Re: [PATCH v6] livepatch: Clear relocation targets on a module removal

Song Liu
2022-11-17T22:07:29Z

Hi folks,

It seems we forgot about this work... What shall we do to move forward?

Thanks,
Song

On Thu, Sep 1, 2022 at 10:16 AM Song Liu <song@kernel.org> wrote:
>
> From: Miroslav Benes <mbenes@suse.cz>
>
> 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). 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>
> Signed-off-by: Song Liu <song@kernel.org>
>
> ---
>
> NOTE: powerpc32 code is only compile tested.
>
> 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
> ---
> arch/powerpc/kernel/module_32.c | 10 ++++
> arch/powerpc/kernel/module_64.c | 49 +++++++++++++++
> arch/s390/kernel/module.c | 8 +++
> arch/x86/kernel/module.c | 102 +++++++++++++++++++++++---------
> include/linux/moduleloader.h | 7 +++
> kernel/livepatch/core.c | 41 ++++++++++++-
> 6 files changed, 189 insertions(+), 28 deletions(-)
>
> diff --git a/arch/powerpc/kernel/module_32.c b/arch/powerpc/kernel/module_32.c
> index ea6536171778..e3c312770453 100644
> --- a/arch/powerpc/kernel/module_32.c
> +++ b/arch/powerpc/kernel/module_32.c
> @@ -285,6 +285,16 @@ int apply_relocate_add(Elf32_Shdr *sechdrs,
> return 0;
> }
>
> +#ifdef CONFIG_LIVEPATCH
> +void clear_relocate_add(Elf32_Shdr *sechdrs,
> + const char *strtab,
> + unsigned int symindex,
> + unsigned int relsec,
> + struct module *me)
> +{
> +}
> +#endif
> +
> #ifdef CONFIG_DYNAMIC_FTRACE
> notrace int module_trampoline_target(struct module *mod, unsigned long addr,
> unsigned long *target)
> diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
> index 7e45dc98df8a..514951f97391 100644
> --- a/arch/powerpc/kernel/module_64.c
> +++ b/arch/powerpc/kernel/module_64.c
> @@ -739,6 +739,55 @@ 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("Clearing 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;
> + /*
> + * reverse the operations in apply_relocate_add() for case
> + * R_PPC_REL24.
> + */
> + if (sym->st_shndx != SHN_UNDEF &&
> + sym->st_shndx != SHN_LIVEPATCH)
> + continue;
> +
> + instruction = (u32 *)location;
> + if (is_mprofile_ftrace_call(symname))
> + continue;
> +
> + if (!instr_is_relative_link_branch(ppc_inst(*instruction)))
> + continue;
> +
> + instruction += 1;
> + patch_instruction(instruction, ppc_inst(PPC_RAW_NOP()));
> + }
> +
> +}
> +#endif
> +
> #ifdef CONFIG_DYNAMIC_FTRACE
> int module_trampoline_target(struct module *mod, unsigned long addr,
> unsigned long *target)
> diff --git a/arch/s390/kernel/module.c b/arch/s390/kernel/module.c
> index 2d159b32885b..cc6784fbc1ac 100644
> --- a/arch/s390/kernel/module.c
> +++ b/arch/s390/kernel/module.c
> @@ -500,6 +500,14 @@ static int module_alloc_ftrace_hotpatch_trampolines(struct module *me,
> }
> #endif /* CONFIG_FUNCTION_TRACER */
>
> +#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 b1abf663417c..f9632afbb84c 100644
> --- a/arch/x86/kernel/module.c
> +++ b/arch/x86/kernel/module.c
> @@ -128,18 +128,20 @@ int apply_relocate(Elf32_Shdr *sechdrs,
> return 0;
> }
> #else /*X86_64*/
> -static int __apply_relocate_add(Elf64_Shdr *sechdrs,
> +static int __apply_clear_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 clear)
> {
> 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",
> relsec, sechdrs[relsec].sh_info);
> @@ -163,40 +165,60 @@ static int __apply_relocate_add(Elf64_Shdr *sechdrs,
> case R_X86_64_NONE:
> break;
> case R_X86_64_64:
> - if (*(u64 *)loc != 0)
> - goto invalid_relocation;
> - write(loc, &val, 8);
> + if (!clear) {
> + if (*(u64 *)loc != 0)
> + goto invalid_relocation;
> + write(loc, &val, 8);
> + } else {
> + write(loc, &zero, 8);
> + }
> break;
> case R_X86_64_32:
> - if (*(u32 *)loc != 0)
> - goto invalid_relocation;
> - write(loc, &val, 4);
> - if (val != *(u32 *)loc)
> - goto overflow;
> + if (!clear) {
> + if (*(u32 *)loc != 0)
> + goto invalid_relocation;
> + write(loc, &val, 4);
> + if (val != *(u32 *)loc)
> + goto overflow;
> + } else {
> + write(loc, &zero, 4);
> + }
> break;
> case R_X86_64_32S:
> - if (*(s32 *)loc != 0)
> - goto invalid_relocation;
> - write(loc, &val, 4);
> - if ((s64)val != *(s32 *)loc)
> - goto overflow;
> + if (!clear) {
> + if (*(s32 *)loc != 0)
> + goto invalid_relocation;
> + write(loc, &val, 4);
> + if ((s64)val != *(s32 *)loc)
> + goto overflow;
> + } else {
> + write(loc, &zero, 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);
> + if (!clear) {
> + if (*(u32 *)loc != 0)
> + goto invalid_relocation;
> + val -= (u64)loc;
> + write(loc, &val, 4);
> #if 0
> - if ((s64)val != *(s32 *)loc)
> - goto overflow;
> + if ((s64)val != *(s32 *)loc)
> + goto overflow;
> #endif
> + } else {
> + write(loc, &zero, 4);
> + }
> break;
> case R_X86_64_PC64:
> - if (*(u64 *)loc != 0)
> - goto invalid_relocation;
> - val -= (u64)loc;
> - write(loc, &val, 8);
> + if (!clear) {
> + if (*(u64 *)loc != 0)
> + goto invalid_relocation;
> + val -= (u64)loc;
> + write(loc, &val, 8);
> + } else {
> + write(loc, &zero, 8);
> + }
> break;
> default:
> pr_err("%s: Unknown rela relocation: %llu\n",
> @@ -234,8 +256,8 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
> mutex_lock(&text_mutex);
> }
>
> - ret = __apply_relocate_add(sechdrs, strtab, symindex, relsec, me,
> - write);
> + ret = __apply_clear_relocate_add(sechdrs, strtab, symindex, relsec, me,
> + write, false /* clear */);
>
> if (!early) {
> text_poke_sync();
> @@ -245,6 +267,32 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
> return ret;
> }
>
> +#ifdef CONFIG_LIVEPATCH
> +
> +void clear_relocate_add(Elf64_Shdr *sechdrs,
> + const char *strtab,
> + unsigned int symindex,
> + unsigned int relsec,
> + struct module *me)
> +{
> + bool early = me->state == MODULE_STATE_UNFORMED;
> + void *(*write)(void *, const void *, size_t) = memcpy;
> +
> + if (!early) {
> + write = text_poke;
> + mutex_lock(&text_mutex);
> + }
> +
> + __apply_clear_relocate_add(sechdrs, strtab, symindex, relsec, me,
> + write, true /* clear */);
> +
> + if (!early) {
> + text_poke_sync();
> + mutex_unlock(&text_mutex);
> + }
> +}
> +#endif
> +
> #endif
>
> int module_finalize(const Elf_Ehdr *hdr,
> diff --git a/include/linux/moduleloader.h b/include/linux/moduleloader.h
> index 9e09d11ffe5b..958e6da7f475 100644
> --- a/include/linux/moduleloader.h
> +++ b/include/linux/moduleloader.h
> @@ -72,6 +72,13 @@ int apply_relocate_add(Elf_Shdr *sechdrs,
> unsigned int symindex,
> unsigned int relsec,
> struct module *mod);
> +#ifdef CONFIG_LIVEPATCH
> +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 bc475e62279d..5c0d8a4eba13 100644
> --- a/kernel/livepatch/core.c
> +++ b/kernel/livepatch/core.c
> @@ -316,6 +316,45 @@ int klp_apply_section_relocs(struct module *pmod, Elf_Shdr *sechdrs,
> return apply_relocate_add(sechdrs, strtab, symndx, secndx, pmod);
> }
>
> +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
> *
> @@ -1154,7 +1193,7 @@ static void klp_cleanup_module_patches_limited(struct module *mod,
> klp_unpatch_object(obj);
>
> klp_post_unpatch_callback(obj);
> -
> + klp_clear_object_relocations(patch->mod, obj);
> klp_free_object_loaded(obj);
> break;
> }
> --
> 2.30.2
>