[RFC] xfrm: fix regression introduced by xdst pcpu cache
diff mbox

Message ID 20171030145843.13496-1-sds@tycho.nsa.gov
State New
Headers show

Commit Message

Stephen Smalley Oct. 30, 2017, 2:58 p.m. UTC
Since 4.14-rc1, the selinux-testsuite has been encountering sporadic
failures during testing of labeled IPSEC. git bisect pointed to
commit ec30d78c14a813db39a647b6a348b4286 ("xfrm: add xdst pcpu cache").
The xdst pcpu cache is only checking that the policies are the same,
but does not validate that the policy, state, and flow match with respect
to security context labeling.  As a result, the wrong SA could be used
and the receiver could end up performing permission checking and
providing SO_PEERSEC or SCM_SECURITY values for the wrong security context.
security_xfrm_state_pol_flow_match() exists for this purpose and is
already called from xfrm_state_look_at() for matching purposes.
Further, xfrm_state_look_at() also performs a xfrm_selector_match() test,
which is also missing from the xdst pcpu cache logic.  Add calls to both
of these functions when validating the cache entry.  With these changes,
the selinux-testsuite passes all tests again.

Fixes: ec30d78c14a813db39a647b6a348b4286ba4abf5 ("xfrm: add xdst pcpu cache")
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
---
This is an RFC because I am not entirely confident in the fix, e.g. is it
sufficient to perform this matching only on the first xfrm or do they all
need to be walked as in xfrm_bundle_ok()?  Also, should we perform this
matching before (as in this patch) or after calling xfrm_bundle_ok()? Also,
do we need to test xfrm->sel.family before calling xfrm_selector_match
(as in this patch) or not - xfrm_state_look_at() does so when the
state is XFRM_STATE_VALID but not when it is _ERROR or _EXPIRED?

 net/xfrm/xfrm_policy.c | 5 +++++
 1 file changed, 5 insertions(+)

Comments

Florian Westphal Oct. 31, 2017, 11:11 a.m. UTC | #1
Stephen Smalley <sds@tycho.nsa.gov> wrote:
> Since 4.14-rc1, the selinux-testsuite has been encountering sporadic
> failures during testing of labeled IPSEC. git bisect pointed to
> commit ec30d78c14a813db39a647b6a348b4286 ("xfrm: add xdst pcpu cache").
> The xdst pcpu cache is only checking that the policies are the same,
> but does not validate that the policy, state, and flow match with respect
> to security context labeling.  As a result, the wrong SA could be used
> and the receiver could end up performing permission checking and
> providing SO_PEERSEC or SCM_SECURITY values for the wrong security context.
> security_xfrm_state_pol_flow_match() exists for this purpose and is
> already called from xfrm_state_look_at() for matching purposes.
> Further, xfrm_state_look_at() also performs a xfrm_selector_match() test,
> which is also missing from the xdst pcpu cache logic.  Add calls to both
> of these functions when validating the cache entry.  With these changes,
> the selinux-testsuite passes all tests again.
> 
> Fixes: ec30d78c14a813db39a647b6a348b4286ba4abf5 ("xfrm: add xdst pcpu cache")
> Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
> ---
> This is an RFC because I am not entirely confident in the fix, e.g. is it
> sufficient to perform this matching only on the first xfrm or do they all
> need to be walked as in xfrm_bundle_ok()?  Also, should we perform this
> matching before (as in this patch) or after calling xfrm_bundle_ok()? Also,
> do we need to test xfrm->sel.family before calling xfrm_selector_match
> (as in this patch) or not - xfrm_state_look_at() does so when the
> state is XFRM_STATE_VALID but not when it is _ERROR or _EXPIRED?

No idea.

I looked at the old flow cache but i don't see any of these extra
checks there either.

However, old flow cache stored flowi struct as key, and that contains a
flowi_secid,  populated by the decode_session hooks.

Was it enough to check for identical flowi_secid in the flowi structs to
avoid this problem or am i missing something?
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Stephen Smalley Oct. 31, 2017, 1:43 p.m. UTC | #2
On Tue, 2017-10-31 at 12:11 +0100, Florian Westphal wrote:
> Stephen Smalley <sds@tycho.nsa.gov> wrote:
> > Since 4.14-rc1, the selinux-testsuite has been encountering
> > sporadic
> > failures during testing of labeled IPSEC. git bisect pointed to
> > commit ec30d78c14a813db39a647b6a348b4286 ("xfrm: add xdst pcpu
> > cache").
> > The xdst pcpu cache is only checking that the policies are the
> > same,
> > but does not validate that the policy, state, and flow match with
> > respect
> > to security context labeling.  As a result, the wrong SA could be
> > used
> > and the receiver could end up performing permission checking and
> > providing SO_PEERSEC or SCM_SECURITY values for the wrong security
> > context.
> > security_xfrm_state_pol_flow_match() exists for this purpose and is
> > already called from xfrm_state_look_at() for matching purposes.
> > Further, xfrm_state_look_at() also performs a xfrm_selector_match()
> > test,
> > which is also missing from the xdst pcpu cache logic.  Add calls to
> > both
> > of these functions when validating the cache entry.  With these
> > changes,
> > the selinux-testsuite passes all tests again.
> > 
> > Fixes: ec30d78c14a813db39a647b6a348b4286ba4abf5 ("xfrm: add xdst
> > pcpu cache")
> > Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
> > ---
> > This is an RFC because I am not entirely confident in the fix, e.g.
> > is it
> > sufficient to perform this matching only on the first xfrm or do
> > they all
> > need to be walked as in xfrm_bundle_ok()?  Also, should we perform
> > this
> > matching before (as in this patch) or after calling
> > xfrm_bundle_ok()? Also,
> > do we need to test xfrm->sel.family before calling
> > xfrm_selector_match
> > (as in this patch) or not - xfrm_state_look_at() does so when the
> > state is XFRM_STATE_VALID but not when it is _ERROR or _EXPIRED?
> 
> No idea.
> 
> I looked at the old flow cache but i don't see any of these extra
> checks there either.
> 
> However, old flow cache stored flowi struct as key, and that contains
> a
> flowi_secid,  populated by the decode_session hooks.
> 
> Was it enough to check for identical flowi_secid in the flowi structs
> to
> avoid this problem or am i missing something?

I'm not sure, but security_xfrm_state_pol_flow_match() ->
selinux_xfrm_state_pol_flow_match() does more than just compare flow
secids.

Also, there is the separate issue of the missing xfrm_selector_match()
call, which can also cause the wrong SA to be used independent of
anything LSM/SELinux-related.

It is a regression; the correct SA was being used prior to the xdst
pcpu cache commit.  Reproducible using the selinux-testsuite, most
easily run on a Fedora VM,
git clone https://github.com/SELinuxProject/selinux-testsuite/
sudo dnf install perl-Test perl-Test-Harness perl-Test-Simple selinux-policy-devel gcc libselinux-devel net-tools netlabel_tools iptables
sudo make -C policy load
cd tests/inet_socket
while sudo ./test; do : ; done

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Stephen Smalley Oct. 31, 2017, 2 p.m. UTC | #3
On Tue, 2017-10-31 at 09:43 -0400, Stephen Smalley wrote:
> On Tue, 2017-10-31 at 12:11 +0100, Florian Westphal wrote:
> > Stephen Smalley <sds@tycho.nsa.gov> wrote:
> > > Since 4.14-rc1, the selinux-testsuite has been encountering
> > > sporadic
> > > failures during testing of labeled IPSEC. git bisect pointed to
> > > commit ec30d78c14a813db39a647b6a348b4286 ("xfrm: add xdst pcpu
> > > cache").
> > > The xdst pcpu cache is only checking that the policies are the
> > > same,
> > > but does not validate that the policy, state, and flow match with
> > > respect
> > > to security context labeling.  As a result, the wrong SA could be
> > > used
> > > and the receiver could end up performing permission checking and
> > > providing SO_PEERSEC or SCM_SECURITY values for the wrong
> > > security
> > > context.
> > > security_xfrm_state_pol_flow_match() exists for this purpose and
> > > is
> > > already called from xfrm_state_look_at() for matching purposes.
> > > Further, xfrm_state_look_at() also performs a
> > > xfrm_selector_match()
> > > test,
> > > which is also missing from the xdst pcpu cache logic.  Add calls
> > > to
> > > both
> > > of these functions when validating the cache entry.  With these
> > > changes,
> > > the selinux-testsuite passes all tests again.
> > > 
> > > Fixes: ec30d78c14a813db39a647b6a348b4286ba4abf5 ("xfrm: add xdst
> > > pcpu cache")
> > > Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
> > > ---
> > > This is an RFC because I am not entirely confident in the fix,
> > > e.g.
> > > is it
> > > sufficient to perform this matching only on the first xfrm or do
> > > they all
> > > need to be walked as in xfrm_bundle_ok()?  Also, should we
> > > perform
> > > this
> > > matching before (as in this patch) or after calling
> > > xfrm_bundle_ok()? Also,
> > > do we need to test xfrm->sel.family before calling
> > > xfrm_selector_match
> > > (as in this patch) or not - xfrm_state_look_at() does so when the
> > > state is XFRM_STATE_VALID but not when it is _ERROR or _EXPIRED?
> > 
> > No idea.
> > 
> > I looked at the old flow cache but i don't see any of these extra
> > checks there either.
> > 
> > However, old flow cache stored flowi struct as key, and that
> > contains
> > a
> > flowi_secid,  populated by the decode_session hooks.
> > 
> > Was it enough to check for identical flowi_secid in the flowi
> > structs
> > to
> > avoid this problem or am i missing something?
> 
> I'm not sure, but security_xfrm_state_pol_flow_match() ->
> selinux_xfrm_state_pol_flow_match() does more than just compare flow
> secids.
> 
> Also, there is the separate issue of the missing
> xfrm_selector_match()
> call, which can also cause the wrong SA to be used independent of
> anything LSM/SELinux-related.
> 
> It is a regression; the correct SA was being used prior to the xdst
> pcpu cache commit.  Reproducible using the selinux-testsuite, most
> easily run on a Fedora VM,
> git clone https://github.com/SELinuxProject/selinux-testsuite/
> sudo dnf install perl-Test perl-Test-Harness perl-Test-Simple
> selinux-policy-devel gcc libselinux-devel net-tools netlabel_tools
> iptables

Actually, you should just run 'sudo make test' instead of the
individual commands below.  I was breaking out the individual commands
to avoid running the rest of the testsuite unrelated to networking, but
 that won't pick up all of the dependencies the first time.  Sorry.

> sudo make -C policy load
> cd tests/inet_socket
> while sudo ./test; do : ; done

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Florian Westphal Oct. 31, 2017, 2:15 p.m. UTC | #4
Stephen Smalley <sds@tycho.nsa.gov> wrote:
> It is a regression; the correct SA was being used prior to the xdst
> pcpu cache commit.

I don't doubt that at all.  I would like to understand why the flow
cache did not have this problem.

> easily run on a Fedora VM,
> git clone https://github.com/SELinuxProject/selinux-testsuite/
> sudo dnf install perl-Test perl-Test-Harness perl-Test-Simple selinux-policy-devel gcc libselinux-devel net-tools netlabel_tools iptables
> sudo make -C policy load
> cd tests/inet_socket
> while sudo ./test; do : ; done

Thanks, I'll have a look.
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Paul Moore Oct. 31, 2017, 8:39 p.m. UTC | #5
On Mon, Oct 30, 2017 at 10:58 AM, Stephen Smalley <sds@tycho.nsa.gov> wrote:
> Since 4.14-rc1, the selinux-testsuite has been encountering sporadic
> failures during testing of labeled IPSEC. git bisect pointed to
> commit ec30d78c14a813db39a647b6a348b4286 ("xfrm: add xdst pcpu cache").
> The xdst pcpu cache is only checking that the policies are the same,
> but does not validate that the policy, state, and flow match with respect
> to security context labeling.  As a result, the wrong SA could be used
> and the receiver could end up performing permission checking and
> providing SO_PEERSEC or SCM_SECURITY values for the wrong security context.
> security_xfrm_state_pol_flow_match() exists for this purpose and is
> already called from xfrm_state_look_at() for matching purposes.
> Further, xfrm_state_look_at() also performs a xfrm_selector_match() test,
> which is also missing from the xdst pcpu cache logic.  Add calls to both
> of these functions when validating the cache entry.  With these changes,
> the selinux-testsuite passes all tests again.
>
> Fixes: ec30d78c14a813db39a647b6a348b4286ba4abf5 ("xfrm: add xdst pcpu cache")
> Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>

Thanks for chasing this down while I was on vacation :)

> This is an RFC because I am not entirely confident in the fix, e.g. is it
> sufficient to perform this matching only on the first xfrm or do they all
> need to be walked as in xfrm_bundle_ok()?

If you look at how we handle outgoing labeled IPsec traffic, e.g.
selinux_xfrm_skb_sid_egress(), you'll see that we only check the first
xfrm because I don't believe it is ever possible for us to create a
xfrm bundle with mis-matching SELinux labels.

Inbound traffic is another story, we need to check the entire bundle.

> ... Also, should we perform this
> matching before (as in this patch) or after calling xfrm_bundle_ok()?

I would probably make the LSM call the last check, as you've done; but
I have to say that is just so it is consistent with the "LSM last"
philosophy and not because of any performance related argument.

> ... Also,
> do we need to test xfrm->sel.family before calling xfrm_selector_match
> (as in this patch) or not - xfrm_state_look_at() does so when the
> state is XFRM_STATE_VALID but not when it is _ERROR or _EXPIRED?

Speaking purely from a SELinux perspective, I'm not sure it matters:
as long as the labels match we are happy.  However, from a general
IPsec perspective it does seem like a reasonable thing.

Granted I'm probably missing something, but it seems a little odd that
the code isn't already checking that the selectors match (... what am
I missing?).  It does check the policies, maybe that is enough in the
normal IPsec case?

> diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
> index 2746b62..171818b 100644
> --- a/net/xfrm/xfrm_policy.c
> +++ b/net/xfrm/xfrm_policy.c
> @@ -1820,6 +1820,11 @@ xfrm_resolve_and_create_bundle(struct xfrm_policy **pols, int num_pols,
>             !xfrm_pol_dead(xdst) &&
>             memcmp(xdst->pols, pols,
>                    sizeof(struct xfrm_policy *) * num_pols) == 0 &&
> +           (!xdst->u.dst.xfrm->sel.family ||
> +            xfrm_selector_match(&xdst->u.dst.xfrm->sel, fl,
> +                                xdst->u.dst.xfrm->sel.family)) &&
> +           security_xfrm_state_pol_flow_match(xdst->u.dst.xfrm,
> +                                              xdst->pols[0], fl) &&
>             xfrm_bundle_ok(xdst)) {
>                 dst_hold(&xdst->u.dst);
>                 return xdst;
> --
> 2.9.5
>

Patch
diff mbox

diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
index 2746b62..171818b 100644
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -1820,6 +1820,11 @@  xfrm_resolve_and_create_bundle(struct xfrm_policy **pols, int num_pols,
 	    !xfrm_pol_dead(xdst) &&
 	    memcmp(xdst->pols, pols,
 		   sizeof(struct xfrm_policy *) * num_pols) == 0 &&
+	    (!xdst->u.dst.xfrm->sel.family ||
+	     xfrm_selector_match(&xdst->u.dst.xfrm->sel, fl,
+				 xdst->u.dst.xfrm->sel.family)) &&
+	    security_xfrm_state_pol_flow_match(xdst->u.dst.xfrm,
+					       xdst->pols[0], fl) &&
 	    xfrm_bundle_ok(xdst)) {
 		dst_hold(&xdst->u.dst);
 		return xdst;