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Message-ID: <20130605192308.GM32700@dhcp-25-225.brq.redhat.com>
Date: Wed, 5 Jun 2013 21:23:08 +0200
From: Petr Matousek <pmatouse@...hat.com>
To: Stephane Eranian <eranian@...gle.com>
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>,
"ak@...ux.intel.com" <ak@...ux.intel.com>, security@...nel.org,
Marcus Meissner <meissner@...e.de>,
OSS Security List <oss-security@...ts.openwall.com>
Subject: Re: CVE Request: More perf security fixes
On Wed, Jun 05, 2013 at 03:53:52PM +0200, Stephane Eranian wrote:
> On Wed, Jun 5, 2013 at 3:35 PM, Petr Matousek <pmatouse@...hat.com> wrote:
> > On Wed, Jun 05, 2013 at 03:02:53PM +0200, Peter Zijlstra wrote:
> >> On Wed, Jun 05, 2013 at 02:38:56PM +0200, Petr Matousek wrote:
> >> > On Wed, Jun 05, 2013 at 02:15:59PM +0200, Peter Zijlstra wrote:
> >> > > On Wed, Jun 05, 2013 at 02:10:54PM +0200, Petr Matousek wrote:
> >> > > > Hello, Peter.
> >> > > >
> >> > > > On Tue, Jun 04, 2013 at 05:53:16PM +0200, Marcus Meissner wrote:
> >> > > > > 1. Info leak (?) via PERF_SAMPLE_BRANCH_KERNEL
> >> > > > >
> >> > > > > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=7cc23cd6c0c7d7f4bee057607e7ce01568925717
> >> > > > >
> >> > > > > commit 7cc23cd6c0c7d7f4bee057607e7ce01568925717
> >> > > > > Author: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> >> > > > > Date: Fri May 3 14:11:25 2013 +0200
> >> > > > >
> >> > > > > perf/x86/intel/lbr: Demand proper privileges for PERF_SAMPLE_BRANCH_KERNEL
> >> > > > >
> >> > > > > We should always have proper privileges when requesting kernel
> >> > > > > data.
> >> > > > >
> >> > > > > Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> >> > > > > Cc: <stable@...nel.org>
> >> > > > > Cc: Andi Kleen <ak@...ux.intel.com>
> >> > > > > Cc: eranian@...gle.com
> >> > > > > Link: http://lkml.kernel.org/r/20130503121256.230745028@chello.nl
> >> > > > > [ Fix build error reported by fengguang.wu@...el.com, propagate error code back. ]
> >> > > > > Signed-off-by: Ingo Molnar <mingo@...nel.org>
> >> > > > > Link: http://lkml.kernel.org/n/tip-v0x9ky3ahzr6nm3c6ilwrili@git.kernel.org
> >> > > >
> >> > > > There is similar check in perf_copy_attr() which is called from
> >> > > > perf_event_open syscall --
> >> > > >
> >> > > > /* kernel level capture: check permissions */
> >> > > > if ((mask & PERF_SAMPLE_BRANCH_PERM_PLM)
> >> > > > && perf_paranoid_kernel() && !capable(CAP_SYS_ADMIN))
> >> > > > return -EACCES;
> >> > > >
> >> > > > It seems to me that it covers PERF_SAMPLE_BRANCH_KERNEL as well. Am I
> >> > > > missing something?
> >> > > >
> >> > >
> >> > > I overlooked it, also its slightly broken. See the discussion at:
> >> > > https://lkml.org/lkml/2013/5/21/166
> >> >
> >> > Got it, thanks for the pointer. So it is safe to say there never was a
> >> > leak in this case (and thus no security issue worth CVE)?
> >>
> >> There was a leak, notice how Stephane's patch did a
> >> s/PERF_SAMPLE_BRANCH_PERM_PLM/PERF_SAMPLE_BRANCH_KERNEL/
> >
> > PERF_SAMPLE_BRANCH_PERM_PLM is a superset of PERF_SAMPLE_BRANCH_KERNEL:
> >
> > #define PERF_SAMPLE_BRANCH_PERM_PLM \
> > (PERF_SAMPLE_BRANCH_KERNEL |\
> > PERF_SAMPLE_BRANCH_HV)
> >
> >
> >> but also places
> >> the check _after_ we propagate the event PLM levels in the case none
> >> were LBR specific.
> >
> > Assuming the leak does occur only when PERF_SAMPLE_BRANCH_KERNEL is set,
> > that does not matter:
> >
> > /* kernel level capture: check permissions */
> > if ((mask & PERF_SAMPLE_BRANCH_PERM_PLM)
> > && perf_paranoid_kernel() && !capable(CAP_SYS_ADMIN))
> > return -EACCES;
> >
> > ^^^ this assures proper permission check if PERF_SAMPLE_BRANCH_KERNEL
> > is explicitly set
> >
> >
> > /* propagate priv level, when not set for branch */
> > if (!(mask & PERF_SAMPLE_BRANCH_PLM_ALL)) {
> >
> > /* exclude_kernel checked on syscall entry */
> > if (!attr->exclude_kernel)
> > mask |= PERF_SAMPLE_BRANCH_KERNEL;
> >
> > And following check in perf_event_open syscall assures the permission
> > are right for (!(mask & PERF_SAMPLE_BRANCH_PLM_ALL)) code:
> >
> > if (!attr.exclude_kernel) {
> > if (perf_paranoid_kernel() && !capable(CAP_SYS_ADMIN))
> > return -EACCES;
> > }
> >
> Yes, your analysis is correct. If the branch has not explicit priv
> level mask, then
> it is inherited from the event branches are requested from.
I am sorry to re-iterate the question, but does that mean that even
before your and Peter's changes, it was not possible to set
PERF_SAMPLE_BRANCH_KERNEL without passing "perf_paranoid_kernel() &&
!capable(CAP_SYS_ADMIN" check either in perf_copy_attr or
perf_event_open (attr.exclude_kernel check)?
Did your patch change anything at all or it was just refactoring?
I must be missing something.
Thanks for the patience,
--
Petr Matousek / Red Hat Security Response Team
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