From: Greg Kurz <groug@kaod.org> To: Miklos Szeredi <miklos@szeredi.hu> Cc: linux-kernel@vger.kernel.org, Max Reitz <mreitz@redhat.com>, linux-fsdevel@vger.kernel.org, virtio-fs@redhat.com, Vivek Goyal <vgoyal@redhat.com>, Greg Kurz <groug@kaod.org>, stable@vger.kernel.org Subject: [PATCH v2 3/7] fuse: Fix infinite loop in sget_fc() Date: Fri, 4 Jun 2021 18:11:52 +0200 [thread overview] Message-ID: <20210604161156.408496-4-groug@kaod.org> (raw) In-Reply-To: <20210604161156.408496-1-groug@kaod.org> 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@redhat.com Cc: stable@vger.kernel.org # v5.10+ Signed-off-by: Greg Kurz <groug@kaod.org> --- fs/fuse/dir.c | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c index 3fd1b71e546b..3fa8604c21d5 100644 --- a/fs/fuse/dir.c +++ b/fs/fuse/dir.c @@ -352,6 +352,17 @@ static struct vfsmount *fuse_dentry_automount(struct path *path) sb->s_flags |= SB_ACTIVE; fsc->root = dget(sb->s_root); + + /* + * FIXME: setting SB_BORN requires a write barrier for + * super_cache_count(). We should actually come + * up with a proper ->get_tree() implementation + * for submounts and call vfs_get_tree() to take + * care of the write barrier. + */ + smp_wmb(); + sb->s_flags |= SB_BORN; + /* We are done configuring the superblock, so unlock it */ up_write(&sb->s_umount); -- 2.31.1
WARNING: multiple messages have this Message-ID (diff)
From: Greg Kurz <groug@kaod.org> To: Miklos Szeredi <miklos@szeredi.hu> Cc: stable@vger.kernel.org, linux-kernel@vger.kernel.org, virtio-fs@redhat.com, linux-fsdevel@vger.kernel.org, Max Reitz <mreitz@redhat.com>, Vivek Goyal <vgoyal@redhat.com> Subject: [Virtio-fs] [PATCH v2 3/7] fuse: Fix infinite loop in sget_fc() Date: Fri, 4 Jun 2021 18:11:52 +0200 [thread overview] Message-ID: <20210604161156.408496-4-groug@kaod.org> (raw) In-Reply-To: <20210604161156.408496-1-groug@kaod.org> 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@redhat.com Cc: stable@vger.kernel.org # v5.10+ Signed-off-by: Greg Kurz <groug@kaod.org> --- fs/fuse/dir.c | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c index 3fd1b71e546b..3fa8604c21d5 100644 --- a/fs/fuse/dir.c +++ b/fs/fuse/dir.c @@ -352,6 +352,17 @@ static struct vfsmount *fuse_dentry_automount(struct path *path) sb->s_flags |= SB_ACTIVE; fsc->root = dget(sb->s_root); + + /* + * FIXME: setting SB_BORN requires a write barrier for + * super_cache_count(). We should actually come + * up with a proper ->get_tree() implementation + * for submounts and call vfs_get_tree() to take + * care of the write barrier. + */ + smp_wmb(); + sb->s_flags |= SB_BORN; + /* We are done configuring the superblock, so unlock it */ up_write(&sb->s_umount); -- 2.31.1
next prev parent reply other threads:[~2021-06-04 16:12 UTC|newest] Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-06-04 16:11 [PATCH v2 0/7] fuse: Some fixes for submounts Greg Kurz 2021-06-04 16:11 ` [Virtio-fs] " Greg Kurz 2021-06-04 16:11 ` [PATCH v2 1/7] fuse: Fix crash in fuse_dentry_automount() error path Greg Kurz 2021-06-04 16:11 ` [Virtio-fs] " Greg Kurz 2021-06-04 16:11 ` [PATCH v2 2/7] fuse: Fix crash if superblock of submount gets killed early Greg Kurz 2021-06-04 16:11 ` [Virtio-fs] " Greg Kurz 2021-06-08 15:08 ` Max Reitz 2021-06-08 15:08 ` [Virtio-fs] " Max Reitz 2021-06-04 16:11 ` Greg Kurz [this message] 2021-06-04 16:11 ` [Virtio-fs] [PATCH v2 3/7] fuse: Fix infinite loop in sget_fc() Greg Kurz 2021-06-08 15:12 ` Max Reitz 2021-06-08 15:12 ` [Virtio-fs] " Max Reitz 2021-06-04 16:11 ` [PATCH v2 4/7] fuse: Add dedicated filesystem context ops for submounts Greg Kurz 2021-06-04 16:11 ` [Virtio-fs] " Greg Kurz 2021-06-08 15:25 ` Max Reitz 2021-06-08 15:25 ` [Virtio-fs] " Max Reitz 2021-06-04 16:11 ` [PATCH v2 5/7] fuse: Call vfs_get_tree() " Greg Kurz 2021-06-04 16:11 ` [Virtio-fs] " Greg Kurz 2021-06-08 15:40 ` Max Reitz 2021-06-08 15:40 ` [Virtio-fs] " Max Reitz 2021-06-04 16:11 ` [PATCH v2 6/7] fuse: Switch to fc_mount() " Greg Kurz 2021-06-04 16:11 ` [Virtio-fs] " Greg Kurz 2021-06-08 15:51 ` Max Reitz 2021-06-08 15:51 ` [Virtio-fs] " Max Reitz 2021-06-09 7:45 ` Greg Kurz 2021-06-09 7:45 ` [Virtio-fs] " Greg Kurz 2021-06-11 14:44 ` Miklos Szeredi 2021-06-11 14:44 ` [Virtio-fs] " Miklos Szeredi 2021-06-04 16:11 ` [PATCH v2 7/7] fuse: Make fuse_fill_super_submount() static Greg Kurz 2021-06-04 16:11 ` [Virtio-fs] " Greg Kurz 2021-06-08 15:52 ` [PATCH v2 0/7] fuse: Some fixes for submounts Max Reitz 2021-06-08 15:52 ` [Virtio-fs] " Max Reitz 2021-06-09 7:46 ` Greg Kurz 2021-06-09 7:46 ` [Virtio-fs] " Greg Kurz
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