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Subject: Re: [PATCH] MIPS: eBPF: Fix icache flush end address
To:     Paul Burton <paul.burton@mips.com>,
        "linux-mips@vger.kernel.org" <linux-mips@vger.kernel.org>,
        "bpf@vger.kernel.org" <bpf@vger.kernel.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Paul Burton <pburton@wavecomp.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>
References: <20190301225743.8632-1-paul.burton@mips.com>
From:   Daniel Borkmann <daniel@iogearbox.net>
Message-ID: <6319f997-bc83-5515-4b3b-a87f57c65db3@iogearbox.net>
Date:   Sat, 2 Mar 2019 00:07:00 +0100
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On 03/01/2019 11:58 PM, Paul Burton wrote:
> The MIPS eBPF JIT calls flush_icache_range() in order to ensure the
> icache observes the code that we just wrote. Unfortunately it gets the
> end address calculation wrong due to some bad pointer arithmetic.
> 
> The struct jit_ctx target field is of type pointer to u32, and as such
> adding one to it will increment the address being pointed to by 4 bytes.
> Therefore in order to find the address of the end of the code we simply
> need to add the number of 4 byte instructions emitted, but we mistakenly
> add the number of instructions multiplied by 4. This results in the call
> to flush_icache_range() operating on a memory region 4x larger than
> intended, which is always wasteful and can cause crashes if we overrun
> into an unmapped page.
> 
> Fix this by correcting the pointer arithmetic to remove the bogus
> multiplication, and use braces to remove the need for a set of brackets
> whilst also making it obvious that the target field is a pointer.
> 
> Signed-off-by: Paul Burton <paul.burton@mips.com>
> Fixes: b6bd53f9c4e8 ("MIPS: Add missing file for eBPF JIT.")
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> Cc: Martin KaFai Lau <kafai@fb.com>
> Cc: Song Liu <songliubraving@fb.com>
> Cc: Yonghong Song <yhs@fb.com>
> Cc: netdev@vger.kernel.org
> Cc: bpf@vger.kernel.org
> Cc: linux-mips@vger.kernel.org
> Cc: stable@vger.kernel.org # v4.13+

Good catch, applied to bpf, thanks!