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

Jisheng Zhang
2021-06-17T18:17:29Z

On Fri, 18 Jun 2021 01:46:48 +0800
Jisheng Zhang <jszhang3@mail.ustc.edu.cn> wrote:

> On Fri, 18 Jun 2021 01:27:31 +0800
> Jisheng Zhang <jszhang3@mail.ustc.edu.cn> wrote:
>
> > On Thu, 17 Jun 2021 16:18:54 +0200
> > Alex Ghiti <alex@ghiti.fr> wrote:
> >
> > > 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.
> >
> > I can reproduce this issue.
> >
> > > >
> > > > 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));
> > >
> > If this patch works, maybe we can still use one kasan_populate() to cover
> > kernel, bpf, and module:
> >
> > 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_VADDR + SZ_2G));
> >
>
> I made a mistake. Below patch works:
>
> 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_VADDR + SZ_2G)));

This isn't the key. I knew the reason now. kasan_init() has local vars named
as _start and _end, then MODULES_VADDR is defined as:
#define MODULES_VADDR (PFN_ALIGN((unsigned long)&_end) - SZ_2G)

So MODULES_VADDR isn't what we expected. To fix it, we must rename the local
vars

>
> > However, both can't solve the early boot hang issue. I'm not sure what's missing.
> >
> > I applied your patch on rc6 + solution below "replace kernel_map.virt_addr with kernel_virt_addr and
> > kernel_map.size with load_sz"
> >
> >
> > Thanks
> >
> > >
> > > 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".
> > >
> >
> >
> > _______________________________________________
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> > linux-riscv@lists.infradead.org
> > http://lists.infradead.org/mailman/listinfo/linux-riscv
>