hugetlbfs: fix null-ptr-deref in hugetlbfs_parse_param()

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

hugetlbfs: fix null-ptr-deref in hugetlbfs_parse_param()

Syzkaller reports a null-ptr-deref bug as follows:

======================================================

KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]

RIP: 0010:hugetlbfs_parse_param+0x1dd/0x8e0 fs/hugetlbfs/inode.c:1380

[...]

Call Trace:

<TASK>

vfs_parse_fs_param fs/fs_context.c:148 [inline]

vfs_parse_fs_param+0x1f9/0x3c0 fs/fs_context.c:129

vfs_parse_fs_string+0xdb/0x170 fs/fs_context.c:191

generic_parse_monolithic+0x16f/0x1f0 fs/fs_context.c:231

do_new_mount fs/namespace.c:3036 [inline]

path_mount+0x12de/0x1e20 fs/namespace.c:3370

do_mount fs/namespace.c:3383 [inline]

__do_sys_mount fs/namespace.c:3591 [inline]

__se_sys_mount fs/namespace.c:3568 [inline]

__x64_sys_mount+0x27f/0x300 fs/namespace.c:3568

do_syscall_x64 arch/x86/entry/common.c:50 [inline]

do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80

entry_SYSCALL_64_after_hwframe+0x63/0xcd

[...]

</TASK>

======================================================

According to commit "vfs: parse: deal with zero length string value",

kernel will set the param->string to null pointer in vfs_parse_fs_string()

if fs string has zero length.

Yet the problem is that, hugetlbfs_parse_param() will dereference the

param->string, without checking whether it is a null pointer. To be more

specific, if hugetlbfs_parse_param() parses an illegal mount parameter,

such as "size=,", kernel will constructs struct fs_parameter with null

pointer in vfs_parse_fs_string(), then passes this struct fs_parameter to

hugetlbfs_parse_param(), which triggers the above null-ptr-deref bug.

This patch solves it by adding sanity check on param->string

in hugetlbfs_parse_param().