Re: [PATCH] xfs: Correctly lock inode when removing suid and security marks

[Jan Kara <jack@suse.cz>]

On Tue 02-12-14 11:35:48, Brian Foster wrote:
> On Tue, Dec 02, 2014 at 04:01:29PM +0100, Jan Kara wrote:
> > Currently XFS calls file_remove_suid() without holding i_mutex. This is
> > wrong because that function can end up messing with file permissions and
> > security xattrs for which we need i_mutex held.
> > 
> > Fix the problem by grabbing iolock exclusively when we will need to
> > change anything in permissions / xattrs.
> > 
> > Signed-off-by: Jan Kara <jack@suse.cz>
> > ---
> 
> Hi Jan,
> 
> This doesn't compile... it looks like we need to include the security.h
> header. FWIW, even then I get an undefined symbol error when compiling
> as a module (security_inode_need_killpriv() does not appear to be
> exported).
  Sorry, forgot to amend the include in the commit. Regarding export of
security_inode_need_killpriv() - right, I had security XFS compiled in so I
didn't notice. Before I go and fix this up in the obvious way, does anyone
have better idea how to fix this than to second guess what
file_remove_suid() does? Maybe a VFS helper like file_needs_remove_suid()
will be cleaner than what I did?

								Honza

> >  fs/xfs/xfs_file.c | 12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> > 
> > diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> > index eb596b419942..ad6636ac4943 100644
> > --- a/fs/xfs/xfs_file.c
> > +++ b/fs/xfs/xfs_file.c
> > @@ -521,6 +521,18 @@ restart:
> >  	if (error)
> >  		return error;
> >  
> > +	/* For changing security info in file_remove_suid() we need i_mutex */
> > +	if (!IS_NOSEC(inode) && *iolock == XFS_IOLOCK_SHARED) {
> > +		struct dentry *dentry = file->f_path.dentry;
> > +
> > +		if (should_remove_suid(dentry) ||
> > +		    security_inode_need_killpriv(dentry)) {
> > +			xfs_rw_iunlock(ip, *iolock);
> > +			*iolock = XFS_IOLOCK_EXCL;
> > +			xfs_rw_ilock(ip, *iolock);
> > +			goto restart;
> > +		}
> > +	}
> >  	/*
> >  	 * If the offset is beyond the size of the file, we need to zero any
> >  	 * blocks that fall between the existing EOF and the start of this
> > -- 
> > 1.8.1.4
> > 
> > _______________________________________________
> > xfs mailing list
> > xfs@oss.sgi.com
> > http://oss.sgi.com/mailman/listinfo/xfs
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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