Re: [PATCH bpf-next] bpf: Make trampolines W^X

From: Andy Lutomirski <luto@amacapital.net>
To: KP Singh <kpsingh@chromium.org>,
	Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc: linux-kernel@vger.kernel.org, bpf@vger.kernel.org,
	x86@kernel.org, linux-security-module@vger.kernel.org,
	Kees Cook <keescook@chromium.org>,
	"David S. Miller" <davem@davemloft.net>,
	Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
	Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>, Andrii Nakryiko <andriin@fb.com>,
	Thomas Garnier <thgarnie@chromium.org>,
	Florent Revest <revest@chromium.org>,
	Brendan Jackman <jackmanb@chromium.org>,
	Jann Horn <jannh@google.com>, Matthew Garrett <mjg59@google.com>,
	Michael Halcrow <mhalcrow@google.com>
Subject: Re: [PATCH bpf-next] bpf: Make trampolines W^X
Date: Sat, 4 Jan 2020 09:49:10 +0900	[thread overview]
Message-ID: <F25C9071-A7A7-4221-BC49-A769E1677EE1@amacapital.net> (raw)
In-Reply-To: <20200103234725.22846-1-kpsingh@chromium.org>

> On Jan 4, 2020, at 8:47 AM, KP Singh <kpsingh@chromium.org> wrote:
> 
> From: KP Singh <kpsingh@google.com>
> 
> The image for the BPF trampolines is allocated with
> bpf_jit_alloc_exe_page which marks this allocated page executable. This
> means that the allocated memory is W and X at the same time making it
> susceptible to WX based attacks.
> 
> Since the allocated memory is shared between two trampolines (the
> current and the next), 2 pages must be allocated to adhere to W^X and
> the following sequence is obeyed where trampolines are modified:

Can we please do better rather than piling garbage on top of garbage?

> 
> - Mark memory as non executable (set_memory_nx). While module_alloc for
> x86 allocates the memory as PAGE_KERNEL and not PAGE_KERNEL_EXEC, not
> all implementations of module_alloc do so

How about fixing this instead?

> - Mark the memory as read/write (set_memory_rw)

Probably harmless, but see above about fixing it.

> - Modify the trampoline

Seems reasonable. It’s worth noting that this whole approach is suboptimal: the “module” allocator should really be returning a list of pages to be written (not at the final address!) with the actual executable mapping to be materialized later, but that’s a bigger project that you’re welcome to ignore for now.  (Concretely, it should produce a vmap address with backing pages but with the vmap alias either entirely unmapped or read-only. A subsequent healer would, all at once, make the direct map pages RO or not-present and make the vmap alias RX.)

> - Mark the memory as read-only (set_memory_ro)
> - Mark the memory as executable (set_memory_x)

No, thanks. There’s very little excuse for doing two IPI flushes when one would suffice.

As far as I know, all architectures can do this with a single flush without races  x86 certainly can. The module freeing code gets this sequence right. Please reuse its mechanism or, if needed, export the relevant interfaces.