Re: [PATCH v10 2/2] livepatch,x86: Clear relocation targets on a module removal

[Christophe Leroy]

Le 21/01/2023 à 01:49, Song Liu a écrit :
> 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.

Would it be possible to not add empty clear_relocate_add() on all 
architecture while only x86 implements it ? Can we make it an empty 
static inline in generic livepatch.h for the architectures not 
implementing it, like we do for most mm functions ?

Another solution would be to define an empty generic weak version of 
clear_relocate_add()


Christophe

> 
> Reported-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Originally-by: Miroslav Benes <mbenes@xxxxxxx>
> Signed-off-by: Song Liu <song@xxxxxxxxxx>
> Acked-by: Miroslav Benes <mbenes@xxxxxxx>
> 
> ---
> 
> NOTE: powerpc32 code is only compile tested.
> 
> 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@xxxxxxx/T/#u
> 
> fix
> ---
>   arch/powerpc/kernel/module_32.c | 10 ++++
>   arch/powerpc/kernel/module_64.c | 10 ++++
>   arch/s390/kernel/module.c       |  8 +++
>   arch/x86/kernel/module.c        | 93 +++++++++++++++++++++------------
>   include/linux/moduleloader.h    | 17 ++++++
>   kernel/livepatch/core.c         | 54 ++++++++++++++-----
>   6 files changed, 146 insertions(+), 46 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 ff045644f13f..1096d6b3a62c 100644
> --- a/arch/powerpc/kernel/module_64.c
> +++ b/arch/powerpc/kernel/module_64.c
> @@ -749,6 +749,16 @@ 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)
> +{
> +}
> +#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 1dee3ad82da2..e4aab0395a33 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++) {
> +		int size = 0;
> +
>   		/* 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..c72f378181ce 100644
> --- a/kernel/livepatch/core.c
> +++ b/kernel/livepatch/core.c
> @@ -291,10 +291,10 @@ 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 +316,26 @@ 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);
> +		return apply_relocate_add(sechdrs, strtab, symndx, secndx, pmod);
> +	}
> +
> +	clear_relocate_add(sechdrs, strtab, symndx, secndx, pmod);
> +	return 0;
> +}
> +
> +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 +784,9 @@ 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 +797,10 @@ 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 +808,18 @@ 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 +1207,7 @@ 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;
>   		}