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().

Link: https://lkml.kernel.org/r/20221020231609.4810-1-yin31149@gmail.com
Reported-by: syzbot+a3e6acd85ded5c16a709@syzkaller.appspotmail.com
Tested-by: syzbot+a3e6acd85ded5c16a709@syzkaller.appspotmail.com
  Link: https://lore.kernel.org/all/0000000000005ad00405eb7148c6@google.com/Signed-off-by: Hawkins Jiawei <yin31149@gmail.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Hawkins Jiawei <yin31149@gmail.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/hugetlbfs/inode.c

@@ -1378,7 +1378,7 @@ static int hugetlbfs_parse_param(struct fs_context *fc, struct fs_parameter *par
 
 	case Opt_size:
 		/* memparse() will accept a K/M/G without a digit */
-		if (!isdigit(param->string[0]))
+		if (!param->string || !isdigit(param->string[0]))
 			goto bad_val;
 		ctx->max_size_opt = memparse(param->string, &rest);
 		ctx->max_val_type = SIZE_STD;
@@ -1388,7 +1388,7 @@ static int hugetlbfs_parse_param(struct fs_context *fc, struct fs_parameter *par
 
 	case Opt_nr_inodes:
 		/* memparse() will accept a K/M/G without a digit */
-		if (!isdigit(param->string[0]))
+		if (!param->string || !isdigit(param->string[0]))
 			goto bad_val;
 		ctx->nr_inodes = memparse(param->string, &rest);
 		return 0;
@@ -1404,7 +1404,7 @@ static int hugetlbfs_parse_param(struct fs_context *fc, struct fs_parameter *par
 
 	case Opt_min_size:
 		/* memparse() will accept a K/M/G without a digit */
-		if (!isdigit(param->string[0]))
+		if (!param->string || !isdigit(param->string[0]))
 			goto bad_val;
 		ctx->min_size_opt = memparse(param->string, &rest);
 		ctx->min_val_type = SIZE_STD;