Date: Wed, 21 Aug 2013 17:23:08 +0200 From: Alessandro Cresto Miseroglio <alex179ohm@...il.com> To: Stephen Röttger <stephen.roettger@...il.com> Cc: oss-security@...ts.openwall.com, gcc@....gnu.org Subject: Re: PoC: Function Pointer Protection in C Programs PDF in English? (http://zero-entropy.de/fpp.pdf is in Deutsch) On 21 August 2013 16:43, Stephen Röttger <stephen.roettger@...il.com> wrote: > Hi everyone, > > I'd like to present you my master's thesis "Malicious Code Execution > Prevention through Function Pointer Protection"  and its > proof-of-concept implementation  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 . > 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 ) > > 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 > >  http://zero-entropy.de/fpp.pdf >  git://zero-entropy.de/fpp_autobuild.git >  http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2013-0170 >
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