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

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Date:   Thu, 20 Jun 2019 17:52:12 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     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 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'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?

				- Ted