Re: [PATCH 2/4] x86/sgx: Put enclaves into anonymous files

[Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>]

On Tue, Apr 07, 2020 at 11:48:10AM +0300, Topi Miettinen wrote:
> On 7.4.2020 0.24, Jarkko Sakkinen wrote:
> > In my opinion udev defining the whole /dev as noexec has zero technical
> > merits. It is same as they would say that "we don't trust our own
> > database". There are no real security benefits as long as dev nodes are
> > configured correctly.
> 
> The threat is not that the device nodes would have execute permissions, but
> that a malicious entity with write access to /dev would create a new
> executable and run it, or rather, trick another (perhaps more privileged or
> more vulnerable) entity to do so. The malicious entity does not need any
> capabilities and it can be constrained by any number of typical seccomp
> filters which just don't block such basic system calls as open(), write(),
> [f]chmod() and close(). It simply needs to have UID 0 (possibly something
> else, like suitable GID could also be sufficient for some subdirectories)
> and write access to /dev (or its subdirectories) in its mount namespace.
> 
> My philosophy is that "trust" means confidence that an action will not be
> done even when there's no control over it. "Control" means that it's
> possible to make active decision on whether the action can or cannot be
> allowed to be done. Trust in security mindset is a weak thing, control is
> stronger, but the strongest case is when you don't need trust nor control:
> the action simply can't ever happen because it's impossible or always
> forbidden. This idea is shown in such famous principles as "least
> privilege", "need to know" or compartmentalization. If the additional
> privilege of exec is not needed, it should not exist.
> 
> -Topi

I get the threat scenario, thanks.

The problem (as Jethro correctly pointed out) with noexec /dev is
somewhat broad.

Thank you anyway for taking time describing the threat scenario.

/Jarkko