CMPSC 473 – Fall 2019 – Project 2: Slab Allocation and Defenses
Data structure | 代写project | assignment | 代写C | 代写lab | OS代写 – 这是利用Data structure进行训练的代写, 对Data structure的流程进行训练解析, 涉及了Data structure/IT等代写方面, 这个项目是lab代写的代写题目
- Out:September 23, 2019
- Due:October 11, 2019
In this project, you will write dynamic storage allocator for C programs, i.e., your own version of the mallocandfree. You will implement a particular type of allocator called as lab allocatordescribed below and setup the ability to check pointers for memory you allocate to prevent some attacks: (1) buffer overflow; (2) use-after-free; and (3) type confusion. The only file you need (or should) modify iscmpsc473-mm.c. For the allocator, you will implement the following functions (see functions for defenses in Section 6).
- int mminit( void );
- void *mymalloc( unsigned int size );
- void myfree( void *buf );
You are encouraged to define other helper functions to modularize your code.
3 Slab Allocation
Slab allocation is briefly described in the Three Easy Pieces book in Section 17.4. The idea is that the heap for a slab allocator is designed to allocate objects of predetermined sizes, e.g., for commonly used data structures like tasks (processes) and files. A slab allocator has a particular structure. Please use the data structures forheapt,slabcachet, slabt, andbitmaptdefined incmpsc473-mm.h. Some b IT operations are provided. SeeWORDOFFSET andBITOFFSETincmpsc473-mm.handsetbit,clearbit, andgetbitincmpsc473-mm.c). The relationship among these data structures is shown in Figure 1. A single heap of typeheaptis the root of the slab allocator structure. A heap consists of a contiguous sequence of pages that may be used for allocating data for each structure from the fieldvoid *startfor sizeunsigned int size. Your slab allocator manages heap memory in slabs, which are the same as pages (i.e., same size as a memory page). See Figure 2 to see an example of how pages may appear in the heap memory. Use the heapsbitmap field to track which pages are free and which are allocated. There are three slabs caches of typeslabcachet(seecmpsc473-mm.h) in your slab allocators heap: one for each of the structs A, B, and C. Please use yourcmpsc473-format-X.hfile from project 1 (where X
Figure 1: Slab allocator data structures and relationships
Slab Page free
Slab Page A
Slab Page B
Slab Page free
Slab Page free
Slab Page A
Slab Page C
Figure 2: Layout of slab pages in the heap
is your format file number) to define the object format for the objects that you will be allocating memory for from your slab allocator. The three slab caches each store a circular, doubly-linked list of slabs from a single reference to one slab atslabt *current. The slab cache tracks slabs by the count of slabs allocated (unsigned int ct) and the size of the data structures (for struct A, B, anc C from yourcmpsc473-format-X.hfile). The slab cache also stores stwo function pointers to cache-specific functions for allocating metadata (for canary and free count, see Section 6) and for checking canaries (see Section 6). The slabs of are typeslabt. The slabs store a state:SLABEMPTY,SLABPARTIAL, andSLABFULL defined incmpsc473-mm.h. Each slab has a start address (void *start), which is always page-aligned since we are allocating slabs in pages. There arenextandprevpointers to slabs in the circular, doubly-linked list of slabs (see Figure 1. The format of an individual slab page is shown in Figure 3. Note that the slab Data structure is always at the end of the slab page. From the start of the slab page, a sequence of objects (calledallocXtof type struct X) of sizenumobjsare available for allocation. You must align objects on 16-byte boundaries, as we will use this restriction to enable implementing a defense below. A count (unsigned int ct) of objects have been allocated from the slab at any time. Please track this count. There are two fields that describe the objects size in each slab. Onerealsizestores the size of the structure being stored (i.e.,sizeoffor struct A, B, or C). The otherobjsizestores the size of the slab object. Each slab object includes the structure data and two metadata fields for the free count (ct) and canary (canary) discussed in Section 6 and must be 16-byte aligned as described above.
4 Requirements for Allocator Functions
Implement the following functions. Note that you can get these functions below running without adding or checking the defenses below. Thus, the project can be programmed incrementally once you have basic memory allocation working.
Figure 3: Layout of an individual slab page in memory
- int mminit( void ): Before callingmymallocormyfreeyou need to initialize the heap to handle these requests. Please allocate 1M for slab pages (using the standard memory allocation functions). Please make sure that the heaps slab pages are all page-aligned. You should also initialize the canary value for the process at this time as described in Section 6.
- void *mymalloc( unsigned int size ): Themymallocroutine returns a pointer to an allocated block payload of at least size bytes. The entire allocated block should lie within the heap region and should not overlap with any other allocated chunk. You should allocate an object from a slab page to fulfill this request. If you have no free objects in all your slab pages for a cache, you must allocate a new slab page for the cache.
- void myfree( void *buf ): Themyfreeroutine frees the block pointed to bybuf. It returns nothing. This routine is only guaranteed to work when the passed pointer (buf) was returned by an earlier call tomymalloc. You should make the object whose start address corresponds to this bufpointer free. Other addresses should fail to deallocate the object. If you free all the objects from a slab page, you must free the slab page for use by another cache, unless that page is the only slab page in the cache.
5 Attacks on Heap Data
In recent years, heaps have become the main target for attacks in processes. This is partly because defenses have been added to prevent attacks on stack memory, and partly because standard heaps typically store both the application data and the heap metadata on the heap. Attackers can use memory errors (e.g., overflows) to overwrite the heap metadata enabling attacks. In our case, only theslabtmetadata is stored in the slab pages of a slab allocator. We will examine three types of attacks:
- Overflows: We want to prevent a write to an object field from writing another objects data. Such an attack may compromise the integrity of another object or theslabtmetadata.
- Type Confusion: We want to prevent an adversary from being able to use a pointer to memory of one type (say,struct B) to reference memory of another type (say,struct A). A program vulnerability may allow an adversary to control a pointer value, and the adversary could leverage this vulnerability to reference an object of another type to change its field values using field operations on the first type.
- Use-After-Free: In this attack, an adversary obtains a pointer to memory that can be used in a state- ment after the memory is freed. If the memory is reallocated, the adversary could reference this newly allocated memory – perhaps of a different data type if the entire slab has been assigned to another cache.
6 Requirements for Preventing Attacks
You will develop modest defenses for each of these three attack types. Each of these check functions returns an integer value. You must return 0 on success and any other number on failure (typically -1 is used though).
Acanaryis an unpredictable (i.e., pseudorandom) value that is used to detect overflows. In this project, eachallocXtincludes a canary field after the object data. This is to detect an overflow on a write to an object field that writes beyond the end of the object. You need to implement the following two functions to apply the canary as a defense.
- void canaryinit( void ): This function should be run before anymymallocormyfree call to initialize the pseudorandom canary value. The canary value must differ on each run of the pro- cess annd be pseudorandom. The canary value must be set for each allocated object.
- int checkcanary( void *addr ): You need to define a function to check the canary value of a object reference (addr) and assign that function to the appropriateslabcache. You may define one function for all types or one function each. We will invoke this function after anywrite operation.
6.2 Check Pointer to Object Size
To prevent type confusion attacks, you must implement the functionint checktype( void *addr, char type )to check that the pointeraddris an appropriate reference for an object of typetype, which is a one character value of either A, B, or C for the corresponding structs. We will test by asking for pointers of the wrong data type for particular object locations. You must determine whether it is acceptable for an object reference to be cast to the requested type. Since there may be many pointers and references, you want to do this efficiently.
6.3 Free Count Tracking
A free count tracks the version of the allocation to prevent use-after-free attacks. Each object of type allocXtis associated with a field for the free count (ct). In addition,mymallocmust encode the free count of the pointer returned in the 4 low-order bits of the pointer returned – remember all allocated objects are 16-byte aligned, so we dont need those bits for addressing..
You must implement the functionint checkcount( void *addr )to check that the free count of the object referenced (ataddr) must comply with the free count of the pointer. Note that the free count of the pointer can be no larger than 15 (since you only have 4 bits), so you must figure out how to use the reference count stored in the object to do the check effectively.
7 Doing the Project
Perform the following tasks to complete the project.
7.1 Download Tarball
The tarball for Project 2 is available from Canvas fromhttp://www.cse.psu.edu/ trj1/cmpsc473-19f/ p2.tgz. This tarball includes my format filecmpsc473-format-1.h. You will use your format file from project 1 instead.
7.2 Project Tasks
You will then perform the following steps.
- Task 0: As in Project 1, please change the SNUM value in the Makefile to your format file number from Project 1.
- Task 1: As in Project 1, please change the header file references tocmpsc473-format.1.hto reference your format file.
- Task 2: Develop the three slab allocator functions:mminit,mymalloc, andmyfree. You may develop subroutines for these functions as you see appropriate.
- Task 3: Develop the functions for the three types of defenses for the slab allocator: canaries, type checks, and free counts. You can also using themallocfnandfreefnfunction pointers to define cache-specific (object- specific) functionality for malloc and free to enable defenses, and use thecanaryfnfor cache- specific (object-specific) checking of the canary values. If you dont need them, you are not required to use them.
We will test your submission on machines in the Linux lab in W204 Westgate. The machines are named cse-p204instXX.cse.psu.edu, where XX is a number from 01 to at least 40. We will test your program by supplying input files containing command sequences consisting of com- mands for memory allocation/deallocation and pointer use, which may or may not indicate an attack. All the commands that we will execute are provided by theprocesscmdsfunction in thecmpsc473-p2.cfile. DO NOT EDIT THIS FILE. The memory allocation deallocations are:
- mallocA/B/C id-start id-end: Allocate objects of either type (struct) A, B, or C from indexid-start toid-end. We will refer to these objects subsequently by ID to use the associated pointer values in other commands. We will passmymallocrequests for allocating memory for the associated type for each ID.
- freeid-start id-end: Free all objects of IDs in the range betweenid-startandid-end. We will pass myfreerequests to free the objects associated with those IDs allocated usingmymalloc.
If you can support execution of these two commands as described above, you will complete the memory allocation portion of the project. For performing operations using the allocated references, the following commands will be provided.
- writeid bytes: Thewritecommand writesbytesnumber of bytes to the pointer associated with ID id. Write commands may or may not write beyond the end of the available object memory, so we run checkcanaryto detect whether the canarys integrity is preserved.
- saveA/B/C id: Thesavecommand stores a pointer of type either (struct) A, B, or C for later use for the pointer associated with IDid. Such an operation may try to save a pointer allocated for one type to a pointer allocated to another type, so we runchecktypeto prevent this kind of type confusion.
- use(saved reference): Theuseoperation will perform a one-byte read from the pointer stored in the save operation. If the memory pointed to by the pointer has been freed since the save, we want to prevent such an operation, so we runcheckcount.
You should SSH into those machines to verify that your code works. We developed and tested the project code on those machines, so should work fine, but it is up to you to make sure. You will need to speak to the CSE IT folks if you do not have access to those machines.
Please answer the following questions regarding slab memory allocation and defenses.
- Does slab allocation create either external or internal fragmentation?
- How much memory is not available for allocation given the slab allocator implementation for your objects of struct A per page?
- Describe an overflow attack that would work even given our implementation of canaries.
- How did you implementchecktypeto prevent type confusion attacks?
Please submit the following:
- A tarball created using make tar from the directoryp2-assign, creating a filep2-assign-X.tgz, where X is your format number as in Project 1.
- A file in PDF format providing answers to the questions above.
The assignment is worth 60 points total broken down as follows.
- We can build and run what you have submitted without incident (9 points).
- Slab allocator tests complete correctly (24 points)
- Slab allocator performance and memory utilization are within bounds (12 points). Performance must be within 90% of course standard and memory utilization must not incur more than 25% excess memory in slab pages used by caches.
- Slab allocator defenses complete correctly (15 points)