[PATCH v11 2/2] livepatch,x86: Clear relocation targets on a module removal

Song Liu
2023-01-25T18:55:01Z

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.

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). 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>
Originally-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Song Liu <song@kernel.org>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Reviewed-by: Petr Mladek <pmladek@suse.com>

---

NOTE: powerpc32 code is only compile tested.

Changes v10 => v11:
1. Do not initialize "size" in __write_relocate_add(). (Petr Mladek)
2. Use __weak clear_relocate_add(). (Christophe Leroy)

Changes v9 => v10:
1. Revise commit log. (Josh Poimboeuf)
2. Various improvements in code style, comments, etc. (Josh Poimboeuf)

Changes v8 => v9:
1. Fix overflow check for R_X86_64_PC32 and R_X86_64_PLT32. (Petr Mladek)

Changes v7 = v8:
1. Remove the logic in powerpc/kernel/module_64.c, as there is ongoing
discussions.
2. For x86_64, add check for expected value during clear_relocate_add().
(Petr Mladek)
3. Optimize the logic in klp_write_section_relocs(). (Petr Mladek)
4. Optimize __write_relocate_add (x86_64). (Joe Lawrence)

Changes v6 = v7:
1. Reduce code duplication in livepatch/core.c and x86/kernel/module.c.
2. Add more comments to powerpc/kernel/module_64.c.
3. Added Joe's Tested-by (which I should have added in v6).

Changes v5 = v6:
1. Fix powerpc64.
2. Fix compile for powerpc32.

Changes v4 = v5:
1. Fix compile with powerpc.

Changes v3 = v4:
1. Reuse __apply_relocate_add to make it more reliable in long term.
(Josh Poimboeuf)
2. Add back ppc64 logic from v2, with changes to match current code.
(Josh Poimboeuf)

Changes v2 => v3:
1. Rewrite x86 changes to match current code style.
2. Remove powerpc changes as there is no test coverage in v3.
3. Only keep 1/3 of v2.

v2: https://lore.kernel.org/all/20190905124514.8944-1-mbenes@suse.cz/T/#u

fix
---
arch/x86/kernel/module.c | 93 +++++++++++++++++++++++-------------
include/linux/moduleloader.h | 17 +++++++
kernel/livepatch/core.c | 62 +++++++++++++++++++-----
3 files changed, 126 insertions(+), 46 deletions(-)

diff --git a/arch/x86/kernel/module.c b/arch/x86/kernel/module.c
index 1dee3ad82da2..84ad0e61ba6e 100644
--- a/arch/x86/kernel/module.c
+++ b/arch/x86/kernel/module.c
@@ -129,22 +129,27 @@ int apply_relocate(Elf32_Shdr *sechdrs,
return 0;
}
#else /*X86_64*/
-static int __apply_relocate_add(Elf64_Shdr *sechdrs,
+static int __write_relocate_add(Elf64_Shdr *sechdrs,
const char *strtab,
unsigned int symindex,
unsigned int relsec,
struct module *me,
- void *(*write)(void *dest, const void *src, size_t len))
+ void *(*write)(void *dest, const void *src, size_t len),
+ bool apply)
{
unsigned int i;
Elf64_Rela *rel = (void *)sechdrs[relsec].sh_addr;
Elf64_Sym *sym;
void *loc;
u64 val;
+ u64 zero = 0ULL;

- DEBUGP("Applying relocate section %u to %u\n",
+ DEBUGP("%s relocate section %u to %u\n",
+ apply ? "Applying" : "Clearing",
relsec, sechdrs[relsec].sh_info);
for (i = 0; i < sechdrs[relsec].sh_size / sizeof(*rel); i++) {
+ size_t size;
+
/* This is where to make the change */
loc = (void *)sechdrs[sechdrs[relsec].sh_info].sh_addr
+ rel[i].r_offset;
@@ -162,52 +167,53 @@ static int __apply_relocate_add(Elf64_Shdr *sechdrs,

switch (ELF64_R_TYPE(rel[i].r_info)) {
case R_X86_64_NONE:
- break;
+ continue; /* nothing to write */
case R_X86_64_64:
- if (*(u64 *)loc != 0)
- goto invalid_relocation;
- write(loc, &val, 8);
+ size = 8;
break;
case R_X86_64_32:
- if (*(u32 *)loc != 0)
- goto invalid_relocation;
- write(loc, &val, 4);
- if (val != *(u32 *)loc)
+ if (val != *(u32 *)&val)
goto overflow;
+ size = 4;
break;
case R_X86_64_32S:
- if (*(s32 *)loc != 0)
- goto invalid_relocation;
- write(loc, &val, 4);
- if ((s64)val != *(s32 *)loc)
+ if ((s64)val != *(s32 *)&val)
goto overflow;
+ size = 4;
break;
case R_X86_64_PC32:
case R_X86_64_PLT32:
- if (*(u32 *)loc != 0)
- goto invalid_relocation;
val -= (u64)loc;
- write(loc, &val, 4);
+ size = 4;
break;
case R_X86_64_PC64:
- if (*(u64 *)loc != 0)
- goto invalid_relocation;
val -= (u64)loc;
- write(loc, &val, 8);
+ size = 8;
break;
default:
pr_err("%s: Unknown rela relocation: %llu\n",
me->name, ELF64_R_TYPE(rel[i].r_info));
return -ENOEXEC;
}
+
+ if (apply) {
+ if (memcmp(loc, &zero, size)) {
+ pr_err("x86/modules: Invalid relocation target, existing value is nonzero for type %d, loc %p, val %Lx\n",
+ (int)ELF64_R_TYPE(rel[i].r_info), loc, val);
+ return -ENOEXEC;
+ }
+ write(loc, &val, size);
+ } else {
+ if (memcmp(loc, &val, size)) {
+ pr_warn("x86/modules: Invalid relocation target, existing value does not match expected value for type %d, loc %p, val %Lx\n",
+ (int)ELF64_R_TYPE(rel[i].r_info), loc, val);
+ return -ENOEXEC;
+ }
+ write(loc, &zero, size);
+ }
}
return 0;

-invalid_relocation:
- pr_err("x86/modules: Skipping invalid relocation target, existing value is nonzero for type %d, loc %p, val %Lx\n",
- (int)ELF64_R_TYPE(rel[i].r_info), loc, val);
- return -ENOEXEC;
-
overflow:
pr_err("overflow in relocation type %d val %Lx\n",
(int)ELF64_R_TYPE(rel[i].r_info), val);
@@ -216,11 +222,12 @@ static int __apply_relocate_add(Elf64_Shdr *sechdrs,
return -ENOEXEC;
}

-int apply_relocate_add(Elf64_Shdr *sechdrs,
- const char *strtab,
- unsigned int symindex,
- unsigned int relsec,
- struct module *me)
+static int write_relocate_add(Elf64_Shdr *sechdrs,
+ const char *strtab,
+ unsigned int symindex,
+ unsigned int relsec,
+ struct module *me,
+ bool apply)
{
int ret;
bool early = me->state == MODULE_STATE_UNFORMED;
@@ -231,8 +238,8 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
mutex_lock(&text_mutex);
}

- ret = __apply_relocate_add(sechdrs, strtab, symindex, relsec, me,
- write);
+ ret = __write_relocate_add(sechdrs, strtab, symindex, relsec, me,
+ write, apply);

if (!early) {
text_poke_sync();
@@ -242,6 +249,26 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
return ret;
}

+int apply_relocate_add(Elf64_Shdr *sechdrs,
+ const char *strtab,
+ unsigned int symindex,
+ unsigned int relsec,
+ struct module *me)
+{
+ return write_relocate_add(sechdrs, strtab, symindex, relsec, me, true);
+}
+
+#ifdef CONFIG_LIVEPATCH
+void clear_relocate_add(Elf64_Shdr *sechdrs,
+ const char *strtab,
+ unsigned int symindex,
+ unsigned int relsec,
+ struct module *me)
+{
+ write_relocate_add(sechdrs, strtab, symindex, relsec, me, false);
+}
+#endif
+
#endif

int module_finalize(const Elf_Ehdr *hdr,
diff --git a/include/linux/moduleloader.h b/include/linux/moduleloader.h
index 7b4587a19189..03be088fb439 100644
--- a/include/linux/moduleloader.h
+++ b/include/linux/moduleloader.h
@@ -75,6 +75,23 @@ int apply_relocate_add(Elf_Shdr *sechdrs,
unsigned int symindex,
unsigned int relsec,
struct module *mod);
+#ifdef CONFIG_LIVEPATCH
+/*
+ * Some architectures (namely x86_64 and ppc64) perform sanity checks when
+ * applying relocations. If a patched module gets unloaded and then later
+ * reloaded (and re-patched), klp re-applies relocations to the replacement
+ * function(s). Any leftover relocations from the previous loading of the
+ * patched module might trigger the sanity checks.
+ *
+ * To prevent that, when unloading a patched module, clear out any relocations
+ * that might trigger arch-specific sanity checks on a future module reload.
+ */
+void clear_relocate_add(Elf_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 201f0c0482fb..140997b36025 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -268,6 +268,14 @@ static int klp_resolve_symbols(Elf_Shdr *sechdrs, const char *strtab,
return 0;
}

+void __weak clear_relocate_add(Elf_Shdr *sechdrs,
+ const char *strtab,
+ unsigned int symindex,
+ unsigned int relsec,
+ struct module *me)
+{
+}
+
/*
* 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
@@ -291,10 +299,10 @@ static int klp_resolve_symbols(Elf_Shdr *sechdrs, const char *strtab,
* 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)
+static int klp_write_section_relocs(struct module *pmod, Elf_Shdr *sechdrs,
+ const char *shstrtab, const char *strtab,
+ unsigned int symndx, unsigned int secndx,
+ const char *objname, bool apply)
{
int cnt, ret;
char sec_objname[MODULE_NAME_LEN];
@@ -316,11 +324,26 @@ int klp_apply_section_relocs(struct module *pmod, Elf_Shdr *sechdrs,
if (strcmp(objname ? objname : "vmlinux", sec_objname))
return 0;

- ret = klp_resolve_symbols(sechdrs, strtab, symndx, sec, sec_objname);
- if (ret)
- return ret;
+ if (apply) {
+ ret = klp_resolve_symbols(sechdrs, strtab, symndx,
+ sec, sec_objname);
+ if (ret)
+ return ret;
+
+ return apply_relocate_add(sechdrs, strtab, symndx, secndx, pmod);
+ }
+
+ clear_relocate_add(sechdrs, strtab, symndx, secndx, pmod);
+ return 0;
+}

- return apply_relocate_add(sechdrs, strtab, symndx, secndx, pmod);
+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)
+{
+ return klp_write_section_relocs(pmod, sechdrs, shstrtab, strtab, symndx,
+ secndx, objname, true);
}

/*
@@ -769,8 +792,9 @@ static int klp_init_func(struct klp_object *obj, struct klp_func *func)
func->old_sympos ? func->old_sympos : 1);
}

-static int klp_apply_object_relocs(struct klp_patch *patch,
- struct klp_object *obj)
+static int klp_write_object_relocs(struct klp_patch *patch,
+ struct klp_object *obj,
+ bool apply)
{
int i, ret;
struct klp_modinfo *info = patch->mod->klp_info;
@@ -781,10 +805,10 @@ static int klp_apply_object_relocs(struct klp_patch *patch,
if (!(sec->sh_flags & SHF_RELA_LIVEPATCH))
continue;

- ret = klp_apply_section_relocs(patch->mod, info->sechdrs,
+ ret = klp_write_section_relocs(patch->mod, info->sechdrs,
info->secstrings,
patch->mod->core_kallsyms.strtab,
- info->symndx, i, obj->name);
+ info->symndx, i, obj->name, apply);
if (ret)
return ret;
}
@@ -792,6 +816,18 @@ static int klp_apply_object_relocs(struct klp_patch *patch,
return 0;
}

+static int klp_apply_object_relocs(struct klp_patch *patch,
+ struct klp_object *obj)
+{
+ return klp_write_object_relocs(patch, obj, true);
+}
+
+static void klp_clear_object_relocs(struct klp_patch *patch,
+ struct klp_object *obj)
+{
+ klp_write_object_relocs(patch, obj, false);
+}
+
/* 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)
@@ -1179,7 +1215,7 @@ static void klp_cleanup_module_patches_limited(struct module *mod,
klp_unpatch_object(obj);

klp_post_unpatch_callback(obj);
-
+ klp_clear_object_relocs(patch, obj);
klp_free_object_loaded(obj);
break;
}
--
2.30.2