Re: [PATCH] riscv: Ensure BPF_JIT_REGION_START aligned with PMD size

Alex Ghiti
2021-06-17T14:19:12Z

Le 17/06/2021 à 10:09, Alex Ghiti a écrit :
> Le 17/06/2021 à 09:30, Palmer Dabbelt a écrit :
>> 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.
>
> 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.
>

The following based on "riscv: Introduce structure that group all
variables regarding kernel mapping" fixes the issue:

diff --git a/arch/riscv/mm/kasan_init.c b/arch/riscv/mm/kasan_init.c
index 9daacae93e33..2a45ea909e7f 100644
--- a/arch/riscv/mm/kasan_init.c
+++ b/arch/riscv/mm/kasan_init.c
@@ -199,9 +199,12 @@ void __init kasan_init(void)
kasan_populate(kasan_mem_to_shadow(start),
kasan_mem_to_shadow(end));
}

- /* Populate kernel, BPF, modules mapping */
+ /* Populate BPF and modules mapping: modules mapping encompasses
BPF mapping */
kasan_populate(kasan_mem_to_shadow((const void *)MODULES_VADDR),
- kasan_mem_to_shadow((const void
*)BPF_JIT_REGION_END));
+ kasan_mem_to_shadow((const void *)MODULES_END));
+ /* Populate kernel mapping */
+ kasan_populate(kasan_mem_to_shadow((const void
*)kernel_map.virt_addr),
+ kasan_mem_to_shadow((const void
*)kernel_map.virt_addr + kernel_map.size));

Without the mentioned patch, replace kernel_map.virt_addr with
kernel_virt_addr and kernel_map.size with load_sz. Note that load_sz was
re-exposed in v6 of the patchset "Map the kernel with correct
permissions the first time".

Alex

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