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

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Date:	Thu, 08 May 2014 13:48:19 +0200
From:	Andreas Greve <andreas.greve@...reve.de>
To:	stephen@...workplumber.org
CC:	andreas.greve@...reve.de, netdev@...r.kernel.org
Subject: Bug: tc filter show .... raise segfault if more than one rule with
 action -j MARK exist

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

View attachment "0001-print_ipt-segfault-if-more-then-one-filter-with-acti.patch" of type "text/x-patch" (2853 bytes)