Re: Security question: "Text file busy" overwriting executables but

[Linus Torvalds]

On 5 Oct 2001, Eric W. Biederman wrote:
> > The MAP_DENYWRITE rule was added a long time ago because people found actual
> > workable DoS attacks
>
> Do you have any details.  I would like to figure out what it takes to
> export MAP_DENYWRITE safely to userspace.

I think it literally was /var/run/[uw]tmp, and using MAP_DENYWRITE to
disable all logins.

But it pretty much covers _any_ logfiles that are readable (and thus
openable) by users.

> Currently checking to see if the file is executable looks good
> enough.

[ executable by the user in question, not just anybody ]

Yes, I suspect it is.

> The fix for bad permission (during a DOS attack) is either:
> 	chmod correct_permissions foo
> 	lsof foo | xargs kill

Well, if you cannot log in as root, it doesn't much matter what the "fix"
is, so it's better to be safe than sorry.

		Linus