linux-ext4 - Re: [PATCH 4/6] vfs: don't allow most setxattr to immutable files

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Date:   Thu, 20 Jun 2019 16:03:45 +0200
From:   Jan Kara <jack@...e.cz>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     matthew.garrett@...ula.com, yuchao0@...wei.com, tytso@....edu,
        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 4/6] vfs: don't allow most setxattr to immutable files

On Mon 10-06-19 21:46:45, 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."
> 
> 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@...cle.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@...e.com>
SUSE Labs, CR