Re: [PATCH 4/6] vfs: don't allow most setxattr to immutable files

Jan Kara
2019-06-20T14:04:00Z

On Mon 10-06-19 21:46:45, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.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 inode flags and project ids and extent
> size hints on supposedly immutable files. Therefore, reject setflags
> and fssetxattr calls on an immutable file if the file is immutable and
> will remain that way.
>
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---
> fs/inode.c | 31 +++++++++++++++++++++++++++++++
> 1 file changed, 31 insertions(+)
>
>
> diff --git a/fs/inode.c b/fs/inode.c
> index a3757051fd55..adfb458bf533 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -2184,6 +2184,17 @@ int vfs_ioc_setflags_check(struct inode *inode, int oldflags, int flags)
> !capable(CAP_LINUX_IMMUTABLE))
> return -EPERM;
>
> + /*
> + * We aren't allowed to change any other flags if the immutable flag is
> + * already set and is not being unset.
> + */
> + if ((oldflags & FS_IMMUTABLE_FL) &&
> + (flags & FS_IMMUTABLE_FL)) {
> + if ((oldflags & ~FS_IMMUTABLE_FL) !=
> + (flags & ~FS_IMMUTABLE_FL))

This check looks a bit strange when you've just check FS_IMMUTABLE_FL isn't
changing... Why not just oldflags != flags?

> + if ((old_fa->fsx_xflags & FS_XFLAG_IMMUTABLE) &&
> + (fa->fsx_xflags & FS_XFLAG_IMMUTABLE)) {
> + if ((old_fa->fsx_xflags & ~FS_XFLAG_IMMUTABLE) !=
> + (fa->fsx_xflags & ~FS_XFLAG_IMMUTABLE))

Ditto here...

> + return -EPERM;
> + if (old_fa->fsx_projid != fa->fsx_projid)
> + return -EPERM;
> + if ((fa->fsx_xflags & (FS_XFLAG_EXTSIZE |
> + FS_XFLAG_EXTSZINHERIT)) &&
> + old_fa->fsx_extsize != fa->fsx_extsize)
> + return -EPERM;
> + if ((old_fa->fsx_xflags & FS_XFLAG_COWEXTSIZE) &&
> + old_fa->fsx_cowextsize != fa->fsx_cowextsize)
> + return -EPERM;
> + }
> +
> /* Extent size hints of zero turn off the flags. */
> if (fa->fsx_extsize == 0)
> fa->fsx_xflags &= ~(FS_XFLAG_EXTSIZE | FS_XFLAG_EXTSZINHERIT);

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR