Null Pointer Dereferenced Error With Approveit
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Rule 02. Expressions (EXP) Skip to end of banner JIRA links Go to start of banner EXP01-J. Do not use a null in a case where an object is http://dazzlen.ga/practice/11788-null-pointer-dereferenced-approve-it-army-download.html required Skip to end of metadata Created by Dhruv Mohindra, last modified by David Svoboda on Oct 05, 2016 Go to start of metadata Do not use the null value in any instance where an object is required, including the following cases:Calling the instance method of a null objectAccessing or modifying the field of a null objectTaking the length https://www.securecoding.cert.org/confluence/display/java/EXP01-J.+Do+not+use+a+null+in+a+case+where+an+object+is+required of null as if it were an arrayAccessing or modifying the elements of null as if it were an arrayThrowing null as if it were a Throwable valueUsing a null in cases where an object is required results in a NullPointerException being thrown, which interrupts execution of the program or thread. Code conforming to this coding standard will consequently terminate because ERR08-J. Do not catch NullPointerException or any of its ancestors requires that NullPointerException is not caught. Noncompliant Code ExampleThis noncompliant example shows a bug in Tomcat version 4.1.24, initially discovered by Reasoning [Reasoning 2003]. The cardinality() method was designed to return the number of occurrences of object obj in collection col. One valid use of the cardinality() method is to determine how many objects in the collection are null. However, because membership in the collection is checked using the expression obj.equals(elt), a null pointer dereference is guaranteed whenever obj is null and elt is not null. Compliant SolutionThis compliant solution eliminates the null pointer dereference by adding an explicit check: Noncompliant Code Exa
posted Posted Jul 17, 2009 14:32 UTC (Fri) by trasz (guest, #45786) Parent article: Linux 2.6.30 exploit posted This says a lot about the code quality, btw. Not only this error would be easy to spot if someone actually looked at the code, but it would also be noticed by automated code analysis http://lwn.net/Articles/341812/ tools like Coverity, if someone actually bothered to use them. (Log in to post comments) Linux 2.6.30 https://lwn.net/Articles/341773/ exploit posted Posted Jul 17, 2009 14:40 UTC (Fri) by bluebirch (subscriber, #58264) [Link] From the ISC dairy: Why is it so fascinating? Because a source code audit of the vulnerable code would never find this vulnerability Linux 2.6.30 exploit posted Posted Jul 17, 2009 14:46 UTC (Fri) by trasz (guest, #45786) [Link] Which is false, IMHO - it is readily visible that you're checking smart card the pointer few lines below dereferencing it. Linux 2.6.30 exploit posted Posted Jul 17, 2009 14:53 UTC (Fri) by spender (subscriber, #23067) [Link] But it would be clear from the source that the bug is not exploitable for privilege escalation (it would just cause a crash when dereferencing TUN). Say if I had a page mmaped at 0 in that case and this gcc problem didn't exist: then there would be no crash, and the function would have returned with an error (due null pointer dereferenced to the !tun check). But because that check doesn't exist in the compiled code, not only does no crash occur, but I'm able to do what should be impossible from a review of the source: get arbitrary code execution in kernel context. It's all explained very well in the exploit code (80% of it is comments), if you would have read it. -Brad Linux 2.6.30 exploit posted Posted Jul 17, 2009 15:05 UTC (Fri) by trasz (guest, #45786) [Link] Ok, I agree. So, to sum up, it's a programming error that normally would just make the kernel crash, but due to an compiler optimization it makes the kernel exploitable. One thing I wonder if whether the optimization is actually invalid. What does the C standard say about program behaviour after dereferencing NULL pointer? Does it say anything? C's notion of null is not really relevant Posted Jul 17, 2009 15:45 UTC (Fri) by xoddam (subscriber, #2322) [Link] It's normal, and could make sense, to succeed in dereferencing a pointer containing zero if you have readable memory mapped at address zero. Some older platforms used 'page zero' memory as cache -- it was faster (or took less object code) to reach. So the C standard can say nothing about it. Where the C standard does mention null pointers is in saying that, when assigning (or comparing) the constant zero to a pointer, a specific value must be used that means 'null'. The semantics of this 'null'
2009 by corbet] From: spender-AT-grsecurity.net (Brad Spengler) To: full-disclosure-AT-lists.grok.org.uk Subject: Linux 2.6.30+/SELinux/RHEL5 test kernel 0day, exploiting the unexploitable Date: Thu, 16 Jul 2009 22:26:45 -0400 Message-ID: <20090717022645.GA26706@grsecurity.net> Archive-link: Article Title says it all, exploit is at: http://grsecurity.net/~spender/cheddar_bay.tgz Everything is described and explained in the exploit.c file. I exploit a bug that by looking at the source is unexploitable; I defeat the null ptr dereference protection in the kernel on both systems with SELinux and those without. I proceed to disable SELinux/AppArmor/LSM/auditing Exploit works on both 32bit and 64bit kernels. Links to videos of the exploit in action are present in the exploit code. Greets to vendor-sec, -Brad ----- End forwarded message ----- ----- End forwarded message ----- _______________________________________________ Full-Disclosure - We believe in it. Charter: http://lists.grok.org.uk/full-disclosure-charter.html Hosted and sponsored by Secunia - http://secunia.com/ (Log in to post comments) Linux 2.6.30 exploit posted Posted Jul 17, 2009 13:42 UTC (Fri) by trasz (guest, #45786) [Link] By looking at the source, this is an obvious coding error - kernel first dereferences a pointer, and after that checks whether it's NULL. Where is the compiler bug there? Linux 2.6.30 exploit posted Posted Jul 17, 2009 14:24 UTC (Fri) by regala (guest, #15745) [Link] no, you don't understand. this pointer should not be NULL, and having it NULL in the code executed after the check is the security matter. Thus, the exploit has first to cause this ptr to be NULL, and it has to rely on the fact that gcc wrongly moves this check away assuming it is not necessary, being done after dereferencing the pointer, which may be wrong in certain loads, or in SMP, preemptible configs. Linux 2.6.30 exploit posted Posted Jul 17, 2009 14:39 UTC (Fri) by trasz (guest, #45786) [Link] This code wouldn't be valid in SMP or in preemptible kernel either, unless some locking was added, which would completely change the situation and would probably prevent the problematic optimization from being applied. Other possible way