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

[Amir Goldstein <amir73il@gmail.com>]

On Mon, Jan 4, 2021 at 8:55 AM Chengguang Xu <cgxu519@mykernel.net> wrote:
>
>  ---- 在 星期一, 2021-01-04 13:04:56 Amir Goldstein <amir73il@gmail.com> 撰写 ----
>  > 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.
>  >
>
> You are right, I overlooked the case of dropping cache.
>
>  > 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.
>
> IIUC, the flags will still keep in overlayfs' inode after copy up until
> the inode cleaned by dropping cache. So I think your suggestion will be
> helpful for the case of meta-copyup & dropping cache.

Yes, for the use case of chowning all files sure cannot rely on caches
and I believe those containers are also used as persistent containers
that can be mounted again later after initial ownership fix.

>
> Hi Miklos
>
> Is it worth to change like above?
>

I guess that depends what are the use cases that benefit.
After all it is not a security fix it just increases the amount of
use cases that preserve the append-only flag.

I *think* it could fix a lot of cases like:
chmod foo; drop_caches; touch foo # should fail
mv foo bar; drop_caches; touch bar # should fail

and in order to lose the append-only flag, users will need to
first open with O_APPEND, set noatime flag or some unusual
operations that do not happen by mistake as often as chmod,chown,rename.

>
>  >
>  > ovl_ioctl_set_flags() triggers data copy up, so that will break the link
>  > to lower flags anyway.
>
> I think though ovl_ioctl_set_flags() triggers data copy up but the flags
> will be set correctly to upper file, because chattr(1) will get the flags
> first and set the whole flags(include original flags) to upper file.
>

Sure, unless the user is not privileged to set flags, but copy up will still
happen. But what I meant is if user changes the flags, data copy up
happens and ovl_copyflags() after drop caches will no longer copy the
lower flags.

Thanks,
Amir.