linux-hardening - Re: [PATCH 07/32] mm: Bring back vmalloc

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Date:   Mon, 19 Jun 2023 20:39:49 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Johannes Thumshirn <Johannes.Thumshirn@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-bcachefs@...r.kernel.org" <linux-bcachefs@...r.kernel.org>,
        Kent Overstreet <kent.overstreet@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Uladzislau Rezki <urezki@...il.com>,
        "hch@...radead.org" <hch@...radead.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
Subject: Re: [PATCH 07/32] mm: Bring back vmalloc_exec

On Mon, Jun 19, 2023 at 12:45:43PM -0700, Kees Cook wrote:
> I think there's a misunderstanding here about the threat model I'm
> interested in protecting against for JITs. While making sure the VM of a
> JIT is safe in itself, that's separate from what I'm concerned about.
> 
> The threat model is about flaws _elsewhere_ in the kernel that can
> leverage the JIT machinery to convert a "write anything anywhere anytime"
> exploit primitive into an "execute anything" primitive. Arguments can
> be made to say "a write anything flaw means the total collapse of the
> security model so there's no point defending against it", but both that
> type of flaw and the slippery slope argument don't stand up well to
> real-world situations.

Hey Kees, thanks for the explanation - I don't think this is a concern
for what bcachefs is doing, since we're not doing a full jit. The unpack
functions we generate only write to the 40 bytes pointed to by rsi; not
terribly useful as an execute anything primitive :)