Re: [PATCH] riscv: pageattr: Fixup synchronization problem between init_mm and active_mm

[Guo Ren]

On Wed, Jul 5, 2023 at 3:01 PM Alexandre Ghiti <alex@xxxxxxxx> wrote:
>
>
> On 04/07/2023 04:25, Guo Ren wrote:
> > On Mon, Jul 3, 2023 at 6:17 PM Alexandre Ghiti <alex@xxxxxxxx> wrote:
> >> Hi Guo,
> >>
> >> On 29/06/2023 10:20, guoren@xxxxxxxxxx wrote:
> >>> From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> >>>
> >>> The machine_kexec() uses set_memory_x to add the executable attribute to the
> >>> page table entry of control_code_buffer. It only modifies the init_mm but not
> >>> the current->active_mm. The current kexec process won't use init_mm directly,
> >>> and it depends on minor_pagefault, which is removed by commit 7d3332be011e4
> >>
> >> Is the removal of minor_pagefault an issue? I'm not sure I understand
> >> this part of the changelog.
> > I use two different work-around patches to answer your question:
> > 1st:
> > -----
> > diff --git a/arch/riscv/mm/fault.c b/arch/riscv/mm/fault.c
> > index 705d63a59aec..b8b200c81606 100644
> > --- a/arch/riscv/mm/fault.c
> > +++ b/arch/riscv/mm/fault.c
> > @@ -249,7 +249,7 @@ void handle_page_fault(struct pt_regs *regs)
> > * only copy the information from the master page table,
> > * nothing more.
> > */
> > - if (unlikely((addr >= VMALLOC_START) && (addr < VMALLOC_END))) {
> > + if (unlikely(addr >= 0x8000000000000000UL)) {
> > vmalloc_fault(regs, code, addr);
> > return;
> > }
> > ------
> >
> > 2nd:
> > ------
> > diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> > index 8e65f0a953e5..270f50852886 100644
> > --- a/arch/riscv/mm/init.c
> > +++ b/arch/riscv/mm/init.c
> > @@ -1387,7 +1387,7 @@ static void __init create_linear_mapping_page_table(void)
> > if (end >= __pa(PAGE_OFFSET) + memory_limit)
> > end = __pa(PAGE_OFFSET) + memory_limit;
> >
> > - create_linear_mapping_range(start, end, 0);
> > + create_linear_mapping_range(start, end, PMD_SIZE);
> > }
> >
> > #ifdef CONFIG_STRICT_KERNEL_RWX
> > -----
> >
> > The removal of minor_pagefault could be an issue, but in this case
> > it's the VMALLOC_START/END which prevents the minor_pagefault at
> > first. I didn't say commit 7d3332be011e4 is the problem.
>
>
> Sorry I still don't understand what you mean here and why you mention
> the minor pagefault, could you explain again please?
>
>
> >>
> >>> ("riscv: mm: Pre-allocate PGD entries for vmalloc/modules area") of 64BIT. So,
> >>> when it met pud mapping on an MMU_SV39 machine, it caused the following:
> >>>
> >>>    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 ffffffda23b0d000
> >>>    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: 000000000000000c
> >>>
> >>> Yes, Using set_memory_x API after boot has the limitation, and at least we
> >>> should synchronize the current->active_mm to fix the problem.
> >>>
> >>> Fixes: d3ab332a5021 ("riscv: add ARCH_HAS_SET_MEMORY support")
> >>> Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> >>> Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
> >>> ---
> >>>    arch/riscv/mm/pageattr.c | 7 +++++++
> >>>    1 file changed, 7 insertions(+)
> >>>
> >>> diff --git a/arch/riscv/mm/pageattr.c b/arch/riscv/mm/pageattr.c
> >>> index ea3d61de065b..23d169c4ee81 100644
> >>> --- a/arch/riscv/mm/pageattr.c
> >>> +++ b/arch/riscv/mm/pageattr.c
> >>> @@ -123,6 +123,13 @@ static int __set_memory(unsigned long addr, int numpages, pgprot_t set_mask,
> >>>                                     &masks);
> >>>        mmap_write_unlock(&init_mm);
> >>>
> >>> +     if (current->active_mm != &init_mm) {
> >>> +             mmap_write_lock(current->active_mm);
> >>> +             ret =  walk_page_range_novma(current->active_mm, start, end,
> >>> +                                          &pageattr_ops, NULL, &masks);
> >>> +             mmap_write_unlock(current->active_mm);
> >>> +     }
> >>> +
> >>>        flush_tlb_kernel_range(start, end);
> >>>
> >>>        return ret;
> >>
> >> I don't understand: any page table inherits the entries of
> >> swapper_pg_dir (see pgd_alloc()), so any kernel page table entry is
> >> "automatically" synchronized, so why should we synchronize one 4K entry
> >> explicitly? A PGD entry would need to be synced, but not a PTE entry.
> > The purpose of the second walk_page_range_novma() is for pgd's entries
> > synchronization. I'm a bit lazy here, I agree, it's unnecessary to
> > write lower level entries again. So I would use a simple pgd entries
> > synchronization from vmalloc_fault in the next version of patch, all
> > right?
>
>
> But vmalloc_fault was removed by commit 7d3332be011e4 for CONFIG_64BIT,
> so I don't get it: why would we need to synchronize a PGD entry in your
> case? Where does this new PGD come from? And the trap address is
> ffffffda23b0d000, which lies in the direct mapping, so why do you
> mention vmalloc_fault at all?
The machine_kexec() uses set_memory_x to modify the direct mapping
attributes from RW to RWX. But set_memory_x only changes the init_mm's
attributes, not current->active_mm, so when kexec jumps into
control_buffer, the instruction page fault happens, and there is no
minor_pagefault for it, then panic.

I found the bug on an MMU_sv39 machine, and the direct mapping used a
1GB PUD, the pgd entries. This patch could solve the problem by
synchronizing a PGD entry between init_mm and current->active_mm.

>
> Sorry if I'm missing something, hope you can clarify things!
>
> Thanks,
>
> Alex
>
>
> >
> >
> > --
> > Best Regards
> >   Guo Ren
> >
> > _______________________________________________
> > linux-riscv mailing list
> > linux-riscv@xxxxxxxxxxxxxxxxxxx
> > http://lists.infradead.org/mailman/listinfo/linux-riscv



-- 
Best Regards
 Guo Ren