Re: [PATCH v2] vsprintf: Check real user/group id for %pK

[Joe Perches <joe@perches.com>]

On Wed, 2013-10-09 at 12:55 +1100, Ryan Mallon wrote:
> On 09/10/13 12:30, Joe Perches wrote:
> > On Tue, 2013-10-08 at 17:49 -0700, Joe Perches wrote:
> >> On Wed, 2013-10-09 at 11:15 +1100, Ryan Mallon wrote:
> >>> Some setuid binaries will allow reading of files which have read
> >>> permission by the real user id. This is problematic with files which
> >>> use %pK because the file access permission is checked at open() time,
> >>> but the kptr_restrict setting is checked at read() time. If a setuid
> >>> binary opens a %pK file as an unprivileged user, and then elevates
> >>> permissions before reading the file, then kernel pointer values may be
> >>> leaked.
> >>
> >> I think it should explicitly test 0.
> > 
> > Also, Documentation/sysctl/kernel.txt should be updated too.
> > 
> > Here's a suggested patch:
> > 
> > ---
> >  Documentation/sysctl/kernel.txt | 14 ++++++++------
> >  lib/vsprintf.c                  | 38 ++++++++++++++++++++++++++------------
> >  2 files changed, 34 insertions(+), 18 deletions(-)
> > 
> > diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
> > index 9d4c1d1..eac53d5 100644
> > --- a/Documentation/sysctl/kernel.txt
> > +++ b/Documentation/sysctl/kernel.txt
> > @@ -290,13 +290,15 @@ Default value is "/sbin/hotplug".
> >  kptr_restrict:
> >  
> >  This toggle indicates whether restrictions are placed on
> > -exposing kernel addresses via /proc and other interfaces.  When
> > -kptr_restrict is set to (0), there are no restrictions.  When
> > -kptr_restrict is set to (1), the default, kernel pointers
> > +exposing kernel addresses via /proc and other interfaces.
> > +
> > +When kptr_restrict is set to (0), there are no restrictions.
> > +When kptr_restrict is set to (1), the default, kernel pointers
> >  printed using the %pK format specifier will be replaced with 0's
> > -unless the user has CAP_SYSLOG.  When kptr_restrict is set to
> > -(2), kernel pointers printed using %pK will be replaced with 0's
> > -regardless of privileges.
> > +unless the user has CAP_SYSLOG and effective user and group ids
> > +are equal to the real ids.
> > +When kptr_restrict is set to (2), kernel pointers printed using
> > +%pK will be replaced with 0's regardless of privileges.
> 
> I'll add this, thanks.
> 
> I'm less fussed about the suggestions for the logic. The current test is
> small and concise.

The logic ends up the same to the compiler, but it's
human readers that want simple and clear.

> The original also does the in_irq tests regardless of
> the kptr_restrict setting since they are mostly intended to catch
> internal kernel bugs.

Not so.

http://marc.info/?l=linux-security-module&m=129303800912245&w=4
https://lkml.org/lkml/2012/7/13/428