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Subject[RFC PATCH v2 1/3] livepatch: Clear relocation targets on a module removal
Date
Josh reported a bug:

When the object to be patched is a module, and that module is
rmmod'ed and reloaded, it fails to load with:

module: x86/modules: Skipping invalid relocation target, existing value is nonzero for type 2, loc 00000000ba0302e9, val ffffffffa03e293c
livepatch: failed to initialize patch 'livepatch_nfsd' for module 'nfsd' (-8)
livepatch: patch 'livepatch_nfsd' failed for module 'nfsd', refusing to load module 'nfsd'

The livepatch module has a relocation which references a symbol
in the _previous_ loading of nfsd. When apply_relocate_add()
tries to replace the old relocation with a new one, it sees that
the previous one is nonzero and it errors out.

On ppc64le, we have a similar issue:

module_64: livepatch_nfsd: Expected nop after call, got e8410018 at e_show+0x60/0x548 [livepatch_nfsd]
livepatch: failed to initialize patch 'livepatch_nfsd' for module 'nfsd' (-8)
livepatch: patch 'livepatch_nfsd' failed for module 'nfsd', refusing to load module 'nfsd'

He also proposed three different solutions. We could remove the error
check in apply_relocate_add() introduced by commit eda9cec4c9a1
("x86/module: Detect and skip invalid relocations"). However the check
is useful for detecting corrupted modules.

We could also deny the patched modules to be removed. If it proved to be
a major drawback for users, we could still implement a different
approach. The solution would also complicate the existing code a lot.

We thus decided to reverse the relocation patching (clear all relocation
targets on x86_64, or return back nops on powerpc). The solution is not
universal and is too much arch-specific, but it may prove to be simpler
in the end.

Reported-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Miroslav Benes <mbenes@suse.cz>
---
arch/powerpc/kernel/module_64.c | 45 +++++++++++++++++++++++++++++++++
arch/s390/kernel/module.c | 8 ++++++
arch/x86/kernel/module.c | 43 +++++++++++++++++++++++++++++++
include/linux/moduleloader.h | 7 +++++
kernel/livepatch/core.c | 45 +++++++++++++++++++++++++++++++++
5 files changed, 148 insertions(+)

diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
index a93b10c48000..e461d456e447 100644
--- a/arch/powerpc/kernel/module_64.c
+++ b/arch/powerpc/kernel/module_64.c
@@ -741,6 +741,51 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
return 0;
}

+#ifdef CONFIG_LIVEPATCH
+void clear_relocate_add(Elf64_Shdr *sechdrs,
+ const char *strtab,
+ unsigned int symindex,
+ unsigned int relsec,
+ struct module *me)
+{
+ unsigned int i;
+ Elf64_Rela *rela = (void *)sechdrs[relsec].sh_addr;
+ Elf64_Sym *sym;
+ unsigned long *location;
+ const char *symname;
+ u32 *instruction;
+
+ pr_debug("Applying ADD relocate section %u to %u\n", relsec,
+ sechdrs[relsec].sh_info);
+
+ for (i = 0; i < sechdrs[relsec].sh_size / sizeof(*rela); i++) {
+ location = (void *)sechdrs[sechdrs[relsec].sh_info].sh_addr
+ + rela[i].r_offset;
+ sym = (Elf64_Sym *)sechdrs[symindex].sh_addr
+ + ELF64_R_SYM(rela[i].r_info);
+ symname = me->core_kallsyms.strtab
+ + sym->st_name;
+
+ if (ELF64_R_TYPE(rela[i].r_info) != R_PPC_REL24)
+ continue;
+
+ if (sym->st_shndx != SHN_UNDEF &&
+ sym->st_shndx != SHN_LIVEPATCH)
+ continue;
+
+ instruction = (u32 *)location;
+ if (is_mprofile_mcount_callsite(symname, instruction))
+ continue;
+
+ if (!instr_is_relative_link_branch(*instruction))
+ continue;
+
+ instruction += 1;
+ *instruction = PPC_INST_NOP;
+ }
+}
+#endif
+
#ifdef CONFIG_DYNAMIC_FTRACE

#ifdef CONFIG_MPROFILE_KERNEL
diff --git a/arch/s390/kernel/module.c b/arch/s390/kernel/module.c
index 31889db609e9..56867d052010 100644
--- a/arch/s390/kernel/module.c
+++ b/arch/s390/kernel/module.c
@@ -437,6 +437,14 @@ int apply_relocate_add(Elf_Shdr *sechdrs, const char *strtab,
return 0;
}

+#ifdef CONFIG_LIVEPATCH
+void clear_relocate_add(Elf64_Shdr *sechdrs, const char *strtab,
+ unsigned int symindex, unsigned int relsec,
+ struct module *me)
+{
+}
+#endif
+
int module_finalize(const Elf_Ehdr *hdr,
const Elf_Shdr *sechdrs,
struct module *me)
diff --git a/arch/x86/kernel/module.c b/arch/x86/kernel/module.c
index d5c72cb877b3..b07d71f125e6 100644
--- a/arch/x86/kernel/module.c
+++ b/arch/x86/kernel/module.c
@@ -215,6 +215,49 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
me->name);
return -ENOEXEC;
}
+
+#ifdef CONFIG_LIVEPATCH
+void clear_relocate_add(Elf64_Shdr *sechdrs,
+ const char *strtab,
+ unsigned int symindex,
+ unsigned int relsec,
+ struct module *me)
+{
+ unsigned int i;
+ Elf64_Rela *rel = (void *)sechdrs[relsec].sh_addr;
+ void *loc;
+
+ DEBUGP("Clearing relocate section %u to %u\n",
+ relsec, sechdrs[relsec].sh_info);
+ for (i = 0; i < sechdrs[relsec].sh_size / sizeof(*rel); i++) {
+ loc = (void *)sechdrs[sechdrs[relsec].sh_info].sh_addr
+ + rel[i].r_offset;
+
+ switch (ELF64_R_TYPE(rel[i].r_info)) {
+ case R_X86_64_NONE:
+ break;
+ case R_X86_64_64:
+ *(u64 *)loc = 0;
+ break;
+ case R_X86_64_32:
+ *(u32 *)loc = 0;
+ break;
+ case R_X86_64_32S:
+ *(s32 *)loc = 0;
+ break;
+ case R_X86_64_PC32:
+ case R_X86_64_PLT32:
+ *(u32 *)loc = 0;
+ break;
+ case R_X86_64_PC64:
+ *(u64 *)loc = 0;
+ break;
+ default:
+ break;
+ }
+ }
+}
+#endif
#endif

int module_finalize(const Elf_Ehdr *hdr,
diff --git a/include/linux/moduleloader.h b/include/linux/moduleloader.h
index 31013c2effd3..f1e52692db5f 100644
--- a/include/linux/moduleloader.h
+++ b/include/linux/moduleloader.h
@@ -62,6 +62,13 @@ int apply_relocate_add(Elf_Shdr *sechdrs,
unsigned int symindex,
unsigned int relsec,
struct module *mod);
+#ifdef CONFIG_LIVEPATCH
+void clear_relocate_add(Elf64_Shdr *sechdrs,
+ const char *strtab,
+ unsigned int symindex,
+ unsigned int relsec,
+ struct module *me);
+#endif
#else
static inline int apply_relocate_add(Elf_Shdr *sechdrs,
const char *strtab,
diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index ab4a4606d19b..f0b380d2a17a 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -295,6 +295,45 @@ static int klp_write_object_relocations(struct module *pmod,
return ret;
}

+static void klp_clear_object_relocations(struct module *pmod,
+ struct klp_object *obj)
+{
+ int i, cnt;
+ const char *objname, *secname;
+ char sec_objname[MODULE_NAME_LEN];
+ Elf_Shdr *sec;
+
+ 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.
+ */
+ secname = pmod->klp_info->secstrings + sec->sh_name;
+ cnt = sscanf(secname, ".klp.rela.%55[^.]", sec_objname);
+ if (cnt != 1) {
+ pr_err("section %s has an incorrectly formatted name\n",
+ secname);
+ continue;
+ }
+
+ if (strcmp(objname, sec_objname))
+ continue;
+
+ clear_relocate_add(pmod->klp_info->sechdrs,
+ pmod->core_kallsyms.strtab,
+ pmod->klp_info->symndx, i, pmod);
+ }
+}
+
/*
* Sysfs Interface
*
@@ -1100,6 +1139,12 @@ static void klp_cleanup_module_patches_limited(struct module *mod,

klp_post_unpatch_callback(obj);

+ mutex_lock(&text_mutex);
+ module_disable_ro(patch->mod);
+ klp_clear_object_relocations(patch->mod, obj);
+ module_enable_ro(patch->mod, true);
+ mutex_unlock(&text_mutex);
+
klp_free_object_loaded(obj);
break;
}
--
2.23.0
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