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

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Date:   Thu, 20 Jun 2019 15:13:06 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     "Theodore Ts'o" <tytso@....edu>, matthew.garrett@...ula.com,
        yuchao0@...wei.com, ard.biesheuvel@...aro.org,
        josef@...icpanda.com, clm@...com, adilger.kernel@...ger.ca,
        viro@...iv.linux.org.uk, jack@...e.com, dsterba@...e.com,
        jaegeuk@...nel.org, jk@...abs.org, reiserfs-devel@...r.kernel.org,
        linux-efi@...r.kernel.org, devel@...ts.orangefs.org,
        linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, linux-xfs@...r.kernel.org,
        linux-mm@...ck.org, linux-nilfs@...r.kernel.org,
        linux-mtd@...ts.infradead.org, ocfs2-devel@....oracle.com,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-btrfs@...r.kernel.org
Subject: Re: [PATCH 1/6] mm/fs: don't allow writes to immutable files

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@...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."
> > 
> > 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@...cle.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