Patent Application: US-201414341118-A

Abstract:
a method for hardening the stack - smashing protector technique which prevents information leaking of the protecting guard is disclosed . the reference stack guard secret value is renewed at one or more selected time points during the execution of the application . the technique is non - intrusive and has a negligible run - time cost . the technique reuses the ssp infrastructure , and does not need to recompile the code or modify the binary image of the application . the method prevents any kind of brute force attacks against the ssp technique and most memory leaks affecting the canary guard .

Description:
in order to easily understand the operation of the disclosed invention , the following general observations from previous art shall be considered : most applications , specially networking servers , after a fork operation , the child process executes a different flow of code , which ends with an explicit call to the exit system call . that is , the child process does not return from the function that started the child code . each child process of a network server defines an error confinement region . that is , any error that occurs on a child process does not affect the correct operation of the father or other sibling processes , as far as the temporal and spatial isolation is honored . although there are several variants of the ssp technique , most implementations use a single reference - canary 100 per process , which is saved in a protected area and initialized during the process start up . the reference - canary 100 is copied in the stack frame 101 between the return address and the buffers , which is called frame - canary 103 . depending on some compilation optimizations not all stack frames are protected with a frame - canary . some ssp variants may implement slightly different versions of the basic mechanism , which does not invalidate the applicability of the disclosed invention . the stack integrity ( compare the reference - canary 100 with the frame - canary 103 ) is only done at the end of each function , or block of code , right before the returning instruction or leaving the block of code . only the value of the frame - canary of the current stack frame is compared with the reference - canary . the present invention consists in renewing the value of the reference - canary of the process at selected functions , or block of code during the execution of the process . there is only one single reference - canary for each process . our invention does not use a secondary stack to hold copies of the frame - canaries , and relies on the same infrastructure of the ssp method . typically , there is at most one vulnerable function or vulnerable block of code per process . it is quite odd to have multiple buffer overflow exploitation functions on the same process . therefore , from the point of view of the attacker there is only one frame - canary to defeat , which is the frame - canary of the vulnerable function . and so , there is little benefit on randomizing the canary on all the functions but on the vulnerable one . there are some special functions where the reference - canary can be renewed and not restored , and the program can continue its execution normally . an example of this type of functions , but not limited to it , is the code executed by the child processes right after its creation ( for example , fork and clone ), which matches the concept of error confinement region . for example , each client request is attended by a child process of a networking server when it is configured as a forking server . more generally , it is possible to renew the value of the reference - canary at any time during the execution of a program , as far as the reference - canary is restored to its previous value before the stacks frames holding old canaries values are checked . in what follows , the term “ function ” and the term “ stack - frame ” are used interchangeably . the former is an active element , and the later is the passive data structure which support the function execution . depending on the context where the term is used , it is more natural to use one or the other , but in both cases it refers to the same concept . fig1 and fig2 sketch the content of a typical stack in two different forms : detailed stack ( fig1 ) and simplified stack ( fig2 ). fig1 shows a stack with two frames 101 , 102 filled with some example content on each one : arguments , return addr ., frame ptr ., etc . the frame 101 has a frame - canary value 103 and the frame 102 does not contain a frame - canary . the present invention only depends on the value of the reference - canary 100 and the frame - canary 103 . fig2 shows the same stack as fig1 with two frames 101 , 102 , but only the frame - canary 103 is displayed , the rest of the content is not shown . those of skill in the art will appreciate our invention can be used with other stack frame contents and layouts . in the rest of this document , we will use the simplified stack representation . our invention does not impose any restriction on which stack frames shall be protected by a canary , and which ones shall not be protected . our invention is independent of the exact implementation of the ssp , and can be used with any variant of the ssp . referring to fig3 , fig4 and fig5 ; they represent the state of the stack at three different instants upon renewing the reference - canary . fig3 represents the stack and the reference - canary 100 and its value 301 before the reference - canary is renewed . fig4 represents the stack and the reference - canary 100 right after its value has been renewed 401 ( from the value 301 to the new one 401 ). fig5 represents the state of the stack after a new frame 501 is created using the renewed reference - canary . the frame - canary value 503 of the new frame 501 uses the renewed reference - canary value 401 . fig6 to fig1 , represent the content of the stack for six different type of functions . the stack frames of the functions where the reference - canary 100 is renewed are marked as dashed boxes 601 , 701 , 801 , 901 , 1001 and 1101 . all figures represent the state of the stack after the reference - canary 100 has been renewed . therefore , the frame - canary of the functions previous to the dashed as well as the dashed one contain the old canary value 301 if any , and posterior frame - canaries have the renewed value 401 if any . referring to fig6 ; it represents the content of the stack when a non - returning function , the dashed stack frame 601 , has called some nested functions 602 to 604 . each nested stack frame may ( 602 , 604 ) or may not ( 603 ) have a frame - canary . during the execution of the non - returning function 601 , called type 1 function , the reference - canary 100 can be renewed . the frame - canary value of the following functions ( the said nested ones 602 , 604 ) will be the new reference - canary value 401 . the said nested functions can make normal returns with 605 or without 606 ssp canary value check . they are also allowed to make non - local - jumps 607 to functions within the nested region . a type 1 function must not return to the parent caller 608 . and neither type 1 nor its nested functions 602 to 604 can make non - local - jumps 609 to any parent function of the said type 1 function . referring to fig7 ; it shows the representation of how our finding is used on type 2 functions . a type 2 function 701 does not return 708 but it is allowed to make non - local - jumps 702 from itself 701 or from a nested function 703 to a parent function of the said type 2 function . the destination function of the non - local - jump 702 can be any function 704 which none of its return - reachable 705 functions check the frame - canary value . therefore , the functions that check the old frame - canary 701 , 706 must not return 708 , 707 . referring to fig8 ; it shows the representation of how our finding is used on type 3 functions . a type 3 function 801 may return 802 but does not check the frame - canary integrity neither itself 801 nor on any return - reachable parent functions 803 , 804 . parent functions which check the frame - canary 805 can not return 806 . referring to fig9 ; it shows the representation of how our finding is used on type 4 functions . a type 4 function 901 renews the reference - canary 100 and eventually returns 902 checking the frame - canary . a type 4 function shall save the original value of the reference - canary 301 in a designated location 903 and restore it back ( copy the old value from the saved reference - canary 903 in the reference - canary 100 ) before the current frame - canary is checked 902 . the original reference - canary value must be saved by the said type 4 function or any of its parent functions . the saved reference - canary is represented as a global variable 903 for clarity , but it is not limited to it . the saved reference - canary may be saved as a local variable or on a dedicated memory segment or on a processor register or at any other retrievable location . referring to fig1 ; it shows the representation of how our finding is used on type 5 functions . a type 5 function 1001 renews the reference - canary and eventually returns 1002 . the stack check is not done by the type 5 function , but there are at least one parent function 1003 which checks the integrity of its stack 1004 . the parent function 1003 which checks the stack integrity 1004 shall restore the reference - canary to the original value , by copying the old value from the saved reference - canary 903 in the reference - canary 100 . in order to be able to restore the reference - canary , it has had to be saved , in a designated location 903 , before it is renewed . the original reference - canary value must be restored by the said type 5 function or any of its parent functions up to the function 1003 which checks the frame - canary . referring to fig1 ; it shows the representation of how our finding is used on type 6 functions . a type 6 function 1101 does not return to its caller 1106 , but makes a non - local - jump 1107 from itself 1101 or from a nested function 1102 to a parent function 1103 , which the said parent function 1103 or a previous caller function 1104 checks the integrity of its stack 1105 . then , the value of the reference - canary 100 has to be saved , in a designated location 903 , before renewing it at the type 6 function 1101 , and restored it back before returning from the function 1104 that checks the frame - canary . bulba and kil3r . bypassing stackguard and stackshield . phrack , 56 , 2002 . crispin cowan , calton pu , dave maier , heather hintongif , jonathan walpole , peat bakke , steve beattie , aaron grier , perry wagle , and qian zhang . stackguard : automatic adaptive detection and prevention of buffer - overflow attacks . in proc . of the 7 th usenix security symposium , pages 63 - 78 , january 1998 . h . etoh . gcc extension for protecting applications from stack - smashing attacks ( propolice ), 2003 . url http :// www . trl . ibm . com / projects / security / ssp /. aleph one . smashing the stack for fun and profit . phrack , 7 ( 49 ), 1996 . gerardo richarte . four different tricks to bypass stackshield and stackguard protection . world wide web , 1 , 2002 . hovav shacham , matthew page , ben pfaff , eu - jin goh , nagendra modadugu , and dan boneh . on the effectiveness of address - space randomization . in proceedings of the 11 th acm conference on computer and communications security , ccs &# 39 ; 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