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Date: Wed, 21 Aug 2013 17:23:08 +0200
From: Alessandro Cresto Miseroglio <>
To: Stephen Röttger <>
Subject: Re: PoC: Function Pointer Protection in C Programs

PDF in English?
( is in Deutsch)

On 21 August 2013 16:43, Stephen Röttger <> wrote:

> Hi everyone,
> I'd like to present you my master's thesis "Malicious Code Execution
> Prevention through Function Pointer Protection" [0] and its
> proof-of-concept implementation [1] for the gcc+glibc and would
> appreciate some feedback.
> In my thesis, I tried to find a way to prevent the exploitation of
> memory corruption vulnerabilities, in which an attacker is able to
> control the content of a function pointer variable and also bypass ASLR
> (e.g. by brute force or an information leak). The former is given for
> example in use-after-free scenarios comparable to CVE-2013-0170 [2].
> In the general case, the attacker can either control parameters to the
> function pointer as well and execute e.g. system() directly, or he will
> have to call a stack pivoting gadget to have the stack pointer point to
> attacker-controlled memory.
> Approach:
> The basic idea of the thesis is to record all addresses that are
> assigned to a function pointer variable at some place in the program (or
> in one of the shared libraries) and if a function pointer is called,
> verify that the address has been recorded previously. Thus, if an
> attacker overwrites the fp variable with either the address of system()
> or of a stack pivoting gadget, the fp call will fail, since these
> adresses have never been assigned to a function pointer in the program.
> The security of the approach relies on the assumption that no function
> that can be abused for malicious purposes is ever assigned to a function
> pointer, but this requirement will be weakened under future work.
> How this works:
> The compiler, GCC in my PoC, will register all assignments of function
> pointer variables in the source code and will create a global variable
> for the assigned function, which is initialized to the function's
> address. Then, it replaces the address of the function in the assignment
> with the address of the newly created variable:
>     fp f = &printf;
> becomes:
>   printf_var = &printf;
>     ...
>     fp f = &printf_var;
> Further, a global constructor is created that is run before the main
> function of the program or before the shared library is loaded. This
> constructor allocates a memory area where it stores the address of each
> fp address previously registered. The created global variable is then
> overwritten to point to the new memory area instead. Finally, the memory
> area is mapped read only. Also, the variable where the address of this
> area is stored has to be in read only memory as well to prevent
> malicious overwrites. Putting it all together, the memory layout looks
> like this:
>                                     <read only>
>  +-------+    +------------+    +------------------+    +----------+
>  | fp f  | -> | printf_var | -> | protected memory | -> | printf() |
>  +-------+    +------------+    +------------------+    +----------+
> Additional instructions are emitted by the compiler before function
> pointer calls. They will verify that the global variable (printf_var)
> points to the protected memory region, from which it extracts the real
> function pointer to be called. If an attacker is able to overwrite
> either the function pointer or the global variable, he will only be able
> to execute functions contained in the protected memory area (which he
> can't overwrite since it is mapped read only during normal execution).
> Implementation:
> The protection and verification code is moved to a shared library,
> libgcc at the moment.
> In order to work, the glibc and the runtime linker required some manual
> modifications, for example manual protection of code addresses, but most
> programs should not need any modifications at all. An exception might be
> JIT compilers or any code that wants to call an address that does not
> belong to a function.
> Performance:
> Though my PoC implementation is not free of bugs, I was able to compile
> an nginx webserver and have it serve static websites, which I used for a
> performance evaluation. On my test system, the number of requests per
> second that the nginx could was reduced to 96% compared to a nginx
> without the scheme. Handling of a single request included 71 function
> pointer calls in this case. (More details can be found in my thesis [0])
> Security:
> Unfortunately, I already found a way to bypass the scheme if the
> attacker controls the first parameter passed to the call of the
> overwritten function pointer, but I will present future work that will
> prevent this bypass.
> The problem is that the glibc uses internal dlopen and dlsym functions
> as function pointers. As a consequence, an attacker will be able to
> abuse these functions to execute arbitrary code. (dlopen by providing a
> shared library with a constructor or by using dlsym to acquire an
> arbitrary callable function pointer. section 6.2.2 in my thesis)
> Future work:
> Through the protected memory area, the possibility exists to store
> additional meta information next to the function pointer. This can be
> used to a) store the type of the function pointer and only allow calls
> using compatible types and b) assign groups to function pointers and
> prohibid calls if the groups do not match.
> The first approach will further narrow down the possibilities of an
> attacker. If he can overwrite a single function pointer variable, he
> will only be able to call functions with a matching signature.
> The second approach, requires an extension to the C language that is not
> standard conformant (e.g. using the gcc __attribute__ syntax) as well as
> manual annotation in the source code. But it could be used for example,
> to assign a group to the internally used dynamic linking routines and
> prevent that they're abused by an attacker.
> Feedback and criticism is very welcome, also if anything is unclear feel
> free to ask.
> Regards,
> Stephen
> [0]
> [1] git://
> [2]

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