netdev - Re: [PATCH 7/9] riscv: bpf: Avoid breaking W^X

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Date:   Sun, 13 Jun 2021 21:50:53 +0200
From:   Andreas Schwab <schwab@...ux-m68k.org>
To:     Jisheng Zhang <jszhang3@...l.ustc.edu.cn>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Björn Töpel <bjorn@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Luke Nelson <luke.r.nels@...il.com>,
        Xi Wang <xi.wang@...il.com>, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com,
        netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH 7/9] riscv: bpf: Avoid breaking W^X

On Jun 14 2021, Jisheng Zhang wrote:

> I think I found the root cause: commit 2bfc6cd81bd ("move kernel mapping
> outside of linear mapping") moves BPF JIT region after the kernel:
>
> #define BPF_JIT_REGION_START   PFN_ALIGN((unsigned long)&_end)
>
> The &_end is unlikely aligned with PMD SIZE, so the front bpf jit region
> sits with kernel .data section in one PMD. But kenrel is mapped in PMD SIZE,
> so when bpf_jit_binary_lock_ro() is called to make the first bpf jit prog
> ROX, we will make part of kernel .data section RO too, so when we write, for example
> memset the .data section, MMU will trigger store page fault.
>
> To fix the issue, we need to make the bpf jit region PMD size aligned by either
> patch BPF_JIT_REGION_START to align on PMD size rather than PAGE SIZE, or
> something as below patch to move the BPF region before modules region:
>
> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
> index 9469f464e71a..997b894edbc2 100644
> --- a/arch/riscv/include/asm/pgtable.h
> +++ b/arch/riscv/include/asm/pgtable.h
> @@ -31,8 +31,8 @@
>  #define BPF_JIT_REGION_SIZE	(SZ_128M)
>  #ifdef CONFIG_64BIT
>  /* KASLR should leave at least 128MB for BPF after the kernel */
> -#define BPF_JIT_REGION_START	PFN_ALIGN((unsigned long)&_end)
> -#define BPF_JIT_REGION_END	(BPF_JIT_REGION_START + BPF_JIT_REGION_SIZE)
> +#define BPF_JIT_REGION_START	(BPF_JIT_REGION_END - BPF_JIT_REGION_SIZE)
> +#define BPF_JIT_REGION_END	(MODULES_VADDR)
>  #else
>  #define BPF_JIT_REGION_START	(PAGE_OFFSET - BPF_JIT_REGION_SIZE)
>  #define BPF_JIT_REGION_END	(VMALLOC_END)
> @@ -40,8 +40,8 @@
>  
>  /* Modules always live before the kernel */
>  #ifdef CONFIG_64BIT
> -#define MODULES_VADDR	(PFN_ALIGN((unsigned long)&_end) - SZ_2G)
>  #define MODULES_END	(PFN_ALIGN((unsigned long)&_start))
> +#define MODULES_VADDR	(MODULES_END - SZ_128M)
>  #endif
>  
>  
> can you please try it? Per my test, the issue is fixed.

I can confirm that this fixes the issue.

Andreas.

-- 
Andreas Schwab, schwab@...ux-m68k.org
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