Re: [PATCH 1/6] mm/fs: don't allow writes to immutable files

Darrick J. Wong
2019-06-20T22:14:39Z

On Thu, Jun 20, 2019 at 05:52:12PM -0400, Theodore Ts'o wrote:
> On Mon, Jun 10, 2019 at 09:46:17PM -0700, 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."
> >
> > Once the flag is set, it is enforced for quite a few file operations,
> > such as fallocate, fpunch, fzero, rm, touch, open, etc. However, we
> > don't check for immutability when doing a write(), a PROT_WRITE mmap(),
> > a truncate(), or a write to a previously established mmap.
> >
> > If a program has an open write fd to a file that the administrator
> > subsequently marks immutable, the program still can change the file
> > contents. Weird!
> >
> > The ability to write to an immutable file does not follow the manpage
> > promise that immutable files cannot be modified. Worse yet it's
> > inconsistent with the behavior of other syscalls which don't allow
> > modifications of immutable files.
> >
> > Therefore, add the necessary checks to make the write, mmap, and
> > truncate behavior consistent with what the manpage says and consistent
> > with other syscalls on filesystems which support IMMUTABLE.
> >
> > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
>
> I note that this patch doesn't allow writes to swap files. So Amir's
> generic/554 test will still fail for those file systems that don't use
> copy_file_range.

I didn't add any IS_SWAPFILE checks here, so I'm not sure to what you're
referring?

> I'm indifferent as to whether you add a new patch, or include that
> change in this patch, but perhaps we should fix this while we're
> making changes in these code paths?

The swapfile patches should be in a separate patch, which I was planning
to work on but hadn't really gotten around to it.

--D

> - Ted