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livepatch,x86: Clear relocation targets on a module removal
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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 eda9cec
("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>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Reviewed-by: Joe Lawrence <joe.lawrence@redhat.com>
Tested-by: Joe Lawrence <joe.lawrence@redhat.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20230125185401.279042-2-song@kernel.org
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Song Liu authored and Petr Mladek committed Feb 3, 2023
1 parent bbb9336 commit 0c05e7b
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Showing 3 changed files with 126 additions and 46 deletions.
93 changes: 60 additions & 33 deletions arch/x86/kernel/module.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -128,22 +128,27 @@ 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;
Expand All @@ -161,52 +166,53 @@ 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);
Expand All @@ -215,11 +221,12 @@ 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;
Expand All @@ -230,8 +237,8 @@ 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();
Expand All @@ -241,6 +248,26 @@ 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,
Expand Down
17 changes: 17 additions & 0 deletions include/linux/moduleloader.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -72,6 +72,23 @@ 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,
Expand Down
62 changes: 49 additions & 13 deletions kernel/livepatch/core.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -261,6 +261,14 @@ 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
Expand All @@ -284,10 +292,10 @@ 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];
Expand All @@ -309,11 +317,26 @@ 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);
}

/*
Expand Down Expand Up @@ -762,8 +785,9 @@ 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;
Expand All @@ -774,17 +798,29 @@ 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;
}

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)
Expand Down Expand Up @@ -1172,7 +1208,7 @@ 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;
}
Expand Down

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