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

Joe Lawrence
2023-01-27T13:47:56Z

On Wed, Jan 25, 2023 at 10:54:01AM -0800, Song Liu wrote:
> 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
>

Reviewed-and-tested-by: Joe Lawrence <joe.lawrence@redhat.com>

--
Joe