Re: [syzbot] general protection fault in _parse_integer_fixup_radix

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

syzbot is reporting that "vfs: parse: deal with zero length string value"
in linux-next.git broke tmpfs's mount option parsing, for tmpfs is expecting that
vfs_parse_fs_string() returning 0 implies that param.string != NULL.

The "nr_inodes" parameter for tmpfs is interpreted as "nr_inodes=$integer", but
the addition of

	if (!v_size) {
		param.string = NULL;
		param.type = fs_value_is_empty;
	} else {

to vfs_parse_fs_string() and

	if (param->type == fs_value_is_empty)
		return 0;

to fs_param_is_string() broke expectation by tmpfs.

  Parsing an fs string that has zero length should result in the parameter
  being set to NULL so that downstream processing handles it correctly.

is wrong and

  Parsing an fs string that has zero length should result in invalid argument
  error so that downstream processing does not dereference NULL param.string
  field.

is correct for the "nr_inodes" parameter.



How do we want to fix?
Should we add param.string != NULL checks into the downstream callers (like
Hawkins Jiawei did for https://syzkaller.appspot.com/bug?extid=a3e6acd85ded5c16a709 ) ?
Or should we add

	if (!*param.string)
		param.string = NULL;

rewriting into downstream callers which expect

  For example, the proc mount table processing should print "(none)" in this
  case to preserve mount record field count, but if the value points to the
  NULL string this doesn't happen.

behavior?

On 2022/10/20 15:16, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    a72b55bc981b Add linux-next specific files for 20221019
> git tree:       linux-next
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=1728c644880000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=200524babbc01b2a
> dashboard link: https://syzkaller.appspot.com/bug?extid=db1d2ea936378be0e4ea
> compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12afb08c880000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11001c72880000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/fa24fb5893fd/disk-a72b55bc.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/cf1b7e7b579c/vmlinux-a72b55bc.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+db1d2ea936378be0e4ea@syzkaller.appspotmail.com
>