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<base href="https://bugzilla.netfilter.org/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - vfork() in xtables.c can corrupt stack"
href="https://bugzilla.netfilter.org/show_bug.cgi?id=982">982</a>
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<th>Summary</th>
<td>vfork() in xtables.c can corrupt stack
</td>
</tr>
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<th>Product</th>
<td>iptables
</td>
</tr>
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<th>Version</th>
<td>1.4.x
</td>
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<th>Hardware</th>
<td>x86_64
</td>
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<th>OS</th>
<td>other
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P5
</td>
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<th>Component</th>
<td>iptables-restore
</td>
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<th>Assignee</th>
<td>netfilter-buglog@lists.netfilter.org
</td>
</tr>
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<th>Reporter</th>
<td>dan.wilder@watchguard.com
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=451" name="attach_451" title="Move some code to avoid cautions in vfork man page">attachment 451</a> <a href="attachment.cgi?id=451&action=edit" title="Move some code to avoid cautions in vfork man page">[details]</a></span>
Move some code to avoid cautions in vfork man page
Running iptables-restore on an embedded platform containing no modprobe
program, the following lines in xtables.c lead to corrupted stack frame:
357 switch (vfork()) {
358 case 0:
359 argv[0] = (char *)modprobe;
360 argv[1] = (char *)modname;
361 if (quiet) {
362 argv[2] = "-q";
363 argv[3] = NULL;
364 } else {
365 argv[2] = NULL;
366 argv[3] = NULL;
367 }
368 execv(argv[0], argv);
369
370 /* not usually reached */
371 exit(1);
modprobe pointed to a non-existant program /sbin/modprobe, so execv()
always failed. Not a problem in itself on our platform, as the kernel
modules are pre-loaded before iptables-restore is run, but it took a
bit of headscratching to track this down, as a stack frame was
corrupted, leading to failures quite a while after the function
containing this code had returned!
Relevant caution in man 2 vfork:
"The vfork() function has the same effect as fork(2), except that
the behavior is undefined if the process created by vfork() either
modifies any data ... or calls any other function before
successfully calling _exit(2) or one of the exec(3) family of
functions."
Apparently this has not been a problem for us in earlier versions of
glibc, maybe because vfork was more like fork, maybe because the
stack corruption was innocuous. Ours is a corner case anyway, as
it might not have been a problem had modprobe existed or had
modprobe been a symlink to /bin/true. But it seems odd to disregard
man page cautions, and our problem goes away if they are heeded.
Our solution was move the argv setting up a few lines, just before the
vfork, and replace exit() with _exit(). The code reviewer suggested
contributing the patch upstream, so please find attached, for
whatever it may be worth.
--
Dan Wilder</pre>
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