netdev - Re: [PATCH bpf] bpf: Use pointer type whitelist for XADD

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Message-ID: <9229787E-6243-4F22-B809-819A3F24B5DD@fb.com>
Date:   Thu, 16 Apr 2020 20:52:20 +0000
From:   Song Liu <songliubraving@...com>
To:     Jann Horn <jannh@...gle.com>
CC:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        "Daniel Borkmann" <daniel@...earbox.net>,
        Martin Lau <kafai@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf] bpf: Use pointer type whitelist for XADD



> On Apr 15, 2020, at 1:47 PM, Jann Horn <jannh@...gle.com> wrote:
> 
> At the moment, check_xadd() uses a blacklist to decide whether a given
> pointer type should be usable with the XADD instruction. Out of all the
> pointer types that check_mem_access() accepts, only four are currently let
> through by check_xadd():
> 
> PTR_TO_MAP_VALUE
> PTR_TO_CTX           rejected
> PTR_TO_STACK
> PTR_TO_PACKET        rejected
> PTR_TO_PACKET_META   rejected
> PTR_TO_FLOW_KEYS     rejected
> PTR_TO_SOCKET        rejected
> PTR_TO_SOCK_COMMON   rejected
> PTR_TO_TCP_SOCK      rejected
> PTR_TO_XDP_SOCK      rejected
> PTR_TO_TP_BUFFER
> PTR_TO_BTF_ID
> 
> Looking at the currently permitted ones:
> 
> - PTR_TO_MAP_VALUE: This makes sense and is the primary usecase for XADD.
> - PTR_TO_STACK: This doesn't make much sense, there is no concurrency on
>   the BPF stack. It also causes confusion further down, because the first
>   check_mem_access() won't check whether the stack slot being read from is
>   STACK_SPILL and the second check_mem_access() assumes in
>   check_stack_write() that the value being written is a normal scalar.
>   This means that unprivileged users can leak kernel pointers.
> - PTR_TO_TP_BUFFER: This is a local output buffer without concurrency.
> - PTR_TO_BTF_ID: This is read-only, XADD can't work. When the verifier
>   tries to verify XADD on such memory, the first check_ptr_to_btf_access()
>   invocation gets confused by value_regno not being a valid array index
>   and writes to out-of-bounds memory.
> 
> Limit XADD to PTR_TO_MAP_VALUE, since everything else at least doesn't make
> sense, and is sometimes broken on top of that.
> 
> Fixes: 17a5267067f3 ("bpf: verifier (add verifier core)")
> Signed-off-by: Jann Horn <jannh@...gle.com>
> ---
> I'm just sending this on the public list, since the worst-case impact for
> non-root users is leaking kernel pointers to userspace. In a context where
> you can reach BPF (no sandboxing), I don't think that kernel ASLR is very
> effective at the moment anyway.

IIUC, this is to fix leaking kernel pointers? If this is accurate, we should
include this information in the commit log. 

> 
> This breaks ten unit tests that assume that XADD is possible on the stack,
> and I'm not sure how all of them should be fixed up; I'd appreciate it if
> someone else could figure out how to fix them. I think some of them might
> be using XADD to cast pointers to numbers, or something like that? But I'm
> not sure.

Could you please list which tests are broken by this? We need to be careful
because some tools probably depend on this. 

Thanks,
Song