Re: [PATCH v29 3/9] arm64: kdump: reserve memory for crash dump kernel

[Mark Rutland <mark.rutland@arm.com>]

On Tue, Jan 17, 2017 at 05:20:44PM +0900, AKASHI Takahiro wrote:
> On Fri, Jan 13, 2017 at 11:39:15AM +0000, Mark Rutland wrote:
> > Great! I think it would be better to follow the approach of
> > mark_rodata_ro(), rather than opening up set_memory_*(), but otherwise,
> > it looks like it should work.
> 
> I'm not quite sure what the approach of mark_rodata_ro() means, but
> I found that using create_mapping_late() may cause two problems:
> 
> 1) it fails when PTE_CONT bits mismatch between an old and new mmu entry.
>    This can happen, say, if the memory range for crash dump kernel
>    starts in the mid of _continuous_ pages.

That should only happen if we try to remap a segment different to what
we originally mapped.

I was intending that we'd explicitly map the reserved region separately
in the boot path, like we do for kernel segments in map_kernel(). We
would allow sections and/or CONT entires. 

Then, in __map_memblock() we'd then skip that range as we do for the
linear map alias of the kernel image.

That way, we can later use create_mapping_late for that same region, and
it should handle sections and/or CONT entries in the exact same way as
it does for the kernel image segments in mark_rodata_ro().

> 2) The control code page, of one-page size, is still written out in
>    machine_kexec() which is called at a crash, and this means that
>    the range must be writable even after kexec_load(), but
>    create_mapping_late() does not handle a case of changing attributes
>    for a single page which is in _section_ mapping.
>    We cannot make single-page mapping for the control page since the address
>    of that page is not determined at the boot time.

That is a problem. I'm not sure I follow how set_memory_*() helps here
though?

> As for (1), we need to call memblock_isolate_range() to make the region
> an independent one.
> 
> > Either way, this still leaves us with an RO alias on crashed cores (and
> > potential cache attribute mismatches in future). Do we need to read from
> > the region later,
> 
> I believe not, but the region must be _writable_ as I mentioned in (2) above.
> To avoid this issue, we have to move copying the control code page
> to machine_kexec_prepare() which is called in kexec_load() and so
> the region is writable anyway then.
> I want Geoff to affirm that this change is safe.
> 
> (See my second solution below.)

From a quick scan that looks ok.

> > or could we unmap it entirely?
> 
> given the change above, I think we can.

Great!

> Is there any code to re-use especially for unmapping?

I don't think we have much code useful for unmapping. We could re-use 
create_mapping_late for this, passing a set of prot bits that means the
entries are invalid (e.g. have a PAGE_KERNEL_INVALID).

We'd have to perform the TLB invalidation ourselves, but that shouldn't
be too painful.

Thanks,
Mark.

> ===8<===
> diff --git a/arch/arm64/kernel/machine_kexec.c b/arch/arm64/kernel/machine_kexec.c
> index c0fc3d458195..80a52e9aaf73 100644
> --- a/arch/arm64/kernel/machine_kexec.c
> +++ b/arch/arm64/kernel/machine_kexec.c
> @@ -26,8 +26,6 @@
>  extern const unsigned char arm64_relocate_new_kernel[];
>  extern const unsigned long arm64_relocate_new_kernel_size;
>  
> -static unsigned long kimage_start;
> -
>  /**
>   * kexec_image_info - For debugging output.
>   */
> @@ -68,7 +66,7 @@ void machine_kexec_cleanup(struct kimage *kimage)
>   */
>  int machine_kexec_prepare(struct kimage *kimage)
>  {
> -	kimage_start = kimage->start;
> +	void *reboot_code_buffer;
>  
>  	kexec_image_info(kimage);
>  
> @@ -77,6 +75,21 @@ int machine_kexec_prepare(struct kimage *kimage)
>  		return -EBUSY;
>  	}
>  
> +	reboot_code_buffer =
> +			phys_to_virt(page_to_phys(kimage->control_code_page));
> +
> +	/*
> +	 * Copy arm64_relocate_new_kernel to the reboot_code_buffer for use
> +	 * after the kernel is shut down.
> +	 */
> +	memcpy(reboot_code_buffer, arm64_relocate_new_kernel,
> +		arm64_relocate_new_kernel_size);
> +
> +	/* Flush the reboot_code_buffer in preparation for its execution. */
> +	__flush_dcache_area(reboot_code_buffer, arm64_relocate_new_kernel_size);
> +	flush_icache_range((uintptr_t)reboot_code_buffer,
> +		arm64_relocate_new_kernel_size);
> +
>  	return 0;
>  }
>  
> @@ -147,7 +160,6 @@ static void kexec_segment_flush(const struct kimage *kimage)
>  void machine_kexec(struct kimage *kimage)
>  {
>  	phys_addr_t reboot_code_buffer_phys;
> -	void *reboot_code_buffer;
>  
>  	/*
>  	 * New cpus may have become stuck_in_kernel after we loaded the image.
> @@ -156,7 +168,6 @@ void machine_kexec(struct kimage *kimage)
>  			!WARN_ON(kimage == kexec_crash_image));
>  
>  	reboot_code_buffer_phys = page_to_phys(kimage->control_code_page);
> -	reboot_code_buffer = phys_to_virt(reboot_code_buffer_phys);
>  
>  	kexec_image_info(kimage);
>  
> @@ -164,26 +175,12 @@ void machine_kexec(struct kimage *kimage)
>  		kimage->control_code_page);
>  	pr_debug("%s:%d: reboot_code_buffer_phys:  %pa\n", __func__, __LINE__,
>  		&reboot_code_buffer_phys);
> -	pr_debug("%s:%d: reboot_code_buffer:       %p\n", __func__, __LINE__,
> -		reboot_code_buffer);
>  	pr_debug("%s:%d: relocate_new_kernel:      %p\n", __func__, __LINE__,
>  		arm64_relocate_new_kernel);
>  	pr_debug("%s:%d: relocate_new_kernel_size: 0x%lx(%lu) bytes\n",
>  		__func__, __LINE__, arm64_relocate_new_kernel_size,
>  		arm64_relocate_new_kernel_size);
>  
> -	/*
> -	 * Copy arm64_relocate_new_kernel to the reboot_code_buffer for use
> -	 * after the kernel is shut down.
> -	 */
> -	memcpy(reboot_code_buffer, arm64_relocate_new_kernel,
> -		arm64_relocate_new_kernel_size);
> -
> -	/* Flush the reboot_code_buffer in preparation for its execution. */
> -	__flush_dcache_area(reboot_code_buffer, arm64_relocate_new_kernel_size);
> -	flush_icache_range((uintptr_t)reboot_code_buffer,
> -		arm64_relocate_new_kernel_size);
> -
>  	/* Flush the kimage list and its buffers. */
>  	kexec_list_flush(kimage);
>  
> @@ -206,7 +203,7 @@ void machine_kexec(struct kimage *kimage)
>  	 */
>  
>  	cpu_soft_restart(kimage != kexec_crash_image,
> -		reboot_code_buffer_phys, kimage->head, kimage_start, 0);
> +		reboot_code_buffer_phys, kimage->head, kimage->start, 0);
>  
>  	BUG(); /* Should never get here. */
>  }
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index 569ec3325bc8..e4cc170edc0c 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -90,6 +90,7 @@ early_param("initrd", early_initrd);
>  static void __init reserve_crashkernel(void)
>  {
>  	unsigned long long crash_size, crash_base;
> +	int start_rgn, end_rgn;
>  	int ret;
>  
>  	ret = parse_crashkernel(boot_command_line, memblock_phys_mem_size(),
> @@ -120,6 +121,8 @@ static void __init reserve_crashkernel(void)
>  			return;
>  		}
>  	}
> +	memblock_isolate_range(&memblock.memory, crash_base, crash_size,
> +			&start_rgn, &end_rgn);
>  	memblock_reserve(crash_base, crash_size);
>  
>  	pr_info("Reserving %lldMB of memory at %lldMB for crashkernel\n",
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 17243e43184e..b7c75845407a 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -22,6 +22,8 @@
>  #include <linux/kernel.h>
>  #include <linux/errno.h>
>  #include <linux/init.h>
> +#include <linux/ioport.h>
> +#include <linux/kexec.h>
>  #include <linux/libfdt.h>
>  #include <linux/mman.h>
>  #include <linux/nodemask.h>
> @@ -817,3 +819,27 @@ int pmd_clear_huge(pmd_t *pmd)
>  	pmd_clear(pmd);
>  	return 1;
>  }
> +
> +#ifdef CONFIG_KEXEC_CORE
> +void arch_kexec_protect_crashkres(void)
> +{
> +	flush_tlb_all();
> +
> +	create_mapping_late(crashk_res.start, __phys_to_virt(crashk_res.start),
> +			    resource_size(&crashk_res), PAGE_KERNEL_RO);
> +
> +	/* flush the TLBs after updating live kernel mappings */
> +	flush_tlb_all();
> +}
> +
> +void arch_kexec_unprotect_crashkres(void)
> +{
> +	flush_tlb_all();
> +
> +	create_mapping_late(crashk_res.start, __phys_to_virt(crashk_res.start),
> +			    resource_size(&crashk_res), PAGE_KERNEL);
> +
> +	/* flush the TLBs after updating live kernel mappings */
> +	flush_tlb_all();
> +}
> +#endif
> ===>8===

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