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Re: [PATCH 6/9] xfs: XFS_IOCTL_SETXATTR can run in user namespaces

To: Brian Foster <bfoster@xxxxxxxxxx>
Subject: Re: [PATCH 6/9] xfs: XFS_IOCTL_SETXATTR can run in user namespaces
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Fri, 30 Jan 2015 10:53:15 +1100
Cc: xfs@xxxxxxxxxxx, iustin@xxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20150129153515.GF17652@xxxxxxxxxxxxxx>
References: <1422328486-24661-1-git-send-email-david@xxxxxxxxxxxxx> <1422328486-24661-7-git-send-email-david@xxxxxxxxxxxxx> <20150129153515.GF17652@xxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Thu, Jan 29, 2015 at 10:35:15AM -0500, Brian Foster wrote:
> On Tue, Jan 27, 2015 at 02:14:43PM +1100, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@xxxxxxxxxx>
> > 
> > Currently XFS_IOCTL_SETXATTR will fail if run in a user namespace as
> > it it not allowed to change project IDs. The current code, however,
> > also prevents any other change being made as well, so things like
> > extent size hints cannot be set in user namespaces. This is wrong,
> > so only disallow access to project IDs and related flags from inside
> > the init namespace.
> > 
> > Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> > ---
> >  fs/xfs/xfs_ioctl.c | 22 +++++++++++++---------
> >  1 file changed, 13 insertions(+), 9 deletions(-)
> > 
> > diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
> > index 563d2b4..ae6e1e3 100644
> > --- a/fs/xfs/xfs_ioctl.c
> > +++ b/fs/xfs/xfs_ioctl.c
> > @@ -1120,6 +1120,19 @@ xfs_ioctl_setattr(
> >             return -EINVAL;
> >  
> >     /*
> > +    * Project Quota ID state is only allowed to change from within the init
> > +    * namespace. Enforce that restriction only if we are trying to change
> > +    * the quota ID state. Everything else is allowed in user namespaces.
> > +    */
> > +   if (current_user_ns() != &init_user_ns) {
> > +           if (xfs_get_projid(ip) != fa->fsx_projid)
> > +                   return -EINVAL;
> > +           if ((fa->fsx_xflags & XFS_XFLAG_PROJINHERIT) ^
> > +               (ip->i_d.di_flags & XFS_DIFLAG_PROJINHERIT))
> 
> Why not use != here? Looks fine, anyways:

Because ^ has an implicit cast of the variables to boolean (i.e flag
set or not), whereas != will only work if XFS_XFLAG_PROJINHERIT =
XFS_DIFLAG_PROJINHERIT. Given that the moment we add more DIFLAGs to
the xfs inode, the current "XFLAG value must match DIFLAG value"
rule is going to be broken, I think that logical evaluation is a
much safer practice for these types of comparisons.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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