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Subject: Re: [PATCH] riscv: Ensure BPF_JIT_REGION_START aligned with PMD size
To:     Jisheng Zhang <jszhang3@mail.ustc.edu.cn>,
        Andreas Schwab <schwab@linux-m68k.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>
Cc:     Andrey Ryabinin <ryabinin.a.a@gmail.com>,
        Alexander Potapenko <glider@google.com>,
        Andrey Konovalov <andreyknvl@gmail.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Luke Nelson <luke.r.nels@gmail.com>,
        Xi Wang <xi.wang@gmail.com>, linux-riscv@lists.infradead.org,
        linux-kernel@vger.kernel.org, kasan-dev@googlegroups.com,
        netdev@vger.kernel.org, bpf@vger.kernel.org
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 <20210330022521.2a904a8c@xhacker> <87o8ccqypw.fsf@igel.home>
 <20210612002334.6af72545@xhacker> <87bl8cqrpv.fsf@igel.home>
 <20210614010546.7a0d5584@xhacker> <87im2hsfvm.fsf@igel.home>
 <20210615004928.2d27d2ac@xhacker>
From:   Daniel Borkmann <daniel@iogearbox.net>
Message-ID: <3c7ec52d-7fa3-dfac-239c-989ea1cc37ee@iogearbox.net>
Date:   Tue, 15 Jun 2021 14:29:54 +0200
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On 6/14/21 6:49 PM, Jisheng Zhang wrote:
> From: Jisheng Zhang <jszhang@kernel.org>
> 
> Andreas reported commit fc8504765ec5 ("riscv: bpf: Avoid breaking W^X")
> breaks booting with one kind of config file, I reproduced a kernel panic
> with the config:
> 
> [    0.138553] Unable to handle kernel paging request at virtual address ffffffff81201220
> [    0.139159] Oops [#1]
> [    0.139303] Modules linked in:
> [    0.139601] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.13.0-rc5-default+ #1
> [    0.139934] Hardware name: riscv-virtio,qemu (DT)
> [    0.140193] epc : __memset+0xc4/0xfc
> [    0.140416]  ra : skb_flow_dissector_init+0x1e/0x82
> [    0.140609] epc : ffffffff8029806c ra : ffffffff8033be78 sp : ffffffe001647da0
> [    0.140878]  gp : ffffffff81134b08 tp : ffffffe001654380 t0 : ffffffff81201158
> [    0.141156]  t1 : 0000000000000002 t2 : 0000000000000154 s0 : ffffffe001647dd0
> [    0.141424]  s1 : ffffffff80a43250 a0 : ffffffff81201220 a1 : 0000000000000000
> [    0.141654]  a2 : 000000000000003c a3 : ffffffff81201258 a4 : 0000000000000064
> [    0.141893]  a5 : ffffffff8029806c a6 : 0000000000000040 a7 : ffffffffffffffff
> [    0.142126]  s2 : ffffffff81201220 s3 : 0000000000000009 s4 : ffffffff81135088
> [    0.142353]  s5 : ffffffff81135038 s6 : ffffffff8080ce80 s7 : ffffffff80800438
> [    0.142584]  s8 : ffffffff80bc6578 s9 : 0000000000000008 s10: ffffffff806000ac
> [    0.142810]  s11: 0000000000000000 t3 : fffffffffffffffc t4 : 0000000000000000
> [    0.143042]  t5 : 0000000000000155 t6 : 00000000000003ff
> [    0.143220] status: 0000000000000120 badaddr: ffffffff81201220 cause: 000000000000000f
> [    0.143560] [<ffffffff8029806c>] __memset+0xc4/0xfc
> [    0.143859] [<ffffffff8061e984>] init_default_flow_dissectors+0x22/0x60
> [    0.144092] [<ffffffff800010fc>] do_one_initcall+0x3e/0x168
> [    0.144278] [<ffffffff80600df0>] kernel_init_freeable+0x1c8/0x224
> [    0.144479] [<ffffffff804868a8>] kernel_init+0x12/0x110
> [    0.144658] [<ffffffff800022de>] ret_from_exception+0x0/0xc
> [    0.145124] ---[ end trace f1e9643daa46d591 ]---
> 
> After some investigation, I think I found the root cause: commit
> 2bfc6cd81bd ("move kernel mapping outside of linear mapping") moves
> BPF JIT region after the kernel:
> 
> The &_end is unlikely aligned with PMD size, so the front bpf jit
> region sits with part of kernel .data section in one PMD size mapping.
> But kernel is mapped in PMD SIZE, 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 to, for example memset the
> .data section, MMU will trigger a store page fault.
> 
> To fix the issue, we need to ensure the BPF JIT region is PMD size
> aligned. This patch acchieve this goal by restoring the BPF JIT region
> to original position, I.E the 128MB before kernel .text section.
> 
> Reported-by: Andreas Schwab <schwab@linux-m68k.org>
> Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
> ---
>   arch/riscv/include/asm/pgtable.h | 5 ++---
>   1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
> index 9469f464e71a..380cd3a7e548 100644
> --- a/arch/riscv/include/asm/pgtable.h
> +++ b/arch/riscv/include/asm/pgtable.h
> @@ -30,9 +30,8 @@
>   
>   #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_END)
>   #else
>   #define BPF_JIT_REGION_START	(PAGE_OFFSET - BPF_JIT_REGION_SIZE)
>   #define BPF_JIT_REGION_END	(VMALLOC_END)

I presume this fix will be routed via riscv tree?

Thanks,
Daniel