Subject: Re: Bug: tc filter show .... raise segfault if more than one rule with action -j MARK exist

From: Andreas Greve <[email-masked]>
Date: Sat, 10 May 2014 11:19:18 +0200

sorry its to early for me ;)

here the missing attachment.

On 05/10/14 11:11, Andreas Greve wrote:
> Dear Stephen Hemminger, Dear List Members,
>
> this is a corrected/optimized version of the patch
> Thanks to Stephen Hemminger, for giving me the hint to use structure 
> assignment instead of
> memcpy().
>
> Best wishes
>
> Andreas Greve
>
> On 05/08/14 13:48, Andreas Greve wrote:
>> Dear Stephen Hemminger, Dear List Members,
>>
>> the command
>>
>> tc filter show dev eth1 parent ffff:
>>
>> raise a segmentation fault if there are more than one filter rule 
>> with action -j MARK exists.
>>
>> Reason:
>> In print_ipt(...) xtables will be initialzed with a pointer to the 
>> static struct tcipt_globals at xtables_init_all(). Later on the 
>> fields .opts and .options_offset of tcipt_globals are modified. The 
>> call of xtables_free_opts(1) at the end of print_ipt(...) does not 
>> restore the original values of tcipt_globals for the modified fields. 
>> It only frees some allocated memory and sets .opts to NULL. This 
>> leads to a segmentation fault when print_ipt()
>> is called for the next filter rule with action -j MARK.
>>
>> Fix: (patch attached)
>> Cloneing tcipt_globals on the stack as tmp_tcipt_globals and use it 
>> instead of tcipt_globals, so tcipt_globals will be not modified.
>>
>> Tests:
>> I only have done limit tests for this fix ( action -j MARK ) on a 
>> debian wheezy iproute2 package build from source based on 
>> upstream/3.14.0. The system is a little bit special  kernel 
>> 3.12.17-xen on xen-4.4.1-pre.
>>
>>
>> Andreas Henriksson (debian iproute maintainer) give me an other idea 
>> of how to fix this bug by modifying  xtables_free_opts() so that it 
>> sets .opts = .orig_opts instead .opts = NULL. This should avoid 
>> having to duplicate the entire struct each time.
>>
>> I prefer the cloning  because:
>>
>> 1)  In my understanding the variable tcipt_globals is a complex 
>> "constant" that should never be modified. ( Is that right ?)
>>
>> 2) If we want to do the fix in xtables_free_opts we have to change 
>> the struct xtables_globals because we need for all fields .orig 
>> fields I think this will  make all things much more complicated.
>>
>>
>> additional Brainstorms:
>>
>> a ) Perhaps we have to think about of a deep clone of tcipt_globals 
>> to protect the underlaying option array
>> b ) makes cloning of  tcipt_globals sense in  parse_ipt(...)
>> But for this two decisions my understanding of the whole system is 
>> not enough.
>>
>>
>> I hope the Information helps
>>
>> Thanks
>>
>> Best wishes
>>
>>
>> Andreas Greve
>>
>>
>>
>>
>>
>>
>>
>>
>> Andreas Henriksson
>>
>>
>> Stephen Hemminger
>>
>

>>From fdf33181ade1320a6de79ac57d0f751b571f1dd8 Mon Sep 17 00:00:00 2001
From: Andreas Greve <[email-masked]>
Date: Sat, 10 May 2014 10:47:01 +0200
Subject: [PATCH] print_ipt: segfault if more then one filter with action -j
 MARK.

BUG: tc filter show ... produce a segmentation fault if more than one
filter rule with action -j MARK exists.

Reason: In print_ipt(...) xtables will be initialzed with a
pointer to the static struct tcipt_globals at xtables_init_all().
Later on the fields .opts and .options_offset of tcipt_globals are
modified. The call of xtables_free_opts(1) at the end of print(...)
does not restore the original values of tcipt_globals for the
modified fields. It only frees some allocated memory and sets
.opts to NULL. This leads to a segmentation fault when print_ipt()
is called for the next filter rule with action -j MARK.

Fix: Cloneing tcipt_globals on the stack as tmp_tcipt_globals and
use it instead of tcipt_globals, so tcipt_globals will be not
modified.

Signed-off-by: Andreas Greve <[email-masked]>
---
 tc/m_xt.c |   17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/tc/m_xt.c b/tc/m_xt.c
index 27029c1..274df1d 100644
--- a/tc/m_xt.c
+++ b/tc/m_xt.c
@@ -298,7 +298,14 @@ print_ipt(struct action_util *au,FILE * f, struct rtattr *arg)
 	if (arg == NULL)
 		return -1;
 
-	xtables_init_all(&tcipt_globals, NFPROTO_IPV4);
+	/* 
+	   clone tcipt_globals because .opts and .options_offset  will be modified later and not 
+           restored by iptables. If tcipt_globals is not cloned the modification tcipt_globals will 
+           cause a segmentation fault if more than one filter rule  with action -j  exists.
+        */
+	struct xtables_globals tmp_tcipt_globals = tcipt_globals;
+
+	xtables_init_all(&tmp_tcipt_globals, NFPROTO_IPV4);
 	set_lib_dir();
 
 	parse_rtattr_nested(tb, TCA_IPT_MAX, arg);
@@ -333,12 +340,12 @@ print_ipt(struct action_util *au,FILE * f, struct rtattr *arg)
 			}
 
 #if (XTABLES_VERSION_CODE >= 6)
-		opts = xtables_options_xfrm(tcipt_globals.orig_opts,
-					    tcipt_globals.opts,
+		opts = xtables_options_xfrm(tmp_tcipt_globals.orig_opts,
+					    tmp_tcipt_globals.opts,
 					    m->x6_options,
 					    &m->option_offset);
 #else
-		opts = xtables_merge_options(tcipt_globals.opts,
+		opts = xtables_merge_options(tmp_tcipt_globals.opts,
 					     m->extra_opts,
 					     &m->option_offset);
 #endif
@@ -346,7 +353,7 @@ print_ipt(struct action_util *au,FILE * f, struct rtattr *arg)
 		fprintf(stderr, " failed to find aditional options for target %s\n\n", optarg);
 		return -1;
 	} else
-		tcipt_globals.opts = opts;
+		tmp_tcipt_globals.opts = opts;
 		} else {
 			fprintf(stderr, " failed to find target %s\n\n",
 				t->u.user.name);
-- 
1.7.10.4