[PATCH 4.9 39/39] fs/binfmt_misc.c: do not allow offset overflow

Greg Kroah-Hartman
2018-06-24T15:53:05Z

From: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

commit 5cc41e099504b77014358b58567c5ea6293dd220 upstream.

WHen registering a new binfmt_misc handler, it is possible to overflow
the offset to get a negative value, which might crash the system, or
possibly leak kernel data.

Here is a crash log when 2500000000 was used as an offset:

BUG: unable to handle kernel paging request at ffff989cfd6edca0
IP: load_misc_binary+0x22b/0x470 [binfmt_misc]
PGD 1ef3e067 P4D 1ef3e067 PUD 0
Oops: 0000 [#1] SMP NOPTI
Modules linked in: binfmt_misc kvm_intel ppdev kvm irqbypass joydev input_leds serio_raw mac_hid parport_pc qemu_fw_cfg parpy
CPU: 0 PID: 2499 Comm: bash Not tainted 4.15.0-22-generic #24-Ubuntu
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.1-1 04/01/2014
RIP: 0010:load_misc_binary+0x22b/0x470 [binfmt_misc]
Call Trace:
search_binary_handler+0x97/0x1d0
do_execveat_common.isra.34+0x667/0x810
SyS_execve+0x31/0x40
do_syscall_64+0x73/0x130
entry_SYSCALL_64_after_hwframe+0x3d/0xa2

Use kstrtoint instead of simple_strtoul. It will work as the code
already set the delimiter byte to '\0' and we only do it when the field
is not empty.

Tested with offsets -1, 2500000000, UINT_MAX and INT_MAX. Also tested
with examples documented at Documentation/admin-guide/binfmt-misc.rst
and other registrations from packages on Ubuntu.

Link: http://lkml.kernel.org/r/20180529135648.14254-1-cascardo@canonical.com
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/binfmt_misc.c | 12 +++++++++---
1 file changed, 9 insertions(+), 3 deletions(-)

--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -384,8 +384,13 @@ static Node *create_entry(const char __u
s = strchr(p, del);
if (!s)
goto einval;
- *s++ = '\0';
- e->offset = simple_strtoul(p, &p, 10);
+ *s = '\0';
+ if (p != s) {
+ int r = kstrtoint(p, 10, &e->offset);
+ if (r != 0 || e->offset < 0)
+ goto einval;
+ }
+ p = s;
if (*p++)
goto einval;
pr_debug("register: offset: %#x\n", e->offset);
@@ -425,7 +430,8 @@ static Node *create_entry(const char __u
if (e->mask &&
string_unescape_inplace(e->mask, UNESCAPE_HEX) != e->size)
goto einval;
- if (e->size + e->offset > BINPRM_BUF_SIZE)
+ if (e->size > BINPRM_BUF_SIZE ||
+ BINPRM_BUF_SIZE - e->size < e->offset)
goto einval;
pr_debug("register: magic/mask length: %i\n", e->size);
if (USE_DEBUG) {