livepatch,x86: Clear relocation targets on a module removal

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 eda9cec4
("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>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Reviewed-by: Joe Lawrence <joe.lawrence@redhat.com>
Tested-by: Joe Lawrence <joe.lawrence@redhat.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20230125185401.279042-2-song@kernel.org

arch/x86/kernel/module.c

@@ -128,22 +128,27 @@ 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;
@@ -161,52 +166,53 @@ 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);
@@ -215,11 +221,12 @@ 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;
@@ -230,8 +237,8 @@ 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();
@@ -241,6 +248,26 @@ 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,

include/linux/moduleloader.h

@@ -72,6 +72,23 @@ 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,

kernel/livepatch/core.c

@@ -261,6 +261,14 @@ 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
@@ -284,10 +292,10 @@ 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];
@@ -309,11 +317,26 @@ 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);
 }
 
 /*
@@ -762,8 +785,9 @@ 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;
@@ -774,10 +798,10 @@ 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;
 	}
@@ -785,6 +809,18 @@ 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)
@@ -1172,7 +1208,7 @@ 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;
 		}