Subject: Re: [PATCH 1/3] kexec: Prevent removal of memory in use by a loaded kexec image

From: Baoquan He <[email-masked]>
Date: Fri, 27 Mar 2020 10:34:39 +0800

On 03/26/20 at 06:07pm, James Morse wrote:
> An image loaded for kexec is not stored in place, instead its segments
> are scattered through memory, and are re-assembled when needed. In the
> meantime, the target memory may have been removed.
> 
> Because mm is not aware that this memory is still in use, it allows it
> to be removed.
> 
> Add a memory notifier to prevent the removal of memory regions that
> overlap with a loaded kexec image segment. e.g., when triggered from the
> Qemu console:
> | kexec_core: memory region in use
> | memory memory32: Offline failed.

As I replied to the cover letter, usually we do loading and juming of
kexec-ed kernel at one time. If we expect to do both of them at
different time, I agree we should do something to make thing safer if
someone really want to do since it's allowed, can we do anything with
the existing notifier?

Mem hotplug has got a notifier to notice it will offline a memory
region.
memory_notify(MEM_OFFLINE, &arg);

> 
> Signed-off-by: James Morse <[email-masked]>
> ---
>  kernel/kexec_core.c | 56 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 56 insertions(+)
> 
> diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> index c19c0dad1ebe..ba1d91e868ca 100644
> --- a/kernel/kexec_core.c
> +++ b/kernel/kexec_core.c
> @@ -12,6 +12,7 @@
>  #include <linux/slab.h>
>  #include <linux/fs.h>
>  #include <linux/kexec.h>
> +#include <linux/memory.h>
>  #include <linux/mutex.h>
>  #include <linux/list.h>
>  #include <linux/highmem.h>
> @@ -22,10 +23,12 @@
>  #include <linux/elf.h>
>  #include <linux/elfcore.h>
>  #include <linux/utsname.h>
> +#include <linux/notifier.h>
>  #include <linux/numa.h>
>  #include <linux/suspend.h>
>  #include <linux/device.h>
>  #include <linux/freezer.h>
> +#include <linux/pfn.h>
>  #include <linux/pm.h>
>  #include <linux/cpu.h>
>  #include <linux/uaccess.h>
> @@ -1219,3 +1222,56 @@ void __weak arch_kexec_protect_crashkres(void)
>  
>  void __weak arch_kexec_unprotect_crashkres(void)
>  {}
> +
> +/*
> + * If user-space wants to offline memory that is in use by a loaded kexec
> + * image, it should unload the image first.
> + */
> +static int mem_remove_cb(struct notifier_block *nb, unsigned long action,
> +			 void *data)
> +{
> +	int rv = NOTIFY_OK, i;
> +	struct memory_notify *arg = data;
> +	unsigned long pfn = arg->start_pfn;
> +	unsigned long nr_segments, sstart, send;
> +	unsigned long end_pfn = arg->start_pfn + arg->nr_pages;
> +
> +	might_sleep();
> +
> +	if (action != MEM_GOING_OFFLINE)
> +		return NOTIFY_DONE;
> +
> +	mutex_lock(&kexec_mutex);
> +	if (kexec_image) {
> +		nr_segments = kexec_image->nr_segments;
> +
> +		for (i = 0; i < nr_segments; i++) {
> +			sstart = PFN_DOWN(kexec_image->segment[i].mem);
> +			send = PFN_UP(kexec_image->segment[i].mem +
> +				      kexec_image->segment[i].memsz);
> +
> +			if ((pfn <= sstart && sstart < end_pfn) ||
> +			    (pfn <= send && send < end_pfn)) {
> +				pr_warn("Memory region in use\n");
> +				rv = NOTIFY_BAD;
> +				break;
> +			}
> +		}
> +	}
> +	mutex_unlock(&kexec_mutex);
> +
> +	return rv;
> +}
> +
> +static struct notifier_block mem_remove_nb = {
> +	.notifier_call = mem_remove_cb,
> +};
> +
> +static int __init register_mem_remove_cb(void)
> +{
> +	if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG))
> +		return register_memory_notifier(&mem_remove_nb);
> +
> +	return 0;
> +}
> +device_initcall(register_mem_remove_cb);
> -- 
> 2.25.1
> 
> 

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