Re: [PATCH] proc: don't show nonexistent capabilities

From: Andrew G. Morgan
Date: Fri Oct 05 2012 - 11:54:45 EST


I like the sentiment but have you considered the implications for a
default system root user trying to set all=eip ? Existing code can do
this because all bits are accessible by default. If you set the
bounding set to something less than ~0, then any code that currently
does this will start to fail in the common case.

Cheers

Andrew

On Oct 5, 2012 7:13 AM, "Serge E. Hallyn" <serge@xxxxxxxxxx> wrote:
>
> Quoting Andrew Vagin (avagin@xxxxxxxxxx):
> > Without this patch it is really hard to interpret a bounding set,
> > if CAP_LAST_CAP is unknown for a current kernel.
>
> To be clear, note that you *can* figure it out using
> prctl(PR_CAPBSET_DROP). But this is a nice improvement.
>
> > Non-existant capabilities can not be deleted from a bounding set
> > with help of prctl.
> >
> > E.g.: Here are two examples without/with this patch.
> > CapBnd: ffffffe0fdecffff
> > CapBnd: 00000000fdecffff
> >
> > I suggest to hide non-existent capabilities. Here is two reasons.
> > * It's logically and easier for using.
> > * It helps to checkpoint-restore capabilities of tasks, because tasks
> > can be restored on another kernel, where CAP_LAST_CAP is bigger.
> >
> > Cc: Serge Hallyn <serge.hallyn@xxxxxxxxxxxxx>
>
> Thanks, Andrew. I saw your other email about having run LTP, I didn't
> see any problems from userspace either. Great idea!
>
> Reviewed-by: Serge Hallyn <serge.hallyn@xxxxxxxxxxxxx>
>
> Still it's been like that for so long that, just to be safe, I'm cc:ing
> Andrew Morgan - can you see any problems with this?
>
> > Cc: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
> > Signed-off-by: Andrew Vagin <avagin@xxxxxxxxxx>
> > ---
> > include/linux/capability.h | 3 ++-
> > 1 files changed, 2 insertions(+), 1 deletions(-)
> >
> > diff --git a/include/linux/capability.h b/include/linux/capability.h
> > index d10b7ed..1642778 100644
> > --- a/include/linux/capability.h
> > +++ b/include/linux/capability.h
> > @@ -420,7 +420,8 @@ extern const kernel_cap_t __cap_init_eff_set;
> > #else /* HAND-CODED capability initializers */
> >
> > # define CAP_EMPTY_SET ((kernel_cap_t){{ 0, 0 }})
> > -# define CAP_FULL_SET ((kernel_cap_t){{ ~0, ~0 }})
> > +# define CAP_FULL_SET ((kernel_cap_t){{ ~0, \
> > + CAP_TO_MASK(CAP_LAST_CAP + 1) - 1 } })
> > # define CAP_FS_SET ((kernel_cap_t){{ CAP_FS_MASK_B0 \
> > | CAP_TO_MASK(CAP_LINUX_IMMUTABLE), \
> > CAP_FS_MASK_B1 } })
> > --
> > 1.7.1
> >
> > --
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