Re: [PATCH V2] riscv: kexec: Fixup synchronization problem between init_mm and active_mm

[Alexandre Ghiti]

Hi Guo,


On 10/07/2023 07:40, guoren@xxxxxxxxxx wrote:
> From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
>
> 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 think it needs more details like this:

"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."


> 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 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
>
> The solution is to fix machine_kexec() to remap control code page outside
> the linear mapping.


"Given the current flaw in the set_memory_x implementation, the simplest 
solution is to ..."


> Fixes: 3335068f8721 ("riscv: Use PUD/P4D/PGD pages for the linear mapping")
> Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
> Cc: Alexandre Ghiti <alex@xxxxxxxx>
> ---
> Changelog:
> 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 | 14 ++++++++++----
>   2 files changed, 11 insertions(+), 4 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/machine_kexec.c
> index 2d139b724bc8..eeb209775107 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 != KEXEC_TYPE_CRASH) {
> -		control_code_buffer = page_address(image->control_code_page);
> +		control_code_buffer = vm_map_ram(&image->control_code_page,
> +						 KEXEC_CONTROL_PAGE_SIZE/PAGE_SIZE,
> +						 NUMA_NO_NODE);
> +		if (control_code_buffer == NULL) {
> +			pr_err("Failed to vm_map control page\n");
> +			return -ENOMEM;
> +		}
> +
>   		control_code_buffer_sz = page_size(image->control_code_page);
>   
>   		if (unlikely(riscv_kexec_relocate_size > control_code_buffer_sz)) {
> @@ -97,8 +104,7 @@ machine_kexec_prepare(struct kimage *image)
>   		memcpy(control_code_buffer, riscv_kexec_relocate,
>   			riscv_kexec_relocate_size);
>   
> -		/* Mark the control page executable */
> -		set_memory_x((unsigned long) control_code_buffer, 1);
> +		internal->control_code_buffer = control_code_buffer;


Where is this mapping marked as executable? I see that vm_map_ram() maps 
the pages as PAGE_KERNEL, which does not set PAGE_EXEC.


>   	}
>   
>   	return 0;
> @@ -211,7 +217,7 @@ machine_kexec(struct kimage *image)
>   	unsigned long this_cpu_id = __smp_processor_id();
>   	unsigned long this_hart_id = cpuid_to_hartid_map(this_cpu_id);
>   	unsigned long fdt_addr = internal->fdt_addr;
> -	void *control_code_buffer = page_address(image->control_code_page);
> +	void *control_code_buffer = internal->control_code_buffer;
>   	riscv_kexec_method kexec_method = NULL;
>   
>   #ifdef CONFIG_SMP


Otherwise, you can add:

Reviewed-by: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx>

Thanks,

Alex