linux-kernel - Re: [PATCH v2 3/7] fuse: Fix infinite loop in sget

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Date:   Tue, 8 Jun 2021 17:12:15 +0200
From:   Max Reitz <mreitz@...hat.com>
To:     Greg Kurz <groug@...d.org>, Miklos Szeredi <miklos@...redi.hu>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        virtio-fs@...hat.com, Vivek Goyal <vgoyal@...hat.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH v2 3/7] fuse: Fix infinite loop in sget_fc()

On 04.06.21 18:11, Greg Kurz wrote:
> We don't set the SB_BORN flag on submounts. This is wrong as these
> superblocks are then considered as partially constructed or dying
> in the rest of the code and can break some assumptions.
>
> One such case is when you have a virtiofs filesystem with submounts
> and you try to mount it again : virtio_fs_get_tree() tries to obtain
> a superblock with sget_fc(). The logic in sget_fc() is to loop until
> it has either found an existing matching superblock with SB_BORN set
> or to create a brand new one. It is assumed that a superblock without
> SB_BORN is transient and the loop is restarted. Forgetting to set
> SB_BORN on submounts hence causes sget_fc() to retry forever.
>
> Setting SB_BORN requires special care, i.e. a write barrier for
> super_cache_count() which can check SB_BORN without taking any lock.
> We should call vfs_get_tree() to deal with that but this requires
> to have a proper ->get_tree() implementation for submounts, which
> is a bigger piece of work. Go for a simple bug fix in the meatime.
>
> Fixes: bf109c64040f ("fuse: implement crossmounts")
> Cc: mreitz@...hat.com
> Cc: stable@...r.kernel.org # v5.10+
> Signed-off-by: Greg Kurz <groug@...d.org>
> ---
>   fs/fuse/dir.c | 11 +++++++++++
>   1 file changed, 11 insertions(+)

Reviewed-by: Max Reitz <mreitz@...hat.com>