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References: <20230714103659.3146949-1-guoren@kernel.org> <ad6e6873-5555-624b-6050-dbbcefb91e5a@ghiti.fr>
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From:   Guo Ren <guoren@kernel.org>
Date:   Fri, 21 Jul 2023 20:14:42 -0400
Message-ID: <CAJF2gTTwzYDCnnVHKV7dPccu8+ynxaNR3B8cK7gsLarswoh03A@mail.gmail.com>
Subject: Re: [PATCH V4] riscv: kexec: Fixup synchronization problem between
 init_mm and active_mm
To:     Alexandre Ghiti <alex@ghiti.fr>
Cc:     palmer@rivosinc.com, paul.walmsley@sifive.com, zong.li@sifive.com,
        atishp@atishpatra.org, jszhang@kernel.org, bjorn@kernel.org,
        xingxg2008@163.com, linux-arch@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org,
        Guo Ren <guoren@linux.alibaba.com>,
        Alexandre Ghiti <alexghiti@rivosinc.com>
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On Thu, Jul 20, 2023 at 4:28=E2=80=AFAM Alexandre Ghiti <alex@ghiti.fr> wro=
te:
>
>
> On 18/07/2023 14:30, Guo Ren wrote:
> > On Mon, Jul 17, 2023 at 9:17=E2=80=AFPM Alexandre Ghiti <alex@ghiti.fr>=
 wrote:
> >>
> >> On 14/07/2023 12:36, guoren@kernel.org wrote:
> >>> From: Guo Ren <guoren@linux.alibaba.com>
> >>>
> >>> The machine_kexec() uses set_memory_x to modify the direct mapping
> >>> attributes from RW to RWX. The current implementation of set_memory_x
> >>> does not split hugepages in the linear mapping and then when a PGD
> >>> mapping is used, the whole PGD is marked as executable. But changing
> >>> the permissions at the PGD level must be propagated to all the page
> >>> tables. When kexec jumps into control_buffer, the instruction page
> >>> fault happens, and there is no minor_pagefault for it, then panic.
> >>>
> >>> The bug is found on an MMU_sv39 machine, and the direct mapping used =
a
> >>> 1GB PUD, the pgd entries. Here is the bug output:
> >>>
> >>>    kexec_core: Starting new kernel
> >>>    Will call new kernel at 00300000 from hart id 0
> >>>    FDT image at 747c7000
> >>>    Bye...
> >>>    Unable to handle kernel paging request at virtual address ffffffda=
23b0d000
> >>>    Oops [#1]
> >>>    Modules linked in:
> >>>    CPU: 0 PID: 53 Comm: uinit Not tainted 6.4.0-rc6 #15
> >>>    Hardware name: Sophgo Mango (DT)
> >>>    epc : 0xffffffda23b0d000
> >>>     ra : machine_kexec+0xa6/0xb0
> >>>    epc : ffffffda23b0d000 ra : ffffffff80008272 sp : ffffffc80c173d10
> >>>     gp : ffffffff8150e1e0 tp : ffffffd9073d2c40 t0 : 0000000000000000
> >>>     t1 : 0000000000000042 t2 : 6567616d69205444 s0 : ffffffc80c173d50
> >>>     s1 : ffffffd9076c4800 a0 : ffffffd9076c4800 a1 : 0000000000300000
> >>>     a2 : 00000000747c7000 a3 : 0000000000000000 a4 : ffffffd800000000
> >>>     a5 : 0000000000000000 a6 : ffffffd903619c40 a7 : ffffffffffffffff
> >>>     s2 : ffffffda23b0d000 s3 : 0000000000300000 s4 : 00000000747c7000
> >>>     s5 : 0000000000000000 s6 : 0000000000000000 s7 : 0000000000000000
> >>>     s8 : 0000000000000000 s9 : 0000000000000000 s10: 0000000000000000
> >>>     s11: 0000003f940001a0 t3 : ffffffff815351af t4 : ffffffff815351af
> >>>     t5 : ffffffff815351b0 t6 : ffffffc80c173b50
> >>>    status: 0000000200000100 badaddr: ffffffda23b0d000 cause: 00000000=
0000000c
> >>>
> >>> Given the current flaw in the set_memory_x implementation, the simple=
st
> >>> solution is to fix machine_kexec() to remap control code page outside
> >>> the linear mapping. Because the control code buffer was moved from th=
e
> >>> direct mapping area to the vmalloc location, we need an additional
> >>> va_va_offset to fix up va_pa_offset.
> >>>
> >>> Fixes: 3335068f8721 ("riscv: Use PUD/P4D/PGD pages for the linear map=
ping")
> >>> Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> >>> Reported-by: Xing XiaoGuang <xingxg2008@163.com>
> >>> Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
> >>> Signed-off-by: Guo Ren <guoren@kernel.org>
> >>> ---
> >>> Changelog:
> >>> V4:
> >>>    - Fixup va_pa_offset with additional va_va_offset.
> >>>    - Add Reported-by tag.
> >>>
> >>> V3:
> >>>    - Resume set_memory_x to set the _PAGE_EXEC attribute
> >>>    - Optimize the commit log with Alexandre advice
> >>>
> >>> V2:
> >>>    - Use vm_map_ram instead of modifying set_memory_x
> >>>    - Correct Fixes tag
> >>> ---
> >>>    arch/riscv/include/asm/kexec.h    |  1 +
> >>>    arch/riscv/kernel/machine_kexec.c | 18 +++++++++++++++---
> >>>    2 files changed, 16 insertions(+), 3 deletions(-)
> >>>
> >>> diff --git a/arch/riscv/include/asm/kexec.h b/arch/riscv/include/asm/=
kexec.h
> >>> index 2b56769cb530..17456e91476e 100644
> >>> --- a/arch/riscv/include/asm/kexec.h
> >>> +++ b/arch/riscv/include/asm/kexec.h
> >>> @@ -41,6 +41,7 @@ crash_setup_regs(struct pt_regs *newregs,
> >>>    struct kimage_arch {
> >>>        void *fdt; /* For CONFIG_KEXEC_FILE */
> >>>        unsigned long fdt_addr;
> >>> +     void *control_code_buffer;
> >>>    };
> >>>
> >>>    extern const unsigned char riscv_kexec_relocate[];
> >>> diff --git a/arch/riscv/kernel/machine_kexec.c b/arch/riscv/kernel/ma=
chine_kexec.c
> >>> index 2d139b724bc8..60c1ef3c2232 100644
> >>> --- a/arch/riscv/kernel/machine_kexec.c
> >>> +++ b/arch/riscv/kernel/machine_kexec.c
> >>> @@ -86,7 +86,14 @@ machine_kexec_prepare(struct kimage *image)
> >>>
> >>>        /* Copy the assembler code for relocation to the control page =
*/
> >>>        if (image->type !=3D KEXEC_TYPE_CRASH) {
> >>> -             control_code_buffer =3D page_address(image->control_cod=
e_page);
> >>> +             control_code_buffer =3D vm_map_ram(&image->control_code=
_page,
> >>> +                                              KEXEC_CONTROL_PAGE_SIZ=
E/PAGE_SIZE,
> >>> +                                              NUMA_NO_NODE);
> >>> +             if (control_code_buffer =3D=3D NULL) {
> >>> +                     pr_err("Failed to vm_map control page\n");
> >>> +                     return -ENOMEM;
> >>> +             }
> >>> +
> >>>                control_code_buffer_sz =3D page_size(image->control_co=
de_page);
> >>>
> >>>                if (unlikely(riscv_kexec_relocate_size > control_code_=
buffer_sz)) {
> >>> @@ -99,6 +106,8 @@ machine_kexec_prepare(struct kimage *image)
> >>>
> >>>                /* Mark the control page executable */
> >>>                set_memory_x((unsigned long) control_code_buffer, 1);
> >>> +
> >>> +             internal->control_code_buffer =3D control_code_buffer;
> >>>        }
> >>>
> >>>        return 0;
> >>> @@ -211,7 +220,10 @@ machine_kexec(struct kimage *image)
> >>>        unsigned long this_cpu_id =3D __smp_processor_id();
> >>>        unsigned long this_hart_id =3D cpuid_to_hartid_map(this_cpu_id=
);
> >>>        unsigned long fdt_addr =3D internal->fdt_addr;
> >>> -     void *control_code_buffer =3D page_address(image->control_code_=
page);
> >>> +     void *control_code_buffer =3D internal->control_code_buffer;
> >>> +     unsigned long va_va_offset =3D
> >>> +                     (unsigned long) page_address(image->control_cod=
e_page)
> >>> +                   - (unsigned long) control_code_buffer;
> >>>        riscv_kexec_method kexec_method =3D NULL;
> >>>
> >>>    #ifdef CONFIG_SMP
> >>> @@ -234,6 +246,6 @@ machine_kexec(struct kimage *image)
> >>>        /* Jump to the relocation code */
> >>>        pr_notice("Bye...\n");
> >>>        kexec_method(first_ind_entry, jump_addr, fdt_addr,
> >>> -                  this_hart_id, kernel_map.va_pa_offset);
> >>> +                  this_hart_id, kernel_map.va_pa_offset - va_va_offs=
et);
> >>>        unreachable();
> >>>    }
> >>
> >> I started working on the set_memory fix and the first thing to do is t=
o
> >> prevent the use of PGD mapping, it's too cumbersome to propagate chang=
es
> >> at this level: IIRC x86 keeps a list of page tables to go through
> >> whenever that happens, that's why Bjorn pre-allocated all the PGD
> >> entries to cover the vmalloc region.
> >>
> >> So, to me, the simple fix for this issue is to prevent the use of PGD
> >> mapping. What do you think? Does the following patch work?
> > The PGD mapping is necessary (especially for Sv32, Sv39), and it has
> > been solved under your first advice. I think limit set_memory_x usage
> > is the smart choice for now.
>
>
> If we use PGD mappings, we won't be able to change the protections or
> remove pages from the linear mapping as we don't have a means to
> synchronize all the page tables. Removing pages from the linear mapping
> is used for example for memfd_secret and certainly other things so that
> means using PGD mappings will break a few things.
>
> The benefits of PGD mappings for the linear mapping was not proven, and
We need PGD mappings for Sv32. liner mapping with 4KB would cause a
significant performance gap in the small core (memset stride with 4KB,
PTW with every store).

> even if I agree we should do our best to keep the largest mapping
> possible just in case, implementing a page table synchronization seems
> very cumbersome (x86 does that though).
>
> I'm not opposed to your solution, but:
>
> - either we remove the PGD mappings, and your problem is fixed,
This patch limits control_buffer_code into a 4KB page mapping entry. I
don't think they are the same solution, even you remove the PGD
mappings.

>
> - or we keep PGD mappings, implement a page table synchronization, and
> your problem is fixed.
We need your synchronization, but not for this case.

>
> And since we need to fix this larger problem, I don't see the point of
> this workaround.
This patch is for kexec. See title riscv: kexec: tag.

And I don't think your larger problem needs to be fixed. I agree with
your first point that simply limit the usage of set_memory_x.

>
> Thanks,
>
> Alex
>
>
> >
> >
> >>
> >> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> >> index 70fb31960b63..6dd12443bfa4 100644
> >> --- a/arch/riscv/mm/init.c
> >> +++ b/arch/riscv/mm/init.c
> >> @@ -662,13 +662,12 @@ void __init create_pgd_mapping(pgd_t *pgdp,
> >>    static uintptr_t __init best_map_size(phys_addr_t pa, uintptr_t va,
> >>                                         phys_addr_t size)
> >>    {
> >> -       if (!(pa & (PGDIR_SIZE - 1)) && !(va & (PGDIR_SIZE - 1)) && si=
ze
> >>   >=3D PGDIR_SIZE)
> >> -               return PGDIR_SIZE;
> >> -
> >> -       if (!(pa & (P4D_SIZE - 1)) && !(va & (P4D_SIZE - 1)) && size >=
=3D
> >> P4D_SIZE)
> >> +       if (pgtable_l5_enabled &&
> >> +           !(pa & (P4D_SIZE - 1)) && !(va & (P4D_SIZE - 1)) && size >=
=3D
> >> P4D_SIZE)
> >>                   return P4D_SIZE;
> >>
> >> -       if (!(pa & (PUD_SIZE - 1)) && !(va & (PUD_SIZE - 1)) && size >=
=3D
> >> PUD_SIZE)
> >> +       if (pgtable_l4_enabled &&
> >> +           !(pa & (PUD_SIZE - 1)) && !(va & (PUD_SIZE - 1)) && size >=
=3D
> >> PUD_SIZE)
> >>                   return PUD_SIZE;
> >>
> >>           if (!(pa & (PMD_SIZE - 1)) && !(va & (PMD_SIZE - 1)) && size=
 >=3D
> >> PMD_SIZE)
> >>
> >>
> >> Thanks,
> >>
> >> Alex
> >>
> >


--=20
Best Regards
 Guo Ren