nilfs2: fix kernel-infoleak in nilfs_ioctl_wrap_copy()

In the Linux kernel, the following vulnerability has been resolved:

nilfs2: fix kernel-infoleak in nilfs_ioctl_wrap_copy()

The ioctl helper function nilfs_ioctl_wrap_copy(), which exchanges a
metadata array to/from user space, may copy uninitialized buffer regions
to user space memory for read-only ioctl commands NILFS_IOCTL_GET_SUINFO
and NILFS_IOCTL_GET_CPINFO.

This can occur when the element size of the user space metadata given by
the v_size member of the argument nilfs_argv structure is larger than the
size of the metadata element (nilfs_suinfo structure or nilfs_cpinfo
structure) on the file system side.

KMSAN-enabled kernels detect this issue as follows:

BUG: KMSAN: kernel-infoleak in instrument_copy_to_user
include/linux/instrumented.h:121 [inline]
BUG: KMSAN: kernel-infoleak in _copy_to_user+0xc0/0x100 lib/usercopy.c:33
instrument_copy_to_user include/linux/instrumented.h:121 [inline]
_copy_to_user+0xc0/0x100 lib/usercopy.c:33
copy_to_user include/linux/uaccess.h:169 [inline]
nilfs_ioctl_wrap_copy+0x6fa/0xc10 fs/nilfs2/ioctl.c:99
nilfs_ioctl_get_info fs/nilfs2/ioctl.c:1173 [inline]
nilfs_ioctl+0x2402/0x4450 fs/nilfs2/ioctl.c:1290
nilfs_compat_ioctl+0x1b8/0x200 fs/nilfs2/ioctl.c:1343
__do_compat_sys_ioctl fs/ioctl.c:968 [inline]
__se_compat_sys_ioctl+0x7dd/0x1000 fs/ioctl.c:910
__ia32_compat_sys_ioctl+0x93/0xd0 fs/ioctl.c:910
do_syscall_32_irqs_on arch/x86/entry/common.c:112 [inline]
__do_fast_syscall_32+0xa2/0x100 arch/x86/entry/common.c:178
do_fast_syscall_32+0x37/0x80 arch/x86/entry/common.c:203
do_SYSENTER_32+0x1f/0x30 arch/x86/entry/common.c:246
entry_SYSENTER_compat_after_hwframe+0x70/0x82

Uninit was created at:
__alloc_pages+0x9f6/0xe90 mm/page_alloc.c:5572
alloc_pages+0xab0/0xd80 mm/mempolicy.c:2287
__get_free_pages+0x34/0xc0 mm/page_alloc.c:5599
nilfs_ioctl_wrap_copy+0x223/0xc10 fs/nilfs2/ioctl.c:74
nilfs_ioctl_get_info fs/nilfs2/ioctl.c:1173 [inline]
nilfs_ioctl+0x2402/0x4450 fs/nilfs2/ioctl.c:1290
nilfs_compat_ioctl+0x1b8/0x200 fs/nilfs2/ioctl.c:1343
__do_compat_sys_ioctl fs/ioctl.c:968 [inline]
__se_compat_sys_ioctl+0x7dd/0x1000 fs/ioctl.c:910
__ia32_compat_sys_ioctl+0x93/0xd0 fs/ioctl.c:910
do_syscall_32_irqs_on arch/x86/entry/common.c:112 [inline]
__do_fast_syscall_32+0xa2/0x100 arch/x86/entry/common.c:178
do_fast_syscall_32+0x37/0x80 arch/x86/entry/common.c:203
do_SYSENTER_32+0x1f/0x30 arch/x86/entry/common.c:246
entry_SYSENTER_compat_after_hwframe+0x70/0x82

Bytes 16-127 of 3968 are uninitialized
...

This eliminates the leak issue by initializing the page allocated as
buffer using get_zeroed_page().