capabilities: fix buffer overread on very short xattr
If userspace attempted to set a "security.capability" xattr shorter than
4 bytes (e.g. 'setfattr -n security.capability -v x file'), then
cap_convert_nscap() read past the end of the buffer containing the xattr
value because it accessed the ->magic_etc field without verifying that
the xattr value is long enough to contain that field.
Fix it by validating the xattr value size first.
This bug was found using syzkaller with KASAN. The KASAN report was as
follows (cleaned up slightly):
BUG: KASAN: slab-out-of-bounds in cap_convert_nscap+0x514/0x630 security/commoncap.c:498
Read of size 4 at addr ffff88002d8741c0 by task syz-executor1/2852
CPU: 0 PID: 2852 Comm: syz-executor1 Not tainted 4.15.0-rc6-00200-gcc0aac99d977 #253
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-20171110_100015-anatol 04/01/2014
Call Trace:
__dump_stack lib/dump_stack.c:17 [inline]
dump_stack+0xe3/0x195 lib/dump_stack.c:53
print_address_description+0x73/0x260 mm/kasan/report.c:252
kasan_report_error mm/kasan/report.c:351 [inline]
kasan_report+0x235/0x350 mm/kasan/report.c:409
cap_convert_nscap+0x514/0x630 security/commoncap.c:498
setxattr+0x2bd/0x350 fs/xattr.c:446
path_setxattr+0x168/0x1b0 fs/xattr.c:472
SYSC_setxattr fs/xattr.c:487 [inline]
SyS_setxattr+0x36/0x50 fs/xattr.c:483
entry_SYSCALL_64_fastpath+0x18/0x85
Fixes: 8db6c34f1dbc ("Introduce v3 namespaced file capabilities")
Cc: <stable@vger.kernel.org> # v4.14+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>
1 file changed