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Date:   Thu, 28 Mar 2019 23:24:48 +0200
From:   Amir Goldstein <amir73il@...il.com>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Ext4 <linux-ext4@...r.kernel.org>,
        Linux Btrfs <linux-btrfs@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH 3/3] xfs: don't allow most setxattr to immutable files

On Thu, Mar 28, 2019 at 7:51 PM Darrick J. Wong <darrick.wong@...cle.com> wrote:
>
> From: Darrick J. Wong <darrick.wong@...cle.com>
>
> The chattr manpage has this to say about immutable files:
>
> "A file with the 'i' attribute cannot be modified: it cannot be deleted
> or renamed, no link can be created to this file, most of the file's
> metadata can not be modified, and the file can not be opened in write
> mode."
>
> However, we don't actually check the immutable flag in the setattr code,
> which means that we can update project ids and extent size hints on
> supposedly immutable files.  Therefore, reject a setattr call on an
> immutable file except for the case where we're trying to unset
> IMMUTABLE.
>

I think if preventing modification of projid and extent size hints is what you
are after you should place the check in xfs_ioctl_setattr() and not in
xfs_ioctl_setattr_xflags().

Yes, it sounds tempting to block changes of xfs_ioc_setxflags(),
but it leads you to a trap of 2nd time chattr +i fails on -EPERM,
because chattr(1) doesn't optimize out the SETFLAGS ioctl
in the case of unmodified flags.
I think if you try to fix that, code will get ugly, so I suggest that
you let SETFLAGS slide.

Thanks,
Amir.

> Signed-off-by: Darrick J. Wong <darrick.wong@...cle.com>
> ---
>  fs/xfs/xfs_ioctl.c |    8 ++++++++
>  1 file changed, 8 insertions(+)
>
>
> diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
> index 2bd1c5ab5008..9cf0bc0ae2bd 100644
> --- a/fs/xfs/xfs_ioctl.c
> +++ b/fs/xfs/xfs_ioctl.c
> @@ -1067,6 +1067,14 @@ xfs_ioctl_setattr_xflags(
>             !capable(CAP_LINUX_IMMUTABLE))
>                 return -EPERM;
>
> +       /*
> +        * If immutable is set and we are not clearing it, we're not allowed
> +        * to change anything else in the inode.
> +        */
> +       if ((ip->i_d.di_flags & XFS_DIFLAG_IMMUTABLE) &&
> +           (fa->fsx_xflags & FS_XFLAG_IMMUTABLE))
> +               return -EPERM;
> +
>         /* diflags2 only valid for v3 inodes. */
>         di_flags2 = xfs_flags2diflags2(ip, fa->fsx_xflags);
>         if (di_flags2 && ip->i_d.di_version < 3)
>

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