Date:   Thu, 17 Jun 2021 10:09:22 +0200
From:   Alex Ghiti <alex@...ti.fr>
To:     Palmer Dabbelt <palmer@...belt.com>, jszhang3@...l.ustc.edu.cn
Cc:     schwab@...ux-m68k.org, Paul Walmsley <paul.walmsley@...ive.com>,
        aou@...s.berkeley.edu, ryabinin.a.a@...il.com, glider@...gle.com,
        andreyknvl@...il.com, dvyukov@...gle.com, bjorn@...nel.org,
        ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        kafai@...com, songliubraving@...com, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org,
        luke.r.nels@...il.com, 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] riscv: Ensure BPF_JIT_REGION_START aligned with PMD size

Le 17/06/2021 à 09:30, Palmer Dabbelt a écrit :
> On Tue, 15 Jun 2021 17:03:28 PDT (-0700), jszhang3@...l.ustc.edu.cn wrote:
>> On Tue, 15 Jun 2021 20:54:19 +0200
>> Alex Ghiti <alex@...ti.fr> wrote:
>>
>>> Hi Jisheng,
>>
>> Hi Alex,
>>
>>>
>>> Le 14/06/2021 à 18:49, Jisheng Zhang a écrit :
>>> > From: Jisheng Zhang <jszhang@...nel.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@...ux-m68k.org>
>>> > Signed-off-by: Jisheng Zhang <jszhang@...nel.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.

Not surprising, I took a shortcut when initializing KASAN for modules, 
kernel and BPF:

         kasan_populate(kasan_mem_to_shadow((const void *)MODULES_VADDR),
                        kasan_mem_to_shadow((const void 
*)BPF_JIT_REGION_END));

The kernel is then not covered, I'm taking a look at how to fix that 
properly.

> 
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