Re: [PATCH v4 3/3] kdump: Protect vmcoreinfo data under the crash memory

[Xunlei Pang]

On 04/26/2017 at 03:09 PM, Dave Young wrote:
> On 04/20/17 at 07:39pm, Xunlei Pang wrote:
>> Currently vmcoreinfo data is updated at boot time subsys_initcall(),
>> it has the risk of being modified by some wrong code during system
>> is running.
>>
>> As a result, vmcore dumped may contain the wrong vmcoreinfo. Later on,
>> when using "crash", "makedumpfile", etc utility to parse this vmcore,
>> we probably will get "Segmentation fault" or other unexpected errors.
>>
>> E.g. 1) wrong code overwrites vmcoreinfo_data; 2) further crashes the
>> system; 3) trigger kdump, then we obviously will fail to recognize the
>> crash context correctly due to the corrupted vmcoreinfo.
>>
>> Now except for vmcoreinfo, all the crash data is well protected(including
>> the cpu note which is fully updated in the crash path, thus its correctness
>> is guaranteed). Given that vmcoreinfo data is a large chunk prepared for
>> kdump, we better protect it as well.
>>
>> To solve this, we relocate and copy vmcoreinfo_data to the crash memory
>> when kdump is loading via kexec syscalls. Because the whole crash memory
>> will be protected by existing arch_kexec_protect_crashkres() mechanism,
>> we naturally protect vmcoreinfo_data from write(even read) access under
>> kernel direct mapping after kdump is loaded.
>>
>> Since kdump is usually loaded at the very early stage after boot, we can
>> trust the correctness of the vmcoreinfo data copied.
>>
>> On the other hand, we still need to operate the vmcoreinfo safe copy when
>> crash happens to generate vmcoreinfo_note again, we rely on vmap() to map
>> out a new kernel virtual address and update to use this new one instead in
>> the following crash_save_vmcoreinfo().
>>
>> BTW, we do not touch vmcoreinfo_note, because it will be fully updated
>> using the protected vmcoreinfo_data after crash which is surely correct
>> just like the cpu crash note.
>>
>> Cc: Michael Holzheu <holzheu@xxxxxxxxxxxxxxxxxx>
>> Signed-off-by: Xunlei Pang <xlpang@xxxxxxxxxx>
>> ---
>> v3->v4:
>> -Rebased on the latest linux-next
>> -Copy vmcoreinfo after machine_kexec_prepare()
>>
>>  include/linux/crash_core.h |  2 +-
>>  include/linux/kexec.h      |  2 ++
>>  kernel/crash_core.c        | 17 ++++++++++++++++-
>>  kernel/kexec.c             |  8 ++++++++
>>  kernel/kexec_core.c        | 39 +++++++++++++++++++++++++++++++++++++++
>>  kernel/kexec_file.c        |  8 ++++++++
>>  6 files changed, 74 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/crash_core.h b/include/linux/crash_core.h
>> index 7d6bc7b..5469adb 100644
>> --- a/include/linux/crash_core.h
>> +++ b/include/linux/crash_core.h
>> @@ -23,6 +23,7 @@
>>  
>>  typedef u32 note_buf_t[CRASH_CORE_NOTE_BYTES/4];
>>  
>> +void crash_update_vmcoreinfo_safecopy(void *ptr);
>>  void crash_save_vmcoreinfo(void);
>>  void arch_crash_save_vmcoreinfo(void);
>>  __printf(1, 2)
>> @@ -54,7 +55,6 @@
>>  	vmcoreinfo_append_str("PHYS_BASE=%lx\n", (unsigned long)value)
>>  
>>  extern u32 *vmcoreinfo_note;
>> -extern size_t vmcoreinfo_size;
>>  
>>  Elf_Word *append_elf_note(Elf_Word *buf, char *name, unsigned int type,
>>  			  void *data, size_t data_len);
>> diff --git a/include/linux/kexec.h b/include/linux/kexec.h
>> index c9481eb..3ea8275 100644
>> --- a/include/linux/kexec.h
>> +++ b/include/linux/kexec.h
>> @@ -181,6 +181,7 @@ struct kimage {
>>  	unsigned long start;
>>  	struct page *control_code_page;
>>  	struct page *swap_page;
>> +	void *vmcoreinfo_data_copy; /* locates in the crash memory */
>>  
>>  	unsigned long nr_segments;
>>  	struct kexec_segment segment[KEXEC_SEGMENT_MAX];
>> @@ -250,6 +251,7 @@ extern void *kexec_purgatory_get_symbol_addr(struct kimage *image,
>>  int kexec_should_crash(struct task_struct *);
>>  int kexec_crash_loaded(void);
>>  void crash_save_cpu(struct pt_regs *regs, int cpu);
>> +extern int kimage_crash_copy_vmcoreinfo(struct kimage *image);
>>  
>>  extern struct kimage *kexec_image;
>>  extern struct kimage *kexec_crash_image;
>> diff --git a/kernel/crash_core.c b/kernel/crash_core.c
>> index 43cdb00..a29e9ad 100644
>> --- a/kernel/crash_core.c
>> +++ b/kernel/crash_core.c
>> @@ -15,9 +15,12 @@
>>  
>>  /* vmcoreinfo stuff */
>>  static unsigned char *vmcoreinfo_data;
>> -size_t vmcoreinfo_size;
>> +static size_t vmcoreinfo_size;
>>  u32 *vmcoreinfo_note;
>>  
>> +/* trusted vmcoreinfo, e.g. we can make a copy in the crash memory */
> May make it clearer like:
> /* Trusted vmcoreinfo copy in the kdump reserved memory */

My thought is that it is in crash_core.c now which should be independent of kexec/kdump,
so I used "e.g. ..." just like one use case.

>
>> +static unsigned char *vmcoreinfo_data_safecopy;
>> +
>>  /*
>>   * parsing the "crashkernel" commandline
>>   *
>> @@ -323,11 +326,23 @@ static void update_vmcoreinfo_note(void)
>>  	final_note(buf);
>>  }
>>  
>> +void crash_update_vmcoreinfo_safecopy(void *ptr)
>> +{
>> +	if (ptr)
>> +		memcpy(ptr, vmcoreinfo_data, vmcoreinfo_size);
>> +
>> +	vmcoreinfo_data_safecopy = ptr;
>> +}
>> +
>>  void crash_save_vmcoreinfo(void)
>>  {
>>  	if (!vmcoreinfo_note)
>>  		return;
>>  
>> +	/* Use the safe copy to generate vmcoreinfo note if have */
>> +	if (vmcoreinfo_data_safecopy)
>> +		vmcoreinfo_data = vmcoreinfo_data_safecopy;
>> +
>>  	vmcoreinfo_append_str("CRASHTIME=%ld\n", get_seconds());
>>  	update_vmcoreinfo_note();
>>  }
>> diff --git a/kernel/kexec.c b/kernel/kexec.c
>> index 980936a..e62ec4d 100644
>> --- a/kernel/kexec.c
>> +++ b/kernel/kexec.c
>> @@ -144,6 +144,14 @@ static int do_kexec_load(unsigned long entry, unsigned long nr_segments,
>>  	if (ret)
>>  		goto out;
>>  
>> +	/*
>> +	 * Some architecture(like S390) may touch the crash memory before
>> +	 * machine_kexec_prepare(), we must copy vmcoreinfo data after it.
>> +	 */
>> +	ret = kimage_crash_copy_vmcoreinfo(image);
>> +	if (ret)
>> +		goto out;
>> +
>>  	for (i = 0; i < nr_segments; i++) {
>>  		ret = kimage_load_segment(image, &image->segment[i]);
>>  		if (ret)
>> diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
>> index ae1a3ba..bac1b4f 100644
>> --- a/kernel/kexec_core.c
>> +++ b/kernel/kexec_core.c
>> @@ -481,6 +481,40 @@ struct page *kimage_alloc_control_pages(struct kimage *image,
>>  	return pages;
>>  }
>>  
>> +int kimage_crash_copy_vmcoreinfo(struct kimage *image)
>> +{
>> +	struct page *vmcoreinfo_page;
>> +	void *safecopy;
>> +
>> +	if (image->type != KEXEC_TYPE_CRASH)
>> +		return 0;
>> +
>> +	/*
>> +	 * For kdump, allocate one vmcoreinfo safe copy from the
>> +	 * crash memory. as we have arch_kexec_protect_crashkres()
>> +	 * after kexec syscall, we naturally protect it from write
>> +	 * (even read) access under kernel direct mapping. But on
>> +	 * the other hand, we still need to operate it when crash
>> +	 * happens to generate vmcoreinfo note, hereby we rely on
>> +	 * vmap for this purpose.
>> +	 */
>> +	vmcoreinfo_page = kimage_alloc_control_pages(image, 0);
>> +	if (!vmcoreinfo_page) {
>> +		pr_warn("Could not allocate vmcoreinfo buffer\n");
>> +		return -ENOMEM;
>> +	}
>> +	safecopy = vmap(&vmcoreinfo_page, 1, VM_MAP, PAGE_KERNEL);
>> +	if (!safecopy) {
>> +		pr_warn("Could not vmap vmcoreinfo buffer\n");
>> +		return -ENOMEM;
>> +	}
>> +
>> +	image->vmcoreinfo_data_copy = safecopy;
>> +	crash_update_vmcoreinfo_safecopy(safecopy);
>> +
>> +	return 0;
>> +}
>> +
>>  static int kimage_add_entry(struct kimage *image, kimage_entry_t entry)
>>  {
>>  	if (*image->entry != 0)
>> @@ -598,6 +632,11 @@ void kimage_free(struct kimage *image)
>>  	if (image->file_mode)
>>  		kimage_file_post_load_cleanup(image);
>>  
>> +	if (image->vmcoreinfo_data_copy) {
>> +		crash_update_vmcoreinfo_safecopy(NULL);
>> +		vunmap(image->vmcoreinfo_data_copy);
>> +	}
>> +
> Should move above chunk before the freeing of the actual page?

It should be fine, because it is allocated from the reserved memory, it doesn't
need to be freed. Anyway I can move it above to avoid confusion. Thanks!

Regards,
Xunlei

>
>>  	kfree(image);
>>  }
>>  
>> diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
>> index b118735..f78f719 100644
>> --- a/kernel/kexec_file.c
>> +++ b/kernel/kexec_file.c
>> @@ -304,6 +304,14 @@ void kimage_file_post_load_cleanup(struct kimage *image)
>>  	if (ret)
>>  		goto out;
>>  
>> +	/*
>> +	 * Some architecture(like S390) may touch the crash memory before
>> +	 * machine_kexec_prepare(), we must copy vmcoreinfo data after it.
>> +	 */
>> +	ret = kimage_crash_copy_vmcoreinfo(image);
>> +	if (ret)
>> +		goto out;
>> +
>>  	ret = kexec_calculate_store_digests(image);
>>  	if (ret)
>>  		goto out;
>> -- 
>> 1.8.3.1
>>
>>
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> Thanks
> Dave
>
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