break; - } + ret = klp_resolve_symbols(sechdrs, strtab, symndx, sec); + if (ret) + return ret; - return ret; + return apply_relocate_add(sechdrs, strtab, symndx, secndx, pmod); } /* @@ -730,6 +735,28 @@ void __weak arch_klp_init_object_loaded(struct klp_patch *patch, { } +int klp_apply_object_relocs(struct klp_patch *patch, struct klp_object *obj) +{ + int i, ret; + struct klp_modinfo *info = patch->mod->klp_info; + + for (i = 1; i < info->hdr.e_shnum; i++) { + Elf_Shdr *sec = info->sechdrs + i; + + if (!(sec->sh_flags & SHF_RELA_LIVEPATCH)) + continue; + + ret = klp_apply_section_relocs(patch->mod, info->sechdrs, + info->secstrings, + patch->mod->core_kallsyms.strtab, + info->symndx, i, obj->name); + if (ret) + return ret; + } + + return 0; +} + /* parts of the initialization that is done only when the object is loaded */ static int klp_init_object_loaded(struct klp_patch *patch, struct klp_object *obj) @@ -738,18 +765,26 @@ static int klp_init_object_loaded(struct klp_patch *patch, int ret; mutex_lock(&text_mutex); - module_disable_ro(patch->mod); - ret = klp_write_object_relocations(patch->mod, obj); - if (ret) { - module_enable_ro(patch->mod, true); - mutex_unlock(&text_mutex); - return ret; + + if (klp_is_module(obj)) { + /* + * Only write module-specific relocations here + * (.klp.rela.{module}.*). vmlinux-specific relocations were + * written earlier during the initialization of the klp module + * itself. + */ + ret = klp_apply_object_relocs(patch, obj); + if (ret) { + module_enable_ro(patch->mod, true); + mutex_unlock(&text_mutex); + return ret; + } } arch_klp_init_object_loaded(patch, obj); - module_enable_ro(patch->mod, true); + module_enable_ro(patch->mod, true); mutex_unlock(&text_mutex); klp_for_each_func(obj, func) { -- cgit v1.2.3