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Red Hat Introduces NX Software Support For Linux
abertoll writes "In this story at ZDnet, Red Hat has apparently added NX support to Linux. NX security technology is a hardware attempt at stopping malicious code." (We recently posted about Transmeta's announcement that its chips will incorporate the NX bit as well.)
Standard 386 protected mode controls per segment, where CS (code segment) is executable and DS (data segment) is writable. However, many 32-bit operating systems use a so-called "tiny" memory model, setting CS = DS, and the 386 allows for turning off read and write privileges per page but not execute privileges (if you can read a page in an executable segment, you can execute from it).
However, true W^X (shorthand for "no segment is both writable and executable") support won't work for applications that depend on self-modifying code, such as JIT-compiling virtual machines for Java and .NET platforms.
This new patch is to support NX in 32-bit processors or 64-bit processors running in 32-bit mode.
The 2.6.6 kernel already included an NX patch for x86_64. Details are in the "Non-Exec stack patches" LKML thread here.
Of course, if those programs were written correctly in the first place they wouldn't need to be fixed to work on NX platforms.
Win32 has always had PAGE_EXECUTE flag, and if you wanted to execute dynamically generated code you were supposed to include this flag when allocating memory (or use VirtualProtect afterwards).
Most people didn't bother with PAGE_EXECUTE because it wasn't enforced on x86. But technically it's always been required.
Linux, Windows, BSD, etc. don't use segments, but instead use paging. Intel has dragged their feet on adding NX support because the feature "already exists", but the reality is that hardly anyone uses segments.
Ok, technically everyone uses segments -- they just create a single segment which covers all of the memory space. The GDT (Global Descriptor Table) must be configured when you switch to protected mode. Paging is optional.
The NX flag prevents a page (typically 4k) from executing. By marking all stack pages as NX, buffer overflow attacks won't be able to remotely execute arbitrary code. I assume that an exception will be generated when an attempt is made to execute from an NX page, which will probably cause the running program to halt. So, remote explots turn into DOS attacks.
Buffer overflow attacks have been known about for decades, and solutions such as NX have been known for quite some time too. As has been mentioned elsewhere on /., this does not remove the responsibility of developers to write good, secure code. But, as history has shown, they will probably continue with the long standing practice of writing insecure code.
NX will prevent buffer overflow attacks. NX will not be able to determine whether a program you choose to execute is good or evil. Viruses existed and managed to propogate back in the days before the Internet or even networking were in common use. NX won't solve all security problems, but it is a good tool to help reduce the possibility of remote exploits.
The NX flag isn't new, it's just new to the x86 world. Kudos to AMD for being the first to add this to the x86!