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Date: Fri, 2 Jun 2017 08:56:13 +1000 (AEST)
From: James Morris <>
To: Kees Cook <>
cc: lazytyped <>,
        "" <>
Subject: Re: Re: [PATCH v7 2/2] security: tty: make TIOCSTI
 ioctl require CAP_SYS_ADMIN

On Thu, 1 Jun 2017, Kees Cook wrote:

> All of the reasoning here seems to match the link restrictions from 5
> years ago: a crappy API (sticky bit) is not handled by userspace (open
> /tmp/$$!) and people get attacked. The solution was a sysctl to enable
> the link restrictions that killed the entire class of the common
> attack (though it didn't solve especially egregious bad uses, much

This is the problem -- it doesn't really eliminate the underlying issue.

A better solution (in this case) was to implement a new API which 
addresses the issue at an architectural level, i.e. namespace-based 
private /tmp views, and encourage its adoption.

> like the TIOCSTI fix). Every distro enabled the sysctl, and, while the
> data is noisy, looking a CVEs matching "/tmp symlink", the numbers
> drop from 2013 and later (with none yet for 2017).

I wonder how much of this is due to the sysctl vs. adoption of private 
/tmp, and what may be lurking in the "egregious bad uses" category for 
future CVEs.  And obviously we don't know what various folk may have up 
their sleeves, if anything.

James Morris

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