From: Liu Chao <[email protected]>
elf_validity_cache_secstrings() checks that every section name offset is
inside the section name table, but skips headers of type SHT_NULL, since
ELF leaves their field values undefined.
However, the section lookup helpers do not check sh_type. This is
find_any_unique_sec(), which load_module() uses to locate ".modinfo":
for (i = 1; i < info->hdr->e_shnum; i++) {
if (strcmp(info->secstrings + info->sechdrs[i].sh_name,
name) == 0) {
find_any_sec() and module_enforce_rwx_sections() read sh_name the same
way, the latter from index 0. find_sec() is only shielded by its
SHF_ALLOC test, which a crafted module can satisfy.
ELF also permits SHT_NULL entries above index 0. A module that carries
one with a large sh_name passes validation and then reads out of bounds:
BUG: unable to handle page fault for address: ffffc9004052f988
Oops: 0000 [#1] SMP KASAN PTI
CPU: 0 UID: 0 PID: 261 Comm: insmod Not tainted 6.18.0-rc7 #2
RIP: 0010:strcmp+0xc/0x40
Call Trace:
? find_any_unique_sec+0x89/0xf0
load_module+0x516/0x3d30
init_module_from_file+0xf5/0x180
__x64_sys_finit_module+0x91/0x100
The read happens in elf_validity_cache_copy(), before the blacklist check
in early_mod_check().
Bounds check sh_name for every section header. The name table is already
known to be non-empty, so the sh_name of 0 that ELF requires at index 0
still passes, and a name offset that points outside the name table is
malformed regardless of section type. Neither an Ubuntu 20.04 x86_64
build (GCC 9.4, 6025 modules) nor an aarch64 vendor build (GCC 13.2, 127
modules) has a header with an out-of-bounds sh_name, or any SHT_NULL
header above index 0.
Link:
https://lore.kernel.org/linux-modules/[email protected]/
Cc: [email protected]
Signed-off-by: Liu Chao <[email protected]>
---
The same report lists two more SHT_NULL gaps, both from
elf_validity_cache_sechdrs skipping offset and size validation:
move_module will memcpy such a section, and elf_validity_cache_index_mod
rejects SHT_NOBITS but not SHT_NULL. Sending this one on its own first,
happy to respin all three as a series if you would rather have them
together.
The Oops line says KASAN, but the access faults rather than producing a
KASAN report, because lib/string.o is built with KASAN_SANITIZE disabled.
No Fixes tag. 3c5700aeabd8 ("module: Factor out
elf_validity_cache_secstrings") is where the exemption was added, but the
problem predates it: before that commit sh_name was only checked for
SHF_ALLOC sections and the ".modinfo" strcmp() ran ahead of the check, so
find_any_sec() could already walk off the end. Same conclusion as
9a5ff4568932 ("module: validate string table section types"), which went
in with Cc: stable and no Fixes tag. checkpatch complains about that
combination.
The scan behind that last paragraph applies exactly the predicate this
patch introduces, sh_name >= strhdr->sh_size, to every section header
from index 0. On the x86_64 host it covered 17546 modules across three
kernel trees (5.15.0-181, 5.15.0-190, 5.4.0-182), all built by GCC 9.4.0;
those trees are the same toolchain recompiling much the same driver set,
so the changelog quotes one tree rather than the total. The aarch64 side
is a 5.10 vendor kernel built with Arm GNU Toolchain 13.2. Both are GNU
toolchains - no clang-built module set was available to me.
Reproducer, needs root and an unsigned module on a kernel without
CONFIG_MODULE_SIG_FORCE. Build any minimal module, then append a
SHT_NULL section header with an out-of-bounds sh_name:
python3 - <<'EOF'
import struct
SHDR = 0x40
data = bytearray(open('dummy.ko', 'rb').read())
shoff = struct.unpack_from('<Q', data, 0x28)[0]
shnum = struct.unpack_from('<H', data, 0x3c)[0]
old = bytes(data[shoff:shoff + shnum * SHDR])
evil = bytearray(SHDR)
struct.pack_into('<I', evil, 0, 0x40000000)
data += b'\0' * ((8 - len(data) % 8) % 8)
new_shoff = len(data)
data += old + bytes(evil)
struct.pack_into('<Q', data, 0x28, new_shoff)
struct.pack_into('<H', data, 0x3c, shnum + 1)
open('evil.ko', 'wb').write(data)
EOF
insmod evil.ko
kernel/module/main.c | 3 ---
1 file changed, 3 deletions(-)
diff --git a/kernel/module/main.c b/kernel/module/main.c
index d0e1e0bd2ad0..7d5943cd6543 100644
--- a/kernel/module/main.c
+++ b/kernel/module/main.c
@@ -2060,9 +2060,6 @@ static int elf_validity_cache_secstrings(struct load_info
*info)
for (i = 0; i < info->hdr->e_shnum; i++) {
shdr = &info->sechdrs[i];
- /* SHT_NULL means sh_name has an undefined value */
- if (shdr->sh_type == SHT_NULL)
- continue;
if (shdr->sh_name >= strhdr->sh_size) {
pr_err("Invalid ELF section name in module (section %u
type %u)\n",
i, shdr->sh_type);
