linux-kernel - Re: [RESEND PATCH v3] fuse: Abort waiting for a response if the daemon receives a fatal signal

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Date:   Mon, 9 Nov 2020 16:27:05 +0100
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Alexey Gladkov <gladkov.alexey@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-fsdevel@...r.kernel.org,
        Alexey Gladkov <legion@...nel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [RESEND PATCH v3] fuse: Abort waiting for a response if the
 daemon receives a fatal signal

On Mon, Nov 9, 2020 at 1:48 PM Alexey Gladkov <gladkov.alexey@...il.com> wrote:
>
> This patch removes one kind of the deadlocks inside the fuse daemon. The
> problem appear when the fuse daemon itself makes a file operation on its
> filesystem and receives a fatal signal.
>
> This deadlock can be interrupted via fusectl filesystem. But if you have
> many fuse mountpoints, it will be difficult to figure out which
> connection to break.
>
> This patch aborts the connection if the fuse server receives a fatal
> signal.

The patch itself might be acceptable, but I have some questions.

To logic of this patch says:

"If a task having the fuse device open in it's fd table receives
SIGKILL (and filesystem was initially mounted in a non-init user
namespace), then abort the filesystem operation"

You just say "server" instead of "task having the fuse device open in
it's fd table" which is sloppy to say the least.  It might also lead
to regressions, although I agree that it's unlikely.

Also how is this solving any security issue?   Just create the request
loop using two fuse filesystems and the deadlock avoidance has just
been circumvented.   So AFAICS "selling" this as a CVE fix is not
appropriate.

What's the reason for making this user-ns only?   If we drop the
security aspect, then I don't see any reason not to do this
unconditionally.

Also note, there's a proper solution for making fuse requests always
killable, and that is to introduce a shadow locking that ensures
correct fs operation in the face of requests that have returned and
released their respective VFS locks.   Now this would be a much more
complex solution, but also a much more correct one, not having issues
with correctly defining what a server is (which is not a solvable
problem).

Thanks,
Miklos