module: Factor out elf_validity_cache_sechdrs

Factor out and document the validation of section headers.

Because we now validate all section offsets and lengths before accessing
them, we can remove the ad-hoc checks.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>

kernel/module/main.c

@@ -1708,6 +1708,87 @@ static int elf_validity_ehdr(const struct load_info *info)
 	return 0;
 }
 
+/**
+ * elf_validity_cache_sechdrs() - Cache section headers if valid
+ * @info: Load info to compute section headers from
+ *
+ * Checks:
+ *
+ * * ELF header is valid (see elf_validity_ehdr())
+ * * Section headers are the size we expect
+ * * Section array fits in the user provided data
+ * * Section index 0 is NULL
+ * * Section contents are inbounds
+ *
+ * Then updates @info with a &load_info->sechdrs pointer if valid.
+ *
+ * Return: %0 if valid, negative error code if validation failed.
+ */
+static int elf_validity_cache_sechdrs(struct load_info *info)
+{
+	Elf_Shdr *sechdrs;
+	Elf_Shdr *shdr;
+	int i;
+	int err;
+
+	err = elf_validity_ehdr(info);
+	if (err < 0)
+		return err;
+
+	if (info->hdr->e_shentsize != sizeof(Elf_Shdr)) {
+		pr_err("Invalid ELF section header size\n");
+		return -ENOEXEC;
+	}
+
+	/*
+	 * e_shnum is 16 bits, and sizeof(Elf_Shdr) is
+	 * known and small. So e_shnum * sizeof(Elf_Shdr)
+	 * will not overflow unsigned long on any platform.
+	 */
+	if (info->hdr->e_shoff >= info->len
+	    || (info->hdr->e_shnum * sizeof(Elf_Shdr) >
+		info->len - info->hdr->e_shoff)) {
+		pr_err("Invalid ELF section header overflow\n");
+		return -ENOEXEC;
+	}
+
+	sechdrs = (void *)info->hdr + info->hdr->e_shoff;
+
+	/*
+	 * The code assumes that section 0 has a length of zero and
+	 * an addr of zero, so check for it.
+	 */
+	if (sechdrs[0].sh_type != SHT_NULL
+	    || sechdrs[0].sh_size != 0
+	    || sechdrs[0].sh_addr != 0) {
+		pr_err("ELF Spec violation: section 0 type(%d)!=SH_NULL or non-zero len or addr\n",
+		       sechdrs[0].sh_type);
+		return -ENOEXEC;
+	}
+
+	/* Validate contents are inbounds */
+	for (i = 1; i < info->hdr->e_shnum; i++) {
+		shdr = &sechdrs[i];
+		switch (shdr->sh_type) {
+		case SHT_NULL:
+		case SHT_NOBITS:
+			/* No contents, offset/size don't mean anything */
+			continue;
+		default:
+			err = validate_section_offset(info, shdr);
+			if (err < 0) {
+				pr_err("Invalid ELF section in module (section %u type %u)\n",
+				       i, shdr->sh_type);
+				return err;
+			}
+		}
+	}
+
+	info->sechdrs = sechdrs;
+
+	return 0;
+}
+
 /*
  * Check userspace passed ELF module against our expectations, and cache
  * useful variables for further processing as we go.
@@ -1737,29 +1818,10 @@ static int elf_validity_cache_copy(struct load_info *info, int flags)
 	unsigned int num_info_secs = 0, info_idx;
 	unsigned int num_sym_secs = 0, sym_idx;
 
-	err = elf_validity_ehdr(info);
+	err = elf_validity_cache_sechdrs(info);
 	if (err < 0)
 		return err;
 
-	if (info->hdr->e_shentsize != sizeof(Elf_Shdr)) {
-		pr_err("Invalid ELF section header size\n");
-		goto no_exec;
-	}
-
-	/*
-	 * e_shnum is 16 bits, and sizeof(Elf_Shdr) is
-	 * known and small. So e_shnum * sizeof(Elf_Shdr)
-	 * will not overflow unsigned long on any platform.
-	 */
-	if (info->hdr->e_shoff >= info->len
-	    || (info->hdr->e_shnum * sizeof(Elf_Shdr) >
-		info->len - info->hdr->e_shoff)) {
-		pr_err("Invalid ELF section header overflow\n");
-		goto no_exec;
-	}
-
-	info->sechdrs = (void *)info->hdr + info->hdr->e_shoff;
-
 	/*
 	 * Verify if the section name table index is valid.
 	 */
@@ -1772,11 +1834,6 @@ static int elf_validity_cache_copy(struct load_info *info, int flags)
 	}
 
 	strhdr = &info->sechdrs[info->hdr->e_shstrndx];
-	err = validate_section_offset(info, strhdr);
-	if (err < 0) {
-		pr_err("Invalid ELF section hdr(type %u)\n", strhdr->sh_type);
-		return err;
-	}
 
 	/*
 	 * The section name table must be NUL-terminated, as required
@@ -1793,18 +1850,6 @@ static int elf_validity_cache_copy(struct load_info *info, int flags)
 		goto no_exec;
 	}
 
-	/*
-	 * The code assumes that section 0 has a length of zero and
-	 * an addr of zero, so check for it.
-	 */
-	if (info->sechdrs[0].sh_type != SHT_NULL
-	    || info->sechdrs[0].sh_size != 0
-	    || info->sechdrs[0].sh_addr != 0) {
-		pr_err("ELF Spec violation: section 0 type(%d)!=SH_NULL or non-zero len or addr\n",
-		       info->sechdrs[0].sh_type);
-		goto no_exec;
-	}
-
 	for (i = 1; i < info->hdr->e_shnum; i++) {
 		shdr = &info->sechdrs[i];
 		switch (shdr->sh_type) {
@@ -1823,12 +1868,6 @@ static int elf_validity_cache_copy(struct load_info *info, int flags)
 			sym_idx = i;
 			fallthrough;
 		default:
-			err = validate_section_offset(info, shdr);
-			if (err < 0) {
-				pr_err("Invalid ELF section in module (section %u type %u)\n",
-					i, shdr->sh_type);
-				return err;
-			}
 			if (strcmp(info->secstrings + shdr->sh_name,
 				   ".gnu.linkonce.this_module") == 0) {
 				num_mod_secs++;