[PATCH v3 11/16] module: Additional validation in elf_validity_cache_strtab

[Matthew Maurer <mmaurer@google.com>]

Validate properties of the strtab that are depended on elsewhere, but
were previously unchecked:
* String table nonempty (offset 0 is valid)
* String table has a leading NUL (offset 0 corresponds to "")
* String table is NUL terminated (strfoo functions won't run out of the
  table while reading).
* All symbols names are inbounds of the string table.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
---
 kernel/module/main.c | 38 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 37 insertions(+), 1 deletion(-)

diff --git a/kernel/module/main.c b/kernel/module/main.c
index d70d829b5ab9..7001054c5c4f 100644
--- a/kernel/module/main.c
+++ b/kernel/module/main.c
@@ -2090,17 +2090,53 @@ static int elf_validity_cache_index(struct load_info *info, int flags)
 }
 
 /**
- * elf_validity_cache_strtab() - Cache symbol string table
+ * elf_validity_cache_strtab() - Validate and cache symbol string table
  * @info: Load info to read from and update.
  *        Must have &load_info->sechdrs and &load_info->secstrings populated.
  *        Must have &load_info->index populated.
  *
+ * Checks:
+ *
+ * * The string table is not empty.
+ * * The string table starts and ends with NUL (required by ELF spec).
+ * * Every &Elf_Sym->st_name offset in the symbol table is inbounds of the
+ *   string table.
+ *
+ * And caches the pointer as &load_info->strtab in @info.
+ *
  * Return: 0 on success, negative error code if a check failed.
  */
 static int elf_validity_cache_strtab(struct load_info *info)
 {
 	Elf_Shdr *str_shdr = &info->sechdrs[info->index.str];
+	Elf_Shdr *sym_shdr = &info->sechdrs[info->index.sym];
 	char *strtab = (char *)info->hdr + str_shdr->sh_offset;
+	Elf_Sym *syms = (void *)info->hdr + sym_shdr->sh_offset;
+	int i;
+
+	if (str_shdr->sh_size == 0) {
+		pr_err("empty symbol string table\n");
+		return -ENOEXEC;
+	}
+	if (strtab[0] != '\0') {
+		pr_err("symbol string table missing leading NUL\n");
+		return -ENOEXEC;
+	}
+	if (strtab[str_shdr->sh_size - 1] != '\0') {
+		pr_err("symbol string table isn't NUL terminated\n");
+		return -ENOEXEC;
+	}
+
+	/*
+	 * Now that we know strtab is correctly structured, check symbol
+	 * starts are inbounds before they're used later.
+	 */
+	for (i = 0; i < sym_shdr->sh_size / sizeof(*syms); i++) {
+		if (syms[i].st_name >= str_shdr->sh_size) {
+			pr_err("symbol name out of bounds in string table");
+			return -ENOEXEC;
+		}
+	}
 
 	info->strtab = strtab;
 	return 0;
-- 
2.46.0.rc2.264.g509ed76dc8-goog