linux-kernel - Re: [PATCH v4 1/5] fuse: Fix leak in fuse_dentry

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Message-ID: <20210521101523.4f276dac@bahia.lan>
Date:   Fri, 21 May 2021 10:15:23 +0200
From:   Greg Kurz <groug@...d.org>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        virtualization@...ts.linux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        virtio-fs-list <virtio-fs@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Max Reitz <mreitz@...hat.com>, Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH v4 1/5] fuse: Fix leak in fuse_dentry_automount() error
 path

On Fri, 21 May 2021 09:54:19 +0200
Miklos Szeredi <miklos@...redi.hu> wrote:

> On Thu, 20 May 2021 at 21:45, Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > On Thu, May 20, 2021 at 05:46:50PM +0200, Greg Kurz wrote:
> > > Some rollback was forgotten during the addition of crossmounts.
> >
> > Have you actually tested that?  Because I strongly suspect that
> > by that point the ownership of fc and fm is with sb and those
> > should be taken care of by deactivate_locked_super().
> 
> Not quite.  Patch looks correct because destruction of fm is done in
> fuse_put_super(), which only gets called if the sb initialization gets
> as far as setting up sb->s_root, which only happens after the
> successful fuse_fill_super_submount() call in this case.
> 
> Doing the destruction from the various ->kill_sb() instances instead
> of from ->put_super() would also fix this, but I'm not quite sure that
> that would be any cleaner.
> 

As saying in the answer I've just posted, a failure in
fuse_fill_super_submount() causes an actual crash because
fuse_mount_remove() logically assumes fm to already be in
fc->mounts, which isn't the case at this point.

In the root mount case, this is handled by taking back
the ownership on fm, i.e. do the rollback *and* clear
sb->s_fs_info. It seems that the same should be done
for submounts.

> Thanks,
> Miklos