Re: [RFC PATCH] ovl: keep some file attrubutions after copy-up

[Amir Goldstein <amir73il@gmail.com>]

On Sat, Dec 26, 2020 at 12:48 PM Chengguang Xu <cgxu519@mykernel.net> wrote:
>
> Currently after copy-up, upper file will lose most of file
> attributions except copy-up triggered by setting fsflags.
> Because ioctl operation of underlying file systems does not
> expect calling from kernel component, it seems hard to
> copy fsflags during copy-up.
>
> Overlayfs keeps limited attributions(append-only, etc) in it's
> inode flags after successfully updating attributions. so ater
> copy-up, lsattr(1) does not show correct result but overlayfs
> can still prohibit ramdom write for those files which originally
> have append-only attribution. However, recently I found this
> protection can be easily broken in below operations.
>
> 1, Set append attribution to lower file.
> 2, Mount overlayfs.
> 3, Trigger copy-up by data append.
> 4, Set noatime attributtion to the file.
> 5, The file is random writable.
>
> This patch tries to keep some file attributions after copy-up
> so that overlayfs keeps compatible behavior with local filesystem
> as much as possible.
>

This approach seems quite wrong.
For one thing, mount cycle overlay or drop caches will result in loss
of append only flag after copy-up, so this is not a security fix.

Second, Miklos has already proposed a much more profound change
to address this and similar issues [1] and he has already made some
changes to ioctl handler to master doesn't have ovl_iflags_to_fsflags().

[1] https://lore.kernel.org/linux-fsdevel/20201123141207.GC327006@miu.piliscsaba.redhat.com/

One more thing.
It seems like ovl_copyflags() in ovl_inode_init() would have been better
to copy from ovl_inode_realdata() inode instead of ovl_inode_real().
This way, copy up still loses the append-only flag, but metacopy up
does not. So at least for the common use case of containers that
chown -R won't cause losing all the file flags.

ovl_ioctl_set_flags() triggers data copy up, so that will break the link
to lower flags anyway.

I also noticed that we can lose the immutable flag on lower directory
when copying up parents and it's probably not the only way to
lose an immutable flag on a lower file/dir.

Thanks,
Amir.