Hardening Linux against buffer overflows [LWN.net]

Whenever kernel hacker Ingo Molnar disappears for a while, it is reasonable to expect that he will resurface with some interesting new development. In the past, he has materialized with the TUX in-kernel web server, the O(1) scheduler, and the new kernel threads implementation. His most recent offering, however, has to do with making Linux systems more resilient in the face of buffer overflow vulnerabilities.

The new "Exec Shield" patch (which applies to the 2.4.21 release candidate kernel) takes several steps to fight buffer overflows, including:

Engaging in some x86 segmentation magic to minimize the range of addresses where the processor can execute code. This approach is essentially a hack to work around the inconvenient fact that the x86 architecture lacks a page table flag for controlling execute permission. By playing with segments, this patch makes it impossible to execute code on the stack and in as much of the data area as possible. As a result, the execution of exploit code delivered via buffer overflows becomes far more difficult.
As much executable code as possible is moved into the lowest part of the virtual address space - below 16MB. Addresses in this range begin with a zero byte, making them hard to create with purely ASCII overflows. Many applications which deal with C strings cannot be overflowed with a string containing NULL bytes. Others, of course, will read any sort of data and will not be affected by this particular change. For applications for which this measure is effective, however (and that set includes most web-based applications), this shift into the "ASCII armor" area will block exploits which work by jumping into library code which might, say, execute a shell.
A subsequent release of the patch includes shared library and stack address randomization. These measures make mass exploits harder, since each target process is different.

This approach, note, is different from that used in the recent OpenBSD 3.3 release. OpenBSD does not (yet) provide executable stack protection on the x86 architecture; instead, it relies on detecting buffer overflows with a modified compiler that installs (and checks) "canaries" on the stack. The OpenBSD "W^X" execute permission control will be extended to the x86 architecture in 3.4.

So does this approach bring any real security? Non-executable stack patches have failed to get into the Linux kernel before, since Linus does not believe in them:

In short, anybody who thinks that the non-executable stack gives them any real security is very very much living in a dream world. It may catch a few attacks for old binaries that have security problems, but the basic problem is that the binaries allow you to overwrite their stacks.

The real solution, says Linus, is to fix the applications. That is undoubtedly true, but fixing all of the applications out there (and keeping them fixed) is not an easy task. In the meantime, raising the bar for potential attackers may well be the right thing to do. It would not be surprising to see this patch find its way into the kernels shipped by the major distributors, even if Linus does not accept it into the mainline.

to post comments

Hardening Linux against buffer overflows

Posted May 8, 2003 16:01 UTC (Thu) by brouhaha (guest, #1698) [Link] (2 responses)

I've never understood Linus' attitude about this. It's like saying "putting a lock on the door of your house won't completely prevent burglary, so instead you should get the police to catch and lock up all the burglars."

No single change anywhere can solve the problem, but it's hard to see a downside to kernel patches that can substantially reduce the effectiveness of a whole class of exploits.

Hardening Linux against buffer overflows

Posted May 8, 2003 16:27 UTC (Thu) by smoogen (subscriber, #97) [Link]

When I have run into similar stances, and asked for clarification it is that too many people get caught up that the lock on the door will protect them completely. [For a real world example look at the run of Duct-Tape and Plastic sheeting in various US cities... the realities are that they would help some people but for most are placebos.]

To summarize, some of us know that it will help a bit when used with other software contraceptives... but the majority of computer users will think it is the end-all and be-all of security.

Hardening Linux against buffer overflows

Posted May 8, 2003 20:22 UTC (Thu) by iabervon (subscriber, #722) [Link]

Well, in this case it's a bit like saying, "Putting a lock on your front door won't prevent burglary, because you have an unlocked side door they'll use instead." The problem is that a stack buffer overflow can be exploited in a number of ways, and only the simplest requires an executable stack. On the other hand, the new patch prevents a number of other exploits by putting more of the program out of harm's way. So it's possible that Linus will like it better, assuming he can't come up with an attack which will still work on otherwise correct programs that have a buffer overflow.

Linus hasn't spoken about this one, to my knowledge - it's different.

Posted May 8, 2003 16:35 UTC (Thu) by dwheeler (guest, #1216) [Link]

Linus may not realize it, but Ingo's "Exec Shield" patch
is subtly different from the patch Linus rejected long ago.

Linus' objection was that it was fairly easy to modify attacks so
that they would simply call a library service (say system("/bin/sh").
And, Linus was correct - the old patch was easily worked around.

Ingo's new patch seems to help deal with that. With this new approach,
the library services you want to call are rigged so that they're
either moved to an "unguessable" location, or have a 0 in the address.
Many attacks aren't able to insert bytes with 0 values.
So, Ingo's patch seems to have a leg up on the previous
no-exec patches.

This doesn't mean it's perfect. Perhaps there's another exploit,
and there are attacks which CAN produce zeros (decompression of any
kind, including graphics files, comes to mind). But this one
is worthy of consideration - its concept is an improvement over the old
no-exec patch.

Grsec

Posted May 8, 2003 23:23 UTC (Thu) by yem (guest, #1138) [Link]

How similar is this patch to the memory protection mechanisms included with the grsecurity patch? Will I gain any extra benefit from using both?

http://www.grsecurity.org/features.php