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

[Alexandre Ghiti <alex@ghiti.fr>]

Hi Guo,

On 29/06/2023 10:20, guoren@kernel.org wrote:
> From: Guo Ren <guoren@linux.alibaba.com>
>
> 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.


> ("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@linux.alibaba.com>
> Signed-off-by: Guo Ren <guoren@kernel.org>
> ---
>   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.