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Message-ID: <CABPqkBTw+qoYQmgiTDwgN18fUb2iV3d1F+cfCEzjde0wAGqmcQ@mail.gmail.com>
Date: Wed, 5 Jun 2013 15:53:52 +0200
From: Stephane Eranian <eranian@...gle.com>
To: Peter Zijlstra <a.p.zijlstra@...llo.nl>, Stephane Eranian <eranian@...gle.com>,
"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 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.
> --
> Petr Matousek / Red Hat Security Response Team
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