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