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Date:   Tue, 9 Apr 2019 11:24:10 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     Dave Chinner <david@...morbit.com>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Ext4 <linux-ext4@...r.kernel.org>,
        Linux Btrfs <linux-btrfs@...r.kernel.org>
Subject: Re: [PATCH v2 4/4] xfs: don't allow most setxattr to immutable files

On Tue, Apr 9, 2019 at 6:19 AM 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.
>
> Signed-off-by: Darrick J. Wong <darrick.wong@...cle.com>
> ---
>  fs/xfs/xfs_ioctl.c |   46 ++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 44 insertions(+), 2 deletions(-)
>
> diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
> index 5a1b96dad901..67d12027f563 100644
> --- a/fs/xfs/xfs_ioctl.c
> +++ b/fs/xfs/xfs_ioctl.c
> @@ -1023,6 +1023,40 @@ xfs_ioctl_setattr_flush(
>         return filemap_write_and_wait(inode->i_mapping);
>  }
>
> +/*
> + * If immutable is set and we are not clearing it, we're not allowed to change
> + * anything else in the inode.

This looks correct, but FYI, neither xfs_io nor chattr clears 'immutable'
and sets projid/*extsize in one ioctl/xfsctl, so there is no justification to
making an extra effort to support that use case. You could do with
checking 'immutable' inside xfs_ioctl_setattr_check_projid/*extsize()
and leave only the di_flags check here.

Some would say that will be cleaner code.
Its a matter of taste and its your subsystem, so feel free to dismiss
this comments.

Thanks,
Amir.

> Don't error out if we're only trying to set
> + * immutable on an immutable file.
> + */
> +static int
> +xfs_ioctl_setattr_immutable(
> +       struct xfs_inode        *ip,
> +       struct fsxattr          *fa,
> +       uint16_t                di_flags,
> +       uint64_t                di_flags2)
> +{
> +       struct xfs_mount        *mp = ip->i_mount;
> +
> +       if (!(ip->i_d.di_flags & XFS_DIFLAG_IMMUTABLE) ||
> +           !(fa->fsx_xflags & FS_XFLAG_IMMUTABLE))
> +               return 0;
> +
> +       if ((ip->i_d.di_flags & ~XFS_DIFLAG_IMMUTABLE) !=
> +           (di_flags & ~XFS_DIFLAG_IMMUTABLE))
> +               return -EPERM;
> +       if (ip->i_d.di_version >= 3 && ip->i_d.di_flags2 != di_flags2)
> +               return -EPERM;
> +       if (xfs_get_projid(ip) != fa->fsx_projid)
> +               return -EPERM;
> +       if (ip->i_d.di_extsize != fa->fsx_extsize >> mp->m_sb.sb_blocklog)
> +               return -EPERM;
> +       if (ip->i_d.di_version >= 3 && (di_flags2 & XFS_DIFLAG2_COWEXTSIZE) &&
> +           ip->i_d.di_cowextsize != fa->fsx_cowextsize >> mp->m_sb.sb_blocklog)
> +               return -EPERM;
> +
> +       return 0;
> +}
> +
>  static int
>  xfs_ioctl_setattr_xflags(
>         struct xfs_trans        *tp,
> @@ -1030,7 +1064,9 @@ xfs_ioctl_setattr_xflags(
>         struct fsxattr          *fa)
>  {
>         struct xfs_mount        *mp = ip->i_mount;
> +       uint16_t                di_flags;
>         uint64_t                di_flags2;
> +       int                     error;
>
>         /* Can't change realtime flag if any extents are allocated. */
>         if ((ip->i_d.di_nextents || ip->i_delayed_blks) &&
> @@ -1061,12 +1097,18 @@ xfs_ioctl_setattr_xflags(
>             !capable(CAP_LINUX_IMMUTABLE))
>                 return -EPERM;
>
> -       /* diflags2 only valid for v3 inodes. */
> +       /* Don't allow changes to an immutable inode. */
> +       di_flags = xfs_flags2diflags(ip, fa->fsx_xflags);
>         di_flags2 = xfs_flags2diflags2(ip, fa->fsx_xflags);
> +       error = xfs_ioctl_setattr_immutable(ip, fa, di_flags, di_flags2);
> +       if (error)
> +               return error;
> +
> +       /* diflags2 only valid for v3 inodes. */
>         if (di_flags2 && ip->i_d.di_version < 3)
>                 return -EINVAL;
>
> -       ip->i_d.di_flags = xfs_flags2diflags(ip, fa->fsx_xflags);
> +       ip->i_d.di_flags = di_flags;
>         ip->i_d.di_flags2 = di_flags2;
>
>         xfs_diflags_to_linux(ip);

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