Monday, December 15, 2014

Using LBR (Last Branch Record) feature to detect ret2usr (return-to-user) attack w/ MMU paging structure corruption

SMEP (Supervisor Mode Execution Prevention) is a mitigation that aims to prevent the CPU from running code from user-mode while in kernel-mode, however this post (Windows 8 Kernel Memory Protections Bypass) presents a generic technique for exploiting kernel vulnerabilities with bypassing SMEP. Unlike my previous post (Page Table Structure Corruption Attacks - How to Mitigate it?) that presented a mitigation to that attack, this post will present a solution to detect such a ret2usr attack due to MMU paging structure corruption.

In Intel/x86 recent processors, the LBR (last branch record) feature has some filtering capabilities like CPL (current privilege level) filtering and indirect jmp/call filterings. 

For instance, for a specific suspicious process or application, we can configure LBR to only record last branch recording addresses (like LastBranchToIP) for indirect jmp/call and ret branch instructions in kernel mode (CPL=0). 

Therefore, by analyzing the LastBranchToIP addresses in BTS (branch trace store) buffer resident in system RAM, we can get to know that whether or not a "ret2usr" attack occurred. 

The rule is pretty simple: 
check all the LastBranchToIP addresses, if we can find out that any one or more of addresses are located in the range of 0~2GB, then it indicates that a "ret2usr" attack occurred in a "monitored" process or application.

This is typically because the user mode virtual address space range is 0~2GB by default on a 32-bit Windows Operating system, even if the paging-structure entry (e.g. PTE) U/S bit is corrupted by a write-what-where vulnerability which causes a user mode memory to be interpreted as a kernel memory.  

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