Patent Application
20060225135
The computer industry has suffered from numerous security vulnerabilities over the past several years, and the number of vulnerabilities continues to rise. More of these vulnerabilities have been turned into attacks which compromise the integrity of the machine under attack. The root cause of many of these attacks is programming errors made when creating or modifying code.
Several solutions have been attempted. Courses on writing secure code have been run by many groups. A managed run time environment (MRTE) is effective where used, but does not encompass the entire programming space. None of the efforts has managed to reverse the downward spiral of vulnerability, attack, and patch. Reducing the number of programming errors would result in reducing the number of security vulnerabilities and improve system integrity.
One common vulnerability is a buffer overflow attack. One example of such an attack occurs when malicious code overwrites a return address of a function that is stored in a stack. Upon returning from the function, a modified return address is pushed into the instruction pointer (extended instruction counter (EIP) register). This modified return address may cause execution of malicious code and/or a stack execution error. Such an attack is typically enabled by poor programming practices, such as unchecked buffer transfers. In contrast, a well-structured program maps memory into structured portions, including a text portion to include program code, a data segment to store initialized and uninitialized global data, and a portion shared by the stack and heap. The stack may be used to store function call-by arguments, local variables and values of selected registers, such as the EIP register. The heap holds dynamic variables. Poor programming practices may cause these segments to be overwritten.
There are other places where unchecked buffers can reside. An example of such a place is a buffer overflow to the heap memory. Heap memory is memory allocated from a common pool and used by a program to store variable and other run time data.
A need thus exists to provide for more protection from security vulnerabilities.
Embodiments of the present invention provide a mechanism to mark memory locations as non-accessible. If a program attempts to access a protected memory location, the mechanism may signal a memory access violation. That is, memory protection in accordance with an embodiment of the present invention may provide a fixed size breakpoint associated with a memory location. Locations may be protected by marking the location as inaccessible for certain types of operations. In such manner, more secure operation occurs.
In various embodiments, attribute bits are defined to provide protection of memory locations. These bits indicate whether access can be granted for read or write operations. These attributes may provide extended memory protection (EMP) by providing access control on a variable, which may be defined as 32 bits of memory in some embodiments. A program generated by an EMP-enabled compiler will set the attributes to provide the correct level of support. Legacy programs may operate seamlessly by not setting any of these attribute bits.
It is to be understood that the properties of memory protection may be architecture-specific. The choices may include, for example, the type of access protection provided: read-protection, write-protection, execute-protection, or some mixed selection of all three; the address range covered by each attribute bit; the ability to alias or overlap with other attributes; the operational address space: virtual or physical; and a protection-walk process to translate an address to the corresponding protection value.
In one embodiment, four states are defined based on read/write restrictions, each expressed by a bit. These states may correspond to a buffer usage model having four states, namely unallocated, uninitialized, initialized, and locked. This model can be mapped into the following states, respectively, no_read-no_write; no_read-write_ok; read_okay-write_ok; and read_ok-no_write. As an example, the no_read-no_write state may be defined by setting read and write protection bits. Thus an exception may be generated if a read or write occurs to a location having those protection bits set.
EMP information may be maintained in main memory and may correspond directly with each addressable memory element in the machine. That is, each addressable memory element may have a set of EMP attributes associated with it. The EMP information may be managed as attribute pages in memory. These pages are not accessible in the virtual address space of the program, but instead may be accessed by a special instruction. The attribute bits may be checked on every memory access. If the bits indicate a restricted access location, a memory access violation is generated.
In virtual memory paging systems, programs make memory references in a logical address space. The logical address is then transformed via segmentation into a linear or virtual address. Finally, a processor's paging mechanism translates the virtual address into a physical address corresponding to where the data is stored in physical memory. If a virtual address-based EMP is implemented and enabled, the processor makes an additional translation from the virtual address into an EMP restriction record stored in a separate EMP physical address space.
Referring now to
As shown in
However, prior to being able to access desired data 65, in embodiments in which EMP is enabled, it may first be determined whether the memory location of data 65 is permitted to be accessed. For example, a read or write operation may be prevented if EMP is enabled and access to the requested data 65 is restricted.
That is, the virtual address information from data access request 10 is provided to EMP translation mechanism 30, where the virtual address is translated into a physical address corresponding to an EMP or restriction record 75 (associated with data 65) stored in an EMP page 70 (associated with data page 60). The accessed EMP record 75 is provided to an EMP check mechanism 40, where it may be determined whether access to requested data 65 is permitted. For example, one or more bits of EMP record 75 may indicate that access to the corresponding requested data 65 is not permitted. If such access is not permitted, EMP check mechanism 40 may generate an EMP protection violation (EPV) on a signal line 45. Accordingly, access to data 65 is prevented.
Thus if EMP write protection is set for a given virtual address and there is a data write attempt to that address, the processor may raise an EMP protection violation (EPV) exception. An EMP read protection has the same effect on data read accesses. Memory reads due to code fetches may not be considered a data read access, in some embodiments. In various embodiments, an EMP exception handler may be used to recover from the protection violation in a safe manner.
In various embodiments, checking of EMP restrictions is privilege level agnostic. That is, if an EMP write protection is set for a particular memory location, all write attempts to that location will lead to an EPV exception regardless of the current program's privilege level. In these embodiments, all page restrictions must still be satisfied for the reference to succeed. EMP exceptions do not occur if the page level protections do not allow access at the current privilege level.
An EMP flag may be included within a processor control structure such as a control register (e.g., control register 4 (CR4)). In some embodiments, the default setting of the EMP flag is zero (cleared), meaning that there is no EMP restriction check for any data accesses. When the EMP restriction checking is disabled globally, there is no need for the processor to perform EMP address translations on the virtual address of every data access. To enable EMP restriction checking, the EMP flag may be set to 1.
The EMP physical address space is not visible to software programs in virtual address space. Accordingly, an operating system (OS) may prevent exposing EMP physical addresses through regular paging mechanisms. To modify EMP restriction attributes, a software program may use an instruction to change the memory attributes, such as moving memory restrictions (e.g., a MOVMR instruction), which may move the memory attribute (EMP restriction record) into the EMP memory in physical space.
The EMP protection violation (EPV) may have its own exception vector identifier (ID). The processor raises an EPV exception if a data access fails the EMP restriction check. In addition, an EPV exception may also be raised during a MOVMR instruction. An EPV may be considered an instruction execution fault. The resulting interrupt is precise and prevents the instruction from executing.
When the EPV exception is raised, it places an error code onto the current stack. The error code may include a number of error status bits. Each bit in the error code may be used to indicate a different cause of the EPV exception. When an EPV is raised, the source virtual address that initiated the EMP translation or restriction check is stored in a specified location, for example, within a control register (e.g., CR2).
In various embodiments, such an EMP mechanism may be used in a variety of ways to increase system integrity. While some embodiments may protect stack variables and parameters, as well as heap variables, it is to be understood that the scope of the present invention is not so limited.
In the following example, a byte-size discrete write-protection on a virtual address model is used. In such an embodiment, for each byte of addressable data (in a virtual address space), there is a write protect attribute bit associated therewith. If this bit is set, any attempt to write to this location may cause an architectural exception. For each virtual address page, the operating system (OS) may allocate a protection record, such as a vector of a corresponding size (e.g., 1 bit per byte) in a separate virtual memory location. In one embodiment, each EMP restriction record describes the data access restrictions for 32 bits (4 bytes) of virtual addresses. Multiple EMP restriction records may be used if a data access covers multiple 4-byte data chunks. The location of this vector in physical memory may be provided through a protection-walk mechanism. In one embodiment, this mechanism may be similar to a page walk mechanism, with the last entry pointing to the vector base address, instead of the physical page. In other embodiments, different translation schemes, including a hash table with a software assist are also possible.
While described herein as byte-size write protection, in other embodiments read protection and other protection granularities may be implemented. Restriction records may also be referred to herein as vectors. A vector may be protected from modification by regular store operations. As an example, a special instruction may be used by user applications to modify a particular bit within a vector. This instruction may be referred to as a “set/clear protection” instruction. Given a virtual address, the special set or clear instruction can be used to modify the appropriate attribute bit without causing an exception.
The stack can be protected from buffer overflows and other dangerous overwrites by marking key memory locations such as the return address as inaccessible. As an example, a compiler may emit code upon entry to a routine to mark the return address as inaccessible. Should the routine attempt a write past the assigned variables and into the return address, then an error is detected using an EMP mechanism. The error may be reported as a memory access violation. As a result, the write does not occur, and an error handler may determine an appropriate manner of resolving the error, unlike buffer overflow detection schemes which only detect an overwrite after the buffer has already been corrupted.
The heap memory may be protected in two ways. Unallocated or deallocated memory may be marked as unallocated. Any reference to those locations will generate a fault. Further, the heap manager can mark key pointers and internal variables as inaccessible.
For example, referring now to
In a buffer overflow attack, malicious code overwrites the buffer from one allocated block to a next block and modifies the data therein. To avoid such buffer overflows, embodiments of the present invention may protect the block headers of each buffer block using protection records. For example, headers 105, 110, and 120 of
To minimize a performance penalty due to extra memory accesses for protection information, a protection cache and a protection translation lookaside buffer (PTLB) may be included within a memory pipeline. Referring now to
In the embodiment of
As further shown in
Protection cache 210 includes a data portion having cache-lines of protection vectors. While the length of each cache-line is implementation-specific, the length may be a multiple of the data cache-line size to simplify handling of split addresses. Instead of using the protection vector address as a lookup tag, the original virtual address of a store operation received in the pipeline may be used to access a tag array of protection cache 210. The protection vector cache-line address is stored as the address check data in the cache. With this structure, cache lookup may occur in parallel with DTLB and PTLB lookups, or as soon as the virtual address of the store request is available.
DTLB 220 is coupled to provide an address page to protection cache 210, along with a protection enable signal. Similarly, DTLB 220 provides the protection enable signal to PTLB 230.
The results from DTLB 220 and PTLB 230 may be used to qualify the cache lookup result by comparing the results with the address check data. In addition to sending addresses to cache 210, either DTLB 220 or PTLB 230 can disable the cache lookup result. In such cases, the cache result is discarded with no output driven therefrom. The lookup result may disabled on a DTLB or PTLB miss, page level protection lookup disable (DTLB/PTLB), and processor state protection lookup disable, for example.
If the virtual address corresponding to a request is present in PTLB 230, the physical address of the EMP record is provided to protection cache 210, along with a PTLB hit signal. Analogous to a TLB for a page-walk, PTLB 230 may be used to speed up the protection-walk process. Also, like a TLB for a page-walk, PTLB 230 may be implementation specific. For example, it may be a single or multi-level TLB, it may support one or more walk tree definitions, and may also utilize a hash table with software assists.
A PTLB lookup may begin with the virtual page address of an incoming store operation, which may be done in parallel with a DTLB lookup. The data output of PTLB 230 is the physical start of the protection record for the corresponding page. The PTLB will send this address and a hit (or enable) signal to protection cache 210. The PTLB output is only relevant on a DTLB hit when DTLB 220 has enabled protection for that page. Using the virtual address received along with the information from DTLB 220 and PTLB 230, protection cache 210 may determine whether the requested EMP record is present. Thus a single virtual address can be used to obtain translations for a physical address for the store request (i.e., information in memory) and a physical address of the EMP record corresponding to the store request.
If the cache lookup output is not discarded, and all the protection bits corresponding to the store request are cached within its structure, a cache hit occurs. Besides reporting the cache hit, protection cache 210 may perform a logical “OR” operation on all the protection bits covered by the store request. That is, in addition to caching physical address translations, protection cache 210 performs bit indexing using a byte mask indicative of store size information. If the combined protection is set, a write access violation may be generated (the “Write Protected” signal of
If instead a hit occurs and the requested data is not subject to protection, protection cache 210 outputs a permission signal (the “Write Permitted” signal of
If the cache lookup output is not discarded, but either the store address is not in the cache tag array, or the check address does not match the DTLB/PTLB output, then a cache miss occurs. As a result, protection cache 210 may request a fill operation by sending a miss signal to PTLB 230, along with the necessary address information about the store request. PTLB 230 may then initiate a memory read of the requested cache-line in the corresponding protection record 275 in memory 270. The store operation will then re-access the memory pipeline upon completion of the cache-line fill (occurring on line 280).
If a PTLB lookup fails and protection is enabled for the requested page, a PTLB miss occurs. If a miss occurs in PTLB 230 (i.e., no translation to the physical address of the requested EMP record 275 is present), a PTLB miss signal is generated and sent to a protection miss handler (PMH) 240. PMH 240 may operate similarly to a page miss handler to cause a protection page walk to obtain the physical address corresponding to the EMP record desired. PMH 240 performs a protection-walk with the original request address.
Thus as shown, PMH 240 initiates a page walk in which various page tables may be accessed to obtain the desired physical address and provide it to PTLB 230 for storage. In the embodiment shown in
Of course, the actual protection walk state machine depends on the architectural selection of the protection walk algorithm. Upon completion of the protection walk, PTLB 230 may automatically initiate a cache fill operation for the critical cache-line desired for the current store operation. As described above, the store operation may be retried after the cache fill operation completes. Accordingly, PMH 240 may provide an entry within a page table to PTLB 230. In turn this entry may be used, along with the byte offset provided by protection cache 210, to access the desired EMP record 275 within physical memory 270.
In some embodiments, PTLB entries may be replaced either through explicit instructions or capacity replacement. While the corresponding cache-line need not be flushed, because a PTLB hit is a prerequisite to a cache hit, such flushing may improve cache efficiency. While shown with the particular structure in the embodiment of
Referring now to
Next it may be determined the type of protection desired (e.g., read or write protection) and the location of the protected memory (block 320). For example, the location of a buffer header in the stack may be determined. Using this location information, a protection record corresponding to the memory location may be accessed (block 330). For example, this protection record may be stored in separate protection pages of system memory. Finally, a bit within the protection record may be set corresponding to the memory location (block 340). As an example, a protection record may be a vector having eight bits, each corresponding to a four byte segment of memory. Accordingly, the given bit corresponding to a four byte block including the buffer header may be set accordingly.
Referring now to
Next it may be determined whether there is a protection TLB miss (diamond 430). If so, a protection page miss occurs and a protection page walk mechanism may be performed (block 440). If the protection TLB hits or upon completion of the page walk, next it may be determined whether there is a protection cache hit (diamond 445). If not, a cache fill operation may be performed (block 450), and control returns to block 420. As discussed above, the cache fill may obtain the desired protection record from system memory and provide it to the protection cache.
If instead at diamond 445 there is a protection cache hit, the protection status of the store operation may be determined (block 460). For example, the protection cache or other structure may perform a logical operation to determine whether one or more bits corresponding to the store operation are set, indicating a protected status. Thus it may be determined whether the address corresponding to the store operation is protected (diamond 470). If not, the operation may be permitted (block 480). If instead the address is protected, a memory access violation may be signaled (block 490).
In some embodiments, EMP in accordance with an embodiment of the present invention may provide runtime checking in hardware as a way to lessen the disastrous results from programming errors and ensure a controlled termination of a program with an exposed error. In such manner, better debug detection and validation may increase detection and coverage of errors before code release.
An EMP mechanism may check a program for unexpected behavior. As an example, such checks may include checking for accesses past the end of a variable, to unallocated memory locations, and to un-initialized memory locations. The checks are not limited to a specific language or subroutine. In addition, an EMP mechanism can be used to maintain integrity of key building blocks such as the heap by protecting key data.
In such manner, by providing micro-architectural support, memory protection may be implemented with minimal impact to the critical path of the memory pipeline in a pipelined general-purpose processor. Accordingly, the performance overhead of such protection is much less than a software based overwrite detection mechanism.
In such manner, software integrity may be improved. As one usage scenario, improved buffer overflow detection in data heap and stack structures may be effected. In the context of the data heap, a heap manager may use the protection mechanism to protect the block header of every buffer allocated. If a buffer overflow attack is attempted from one allocated block to the next, the protection mechanism will detect it and prevent the unwanted modification. Accordingly, buffer overflow and other over-write attacks can be prevented from occurring, rather than merely detecting the attack after the fact.
With compiler support, software vulnerabilities can be mitigated using memory protection in accordance with an embodiment of the present invention. For example, the fine-grain nature of protection records can be used to protect other static structures, like a virtual function table pointer, from being modified as part of an execution path re-direction attack. In some embodiments, a hardware canary can also be placed around statically allocated buffers that may be the source of an overflow.
In addition, protection records can be viewed as the limitless implementation of debug breakpoints. Instead of being limited by few available debug registers, a software developer will have as many simultaneous debug breakpoints as desired. Accordingly, fine grain breakpoint detection may be implemented for the whole address space. This detection thus provides a hardware approach to solving certain software integrity issues. That is, detection of unexpected behavior in applications can be achieved via breakpoint coverage, enabling a new programming methodology.
Referring now to
As shown in
In turn, chipset 590 may be coupled to a first bus 516 via an interface 596. In one embodiment, first bus 516 may be a Peripheral Component Interconnect (PCI) bus, as defined by the PCI Local Bus Specification, Production Version, Revision 2.1, dated June 1995 or a bus such as the PCI Express bus or another third generation I/O interconnect bus, although the scope of the present invention is not so limited.
As shown in
Embodiments may be implemented in a computer program that may be stored on a storage medium having instructions to program a computer system to perform the embodiments. The storage medium may include, but is not limited to, any type of disk including floppy disks, optical disks, compact disk read-only memories (CD-ROMs), compact disk rewritables (CD-RWs), and magneto-optical disks, semiconductor devices such as read-only memories (ROMs), random access memories (RAMs) such as dynamic and static RAMs, erasable programmable read-only memories (EPROMs), electrically erasable programmable read-only memories (EEPROMs), flash memories, magnetic or optical cards, or any type of media suitable for storing electronic instructions. Other embodiments may be implemented as software modules executed by a programmable control device.
While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.
