[PATCH] module: validate string table section types

["Thiébaud Weksteen" <tweek@google.com>]

In elf_validity_cache_sechdrs, section sizes and offsets are validated,
unless the section type is SHT_NULL or SHT_NOBITS.

Later, elf_validity_cache_secstrings and elf_validity_cache_index_str
access the section name table (.shstrtab) and symbol string table
(.strtab) headers without first ensuring that their types are
SHT_STRTAB. If a section type is SHT_NULL or SHT_NOBITS, sh_offset has
not been validated and may reference out-of-bounds memory when
dereferenced in elf_validity_cache_secstrings or
elf_validity_cache_strtab.

Validate that both string section headers are of type SHT_STRTAB before
caching them.

Signed-off-by: Thiébaud Weksteen <tweek@google.com>
---
 kernel/module/main.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/kernel/module/main.c b/kernel/module/main.c
index 46dd8d25a605..7cbc8f0e28c6 100644
--- a/kernel/module/main.c
+++ b/kernel/module/main.c
@@ -2011,6 +2011,7 @@ static int elf_validity_cache_sechdrs(struct load_info *info)
  * Specifically checks:
  *
  * * Section name table index is inbounds of section headers
+ * * Section name table type is SHT_STRTAB
  * * Section name table is not empty
  * * Section name table is NUL terminated
  * * All section name offsets are inbounds of the section
@@ -2038,6 +2039,11 @@ static int elf_validity_cache_secstrings(struct load_info *info)
 
 	strhdr = &info->sechdrs[info->hdr->e_shstrndx];
 
+	if (strhdr->sh_type != SHT_STRTAB) {
+		pr_err("Invalid ELF section name table type: %u\n", strhdr->sh_type);
+		return -ENOEXEC;
+	}
+
 	/*
 	 * The section name table must be NUL-terminated, as required
 	 * by the spec. This makes strcmp and pr_* calls that access
@@ -2204,7 +2210,7 @@ static int elf_validity_cache_index_sym(struct load_info *info)
  *        Must have &load_info->index.sym populated.
  *
  * Looks at the symbol table's associated string table, makes sure it is
- * in-bounds, and caches it.
+ * in-bounds and of type SHT_STRTAB, and caches it.
  *
  * Return: %0 if valid, %-ENOEXEC on failure.
  */
@@ -2218,6 +2224,12 @@ static int elf_validity_cache_index_str(struct load_info *info)
 		return -ENOEXEC;
 	}
 
+	if (info->sechdrs[str_idx].sh_type != SHT_STRTAB) {
+		pr_err("Invalid ELF symbol string table type: %u\n",
+		       info->sechdrs[str_idx].sh_type);
+		return -ENOEXEC;
+	}
+
 	info->index.str = str_idx;
 	return 0;
 }
-- 
2.55.0.795.g602f6c329a-goog