Hi, i heard several people looking at the gobbles exploit and believing it can only be fake: here is my little explanation how bsd memcpy can be exploited: first a snipset of the bsd memcpy code: ... 1: addl %ecx,%edi /* copy backwards. */ addl %ecx,%esi std [1] andl $3,%ecx /* any fractional bytes? */ decl %edi decl %esi rep movsb [X] movl 20(%esp),%ecx /* copy remainder by words */ shrl $2,%ecx subl $3,%esi subl $3,%edi rep movsl ... In Apache we trigger exactly this piece of code: bsd thinks the two buffers are overlapping and so it wants to copy backward. The problem is that you are able to overwrite the call to memcpy including the supplied paramters (dst, src, length). With up to 3 bytes ([1]) depending on alignment. if you align everything perfectly you can set the 3 high bytes of length to zero and so change how many dwords memcpy tries to copy in our case 0x000000?? This is only possible because the code reads the length param again from stack [X]... This way you can easily survive the call and overwrite the saved instruction pointer before the memcpy call... just my 0.02 cents Stefan Esser - e-matters Security
