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Date: Wed, 28 Jun 2017 08:58:26 +0200
From: Dominique Martinet <>
Subject: Re: CoreOS membership to linux-distros

Sven Dowideit wrote on Wed, Jun 28, 2017:
> I'm responsible for RancherOS, and think that both I, and my users
> would prefer that I had access to the embargoed information earlier,
> so preparing a response would have been less of a rush.

I can relate to the rush feeling, even with few users/"private" distro
here, having a custom kernel makes this kind of fixes annoying...
But given the delayed exploit release I'd say it does not really matter
if you take a few days for this, especially in this case with the low
success rate on 64bit linux. As soon as reasonably possible does not
necessarily mean rush.

As a rhel/centos spin-off though we would have liked the bug brought up
here ( ) to have its
fix published faster though, it's apparently been ready for a week but
not been published... I don't mind bugs, but if it's fixed it's annoying
to keep it behind closed doors.

> One of the things that would have made my last week less worrying, is
> to have some access to exploit code - so as to verify the changes
> actually had a useful effect.

You don't need an actual exploit to test this. You're not the first
person who have told me this so I actually took some time this morning
to whip up a "tester" -- it's probably far from perfect but will run
successfully on older debian/rhel and crash with a patched kernel as
expected, and is as inoffensive as it can get.

I'm sure there are other better testers online, I didn't try looking as
I don't get much chance to play with this kind of stuff :)

Qualys gave a lot of details in their report (kudos to well written
advisories like that!), I agree having everything on a golden plate is
better but it really isn't much work left for smaller distros if you
trust the big ones or even just upstream, once bugs got steamed out.

Asmadeus | Dominique Martinet

/** Stack Clash kernel "stack gap" patch tester
 * WTFPLv2 - do whatever the fuck you want with this
 * Dominique Martinet <>
 * gcc -o teststackclash teststackclash.c && ./teststackclash

#include <stdio.h>
#include <sys/mman.h>
#include <alloca.h>
#include <unistd.h>
#include <errno.h>
#include <inttypes.h>

#define MMAP_OFFSET (1024*1024)

int main(int argc, char argv[]) {
	void *stackedge, *stackaddr, *mmapaddr;
	char ispresent;

	printf("Address within stack: %p, looking for stack start edge...\n", &stackedge);

	stackedge = (void*)((uintptr_t)&stackedge & 0xfffffffffffff000);
       	while (mincore(stackedge, 1, &ispresent) == 0)
		stackedge -= 4096;

	if (errno != ENOMEM) {
		printf("unexpected mincore errno: %d\n", errno);
		return -1;

	printf("Stack starts here: %p, doing mmap largely before it (%p)\n", stackedge+4096, stackedge-MMAP_OFFSET);
	errno = 0;

	mmapaddr = mmap(stackedge-MMAP_OFFSET, 4096, PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED, 0, 0); 
	if (mmapaddr == MAP_FAILED) {
		printf("mmap failed: %d\n", errno);
		return -1;

	if (mmapaddr != stackedge - MMAP_OFFSET) {
		printf("mmap ok, but didn't respect MAP_FIXED hint: %p\n", mmapaddr);
		return -1;

	printf("mmap ok, allocating within stack until it grows (should crash with that mmap)\n");
	while ((stackaddr = alloca(4096)) > stackedge);

	// ((char*)mmapaddr)[0] = 1; // actually didn't even need to write in map
	printf("Didn't crash, stack gap guard <1MB or didn't care about our map\n");

	return 0;

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