Re: [Bugme-new] [Bug 16517] New: rp_filter fails to filter with CONFIG_IP_ROUTE_MULTIPATH and more than one 0/0 nexthop dev

Re: [Bugme-new] [Bug 16517] New: rp_filter fails to filter with CONFIG_IP_ROUTE_MULTIPATH and more than one 0/0 nexthop dev

[Andrew Morton]

(switched to email.  Please respond via emailed reply-to-all, not via the
bugzilla web interface).

On Thu, 5 Aug 2010 01:48:01 GMT
bugzilla-daemon@bugzilla.kernel.org wrote:

> https://bugzilla.kernel.org/show_bug.cgi?id=16517
> 
>            Summary: rp_filter fails to filter with
>                     CONFIG_IP_ROUTE_MULTIPATH and more than one 0/0
>                     nexthop dev
>            Product: Networking
>            Version: 2.5
>     Kernel Version: at least 2.6.18 and newer
>           Platform: All
>         OS/Version: Linux
>               Tree: Mainline
>             Status: NEW
>           Severity: high
>           Priority: P1
>          Component: IPV4
>         AssignedTo: shemminger@linux-foundation.org
>         ReportedBy: for.poige+bugzilla.kernel.org@gmail.com
>         Regression: No
> 
> 
> I think the problem is net/ipv4/fib_frontend.c fib_validate_source()
> 
> ...
> #ifdef CONFIG_IP_ROUTE_MULTIPATH
>     if (FIB_RES_DEV(res) == dev || res.fi->fib_nhs > 1)
> #else
>     if (FIB_RES_DEV(res) == dev)
> #endif
> ...
> 
> I'm not sure, but this code is quite trivial and self-speaking. In case we have
> several default routes, we'd better iterate over each of them and compare
> resulting devices with the input one. So, fix is also trivial, specially for
> network kernel developers. ;-)
> 
> I've set priority "High" cause it doesn't affects usual Linux users directly,
> but indirectly it can affect any host on Internet, so the problem is
> significant, of course.
> 
> P. S. Kernel docs say: {
> rp_filter - INTEGER
>         0 - No source validation.
>         1 - Strict mode as defined in RFC3704 Strict Reverse Path
>             Each incoming packet is tested against the FIB and if the interface
>             is not the best reverse path the packet check will fail.
>             By default failed packets are discarded.
>         2 - Loose mode as defined in RFC3704 Loose Reverse Path
>             Each incoming packet's source address is also tested against the
> FIB
>             and if the source address is not reachable via any interface
>             the packet check will fail.
> 
>         Current recommended practice in RFC3704 is to enable strict mode
>         to prevent IP spoofing from DDos attacks. If using asymmetric routing
>         or other complicated routing, then loose mode is recommended.
> 
>         conf/all/rp_filter must also be set to non-zero to do source validation
>         on the interface
> 
>         Default value is 0. Note that some distributions enable it
>         in startup scripts.
> }, but is in reality level "2" of rp_filtering implemented?
> 
> P. P. S. netfilter would be the best place to have Reverse Path checks. But
> that's another story.
> 

Re: [Bugme-new] [Bug 16517] New: rp_filter fails to filter with CONFIG_IP_ROUTE_MULTIPATH and more than one 0/0 nexthop dev

[David Miller]

From: Andrew Morton <akpm@linux-foundation.org>
Date: Thu, 5 Aug 2010 13:46:53 -0700

>> I think the problem is net/ipv4/fib_frontend.c fib_validate_source()
>> 
>> ...
>> #ifdef CONFIG_IP_ROUTE_MULTIPATH
>>     if (FIB_RES_DEV(res) == dev || res.fi->fib_nhs > 1)
>> #else
>>     if (FIB_RES_DEV(res) == dev)
>> #endif
>> ...
>> 
>> I'm not sure, but this code is quite trivial and self-speaking. In case we have
>> several default routes, we'd better iterate over each of them and compare
>> resulting devices with the input one. So, fix is also trivial, specially for
>> network kernel developers. ;-)

Please test this patch:

ipv4: Fix reverse path filtering with multipath routing.

Actually iterate over the next-hops to make sure we have
a device match.  Otherwise RP filtering is always elided
when the route matched has multiple next-hops.

Reported-by: Igor M Podlesny <for.poige@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/fib_frontend.c b/net/ipv4/fib_frontend.c
index a439689..7d02a9f 100644
--- a/net/ipv4/fib_frontend.c
+++ b/net/ipv4/fib_frontend.c
@@ -246,6 +246,7 @@ int fib_validate_source(__be32 src, __be32 dst, u8 tos, int oif,
 
 	struct fib_result res;
 	int no_addr, rpf, accept_local;
+	bool dev_match;
 	int ret;
 	struct net *net;
 
@@ -273,12 +274,22 @@ int fib_validate_source(__be32 src, __be32 dst, u8 tos, int oif,
 	}
 	*spec_dst = FIB_RES_PREFSRC(res);
 	fib_combine_itag(itag, &res);
+	dev_match = false;
+
 #ifdef CONFIG_IP_ROUTE_MULTIPATH
-	if (FIB_RES_DEV(res) == dev || res.fi->fib_nhs > 1)
+	for (ret = 0; ret < res.fi->fib_nhs; ret++) {
+		struct fib_nh *nh = &res.fi->fib_nh[ret];
+
+		if (nh->nh_dev == dev) {
+			dev_match = true;
+			break;
+		}
+	}
 #else
 	if (FIB_RES_DEV(res) == dev)
+		dev_match = true;
 #endif
-	{
+	if (dev_match) {
 		ret = FIB_RES_NH(res).nh_scope >= RT_SCOPE_HOST;
 		fib_res_put(&res);
 		return ret;