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

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Message-ID: <ZGP54T0d89TMySsf@casper.infradead.org>
Date:   Tue, 16 May 2023 22:47:13 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Kent Overstreet <kent.overstreet@...ux.dev>
Cc:     Kees Cook <keescook@...omium.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 Tue, May 16, 2023 at 05:20:33PM -0400, Kent Overstreet wrote:
> On Tue, May 16, 2023 at 02:02:11PM -0700, Kees Cook wrote:
> > For something that small, why not use the text_poke API?
> 
> This looks like it's meant for patching existing kernel text, which
> isn't what I want - I'm generating new functions on the fly, one per
> btree node.
> 
> I'm working up a new allocator - a (very simple) slab allocator where
> you pass a buffer, and it gives you a copy of that buffer mapped
> executable, but not writeable.
> 
> It looks like we'll be able to convert bpf, kprobes, and ftrace
> trampolines to it; it'll consolidate a fair amount of code (particularly
> in bpf), and they won't have to burn a full page per allocation anymore.
> 
> bpf has a neat trick where it maps the same page in two different
> locations, one is the executable location and the other is the writeable
> location - I'm stealing that.

How does that avoid the problem of being able to construct an arbitrary
gadget that somebody else will then execute?  IOW, what bpf has done
seems like it's working around & undoing the security improvements.

I suppose it's an improvement that only the executable address is
passed back to the caller, and not the writable address.

> external api will be:
> 
> void *jit_alloc(void *buf, size_t len, gfp_t gfp);
> void jit_free(void *buf);
> void jit_update(void *buf, void *new_code, size_t len); /* update an existing allocation */
>