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Date:   Thu, 17 Jun 2021 00:30:35 -0700 (PDT)
Subject:     Re: [PATCH] riscv: Ensure BPF_JIT_REGION_START aligned with PMD size
In-Reply-To: <20210616080328.6548e762@xhacker>
CC:     alex@ghiti.fr, schwab@linux-m68k.org,
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From:   Palmer Dabbelt <palmer@dabbelt.com>
To:     jszhang3@mail.ustc.edu.cn
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On Tue, 15 Jun 2021 17:03:28 PDT (-0700), jszhang3@mail.ustc.edu.cn wrote:
> On Tue, 15 Jun 2021 20:54:19 +0200
> Alex Ghiti <alex@ghiti.fr> wrote:
>
>> Hi Jisheng,
>
> Hi Alex,
>
>> 
>> Le 14/06/2021 à 18:49, Jisheng Zhang a écrit :
>> > 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.  
>> 
>> Good catch, we make sure no physical allocation happens between _end and 
>> the next PMD aligned address, but I missed this one.
>> 
>> > 
>> > 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.  
>> 
>> But I disagree with your solution: I made sure modules and BPF programs 
>> get their own virtual regions to avoid worst case scenario where one 
>> could allocate all the space and leave nothing to the other (we are 
>> limited to +- 2GB offset). Why don't just align BPF_JIT_REGION_START to 
>> the next PMD aligned address?
>
> Originally, I planed to fix the issue by aligning BPF_JIT_REGION_START, but
> IIRC, BPF experts are adding (or have added) "Calling kernel functions from BPF"
> feature, there's a risk that BPF JIT region is beyond the 2GB of module region:
>
> ------
> module
> ------
> kernel
> ------
> BPF_JIT
>
> So I made this patch finally. In this patch, we let BPF JIT region sit
> between module and kernel.
>
> To address "make sure modules and BPF programs get their own virtual regions",
> what about something as below (applied against this patch)?
>
> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
> index 380cd3a7e548..da1158f10b09 100644
> --- a/arch/riscv/include/asm/pgtable.h
> +++ b/arch/riscv/include/asm/pgtable.h
> @@ -31,7 +31,7 @@
>  #define BPF_JIT_REGION_SIZE	(SZ_128M)
>  #ifdef CONFIG_64BIT
>  #define BPF_JIT_REGION_START	(BPF_JIT_REGION_END - BPF_JIT_REGION_SIZE)
> -#define BPF_JIT_REGION_END	(MODULES_END)
> +#define BPF_JIT_REGION_END	(PFN_ALIGN((unsigned long)&_start))
>  #else
>  #define BPF_JIT_REGION_START	(PAGE_OFFSET - BPF_JIT_REGION_SIZE)
>  #define BPF_JIT_REGION_END	(VMALLOC_END)
> @@ -40,7 +40,7 @@
>  /* 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_END	(BPF_JIT_REGION_END)
>  #endif
>  
>
>
>> 
>> Again, good catch, thanks,
>> 
>> Alex
>> 
>> > 
>> > 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)
>> >   

This, when applied onto fixes, is breaking early boot on KASAN 
configurations for me.