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

[Song Liu]

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@xxxxxxxxxx> wrote:
>
> From: Miroslav Benes <mbenes@xxxxxxx>
>
> 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@xxxxxxxxxx>
> Signed-off-by: Miroslav Benes <mbenes@xxxxxxx>
> Signed-off-by: Song Liu <song@xxxxxxxxxx>
>
> ---
>
> 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@xxxxxxx/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
>