On Fri, Jan 30, 2015 at 10:53:15AM +1100, Dave Chinner wrote:
> 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.
>
Hrm, I'm not following how a boolean cast occurs here. Isn't ^ a bitwise
operation? If I tweak the DIFLAG_PROJINHERIT to bit 15 (rather than 9)
and regenerate assembly for the code above, I end up with something like
this:
/* -EINVAL */
movl $-22, %eax #, D.54727
...
/* load up the flags vars */
movzwl 354(%rdi), %ecx # ip_5(D)->i_d.di_flags,ip_5(D)->i_d.di_flags
movl (%rsi), %edx # fa_3(D)->fsx_xflags, D.54728
...
/* grab the associated bits and cmp */
andl $512, %edx #, D.54728
andl $32768, %ecx #, D.54728
cmpl %edx, %ecx # D.54728, D.54728
/* ret 0 if values are equal */
movl $0, %edx #, tmp99
cmove %edx, %eax # tmp99,, D.54727
ret
So it seems like this breaks just the same if the bit meaning doesn't
match between fields..?
Brian
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@xxxxxxxxxxxxx
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