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

Darrick J. Wong
2019-06-20T21:39:07Z

On Thu, Jun 20, 2019 at 04:03:45PM +0200, Jan Kara wrote:
> 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...

Good point! I'll fix it.

--D

>
> > + 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