Re: [syzbot] [hfs?] WARNING in hfs_write_inode

[Eric W. Biederman]

Dmitry Vyukov <dvyukov@xxxxxxxxxx> writes:

> On Thu, 5 Jan 2023 at 17:45, Viacheslav Dubeyko <slava@xxxxxxxxxxx> wrote:
>> > On Wed, Jan 04, 2023 at 08:37:16PM -0800, Viacheslav Dubeyko wrote:
>> >> Also, as far as I can see, available volume in report (mount_0.gz) somehow corrupted already:
>> >
>> > Syzbot generates deliberately-corrupted (aka fuzzed) filesystem images.
>> > So basically, you can't trust anything you read from the disc.
>> >
>>
>> If the volume has been deliberately corrupted, then no guarantee that file system
>> driver will behave nicely. Technically speaking, inode write operation should never
>> happened for corrupted volume because the corruption should be detected during
>> b-tree node initialization time. If we would like to achieve such nice state of HFS/HFS+
>> drivers, then it requires a lot of refactoring/implementation efforts. I am not sure that
>> it is worth to do because not so many guys really use HFS/HFS+ as the main file
>> system under Linux.
>
>
> Most popular distros will happily auto-mount HFS/HFS+ from anything
> inserted into USB (e.g. what one may think is a charger). This creates
> interesting security consequences for most Linux users.
> An image may also be corrupted non-deliberately, which will lead to
> random memory corruptions if the kernel trusts it blindly.

I am going to point out that there are no known linux filesystems
that are safe to mount when someone has written a deliberately
corrupted filesystem on a usb stick.

Some filesystems like ext4 make a best effort to fix bugs of this sort
as they are discovered but unless something has changed since last I
looked no one makes the effort to ensure that it is 100% safe to mount
any possible corrupted version of any Linux filesystem.

If there is any filesystem in Linux that is safe to automount from
an untrusted USB stick I really would like to hear about it.  We could
allow mounting them in unprivileged user namespaces and give all kinds
of interesting capabilities to our users.

As it is I respectfully suggest that if there is a security issue it is
the userspace code that automounts any filesystem on an untrusted USB
stick.

Eric