Re: [PATCH] Revert "Revert "kdump, vmcoreinfo: report memory sections virtual addresses""

[Thomas Garnier]

On Thu, Dec 15, 2016 at 10:16 AM, Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
> On Thu, Dec 15, 2016 at 9:50 AM, Eric W. Biederman
> <ebiederm@xxxxxxxxxxxx> wrote:
>> Thomas Garnier <thgarnie@xxxxxxxxxx> writes:
>>
>>> This reverts commit 49fd897573c97b0eaf10f47d850027d78c456cd7.
>>>
>>> Reverting back to commit 0549a3c because the values are used by crash
>>> and other tools already. I expected this commit would not go through given
>>> the unresolved comments. I want it to be easy to resolve major memory
>>> section positions when KASLR memory randomization is enabled.
>>
>> This patch is broken.  The commit referenced is wrong,
>
> Yes, I based them on linux-next. I can update them to linux main tree.
> Sorry about that.
>
>> as is the justification.
>>
>> These values are not in fact widely used by userspace (they are brand new).
>>
>
> They were new and got through to the master tree before and on 4.9. I
> didn't get any feedback on that when it went through.
>
>> This is a very fragile approach relying on kernel implementation
>> details, so if we can do anything else that is more robust it
>> is much more likely to pass the test of time.
>>
>> And yes a more robust implementation has been already discussed.
>>
>
> There were discussed for the PAGE_OFFSET but not VMALLOC_START. What's
> your approach for it?
>

Also with improvement on KASLR memory randomization, I don't think we
should assume memory sections by looking at PT_LOAD first entries. We
will certainly try to randomize the order. That's why I thought
clearly indicating which VA define which memory section was a good
idea.

I also want to find a time proof approach where we can use it no
matter the changes to KASLR.

>> Nacked-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
>>
>>>
>>> Signed-off-by: Thomas Garnier <thgarnie@xxxxxxxxxx>
>>> ---
>>>  arch/x86/kernel/machine_kexec_64.c | 3 +++
>>>  include/linux/kexec.h              | 6 ++++++
>>>  2 files changed, 9 insertions(+)
>>>
>>> diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
>>> index 2e3c34b..05f3367 100644
>>> --- a/arch/x86/kernel/machine_kexec_64.c
>>> +++ b/arch/x86/kernel/machine_kexec_64.c
>>> @@ -339,6 +339,9 @@ void arch_crash_save_vmcoreinfo(void)
>>>                             kaslr_offset());
>>>       VMCOREINFO_NUMBER(KERNEL_IMAGE_SIZE);
>>>       VMCOREINFO_PHYS_BASE(phys_base);
>>> +     VMCOREINFO_PAGE_OFFSET(PAGE_OFFSET);
>>> +     VMCOREINFO_VMALLOC_START(VMALLOC_START);
>>> +     VMCOREINFO_VMEMMAP_START(VMEMMAP_START);
>>>  }
>>>
>>>  /* arch-dependent functionality related to kexec file-based syscall */
>>> diff --git a/include/linux/kexec.h b/include/linux/kexec.h
>>> index e98e546..ff9c876 100644
>>> --- a/include/linux/kexec.h
>>> +++ b/include/linux/kexec.h
>>> @@ -285,6 +285,12 @@ phys_addr_t paddr_vmcoreinfo_note(void);
>>>       vmcoreinfo_append_str("CONFIG_%s=y\n", #name)
>>>  #define VMCOREINFO_PHYS_BASE(value) \
>>>       vmcoreinfo_append_str("PHYS_BASE=%lx\n", (unsigned long)value)
>>> +#define VMCOREINFO_PAGE_OFFSET(value) \
>>> +     vmcoreinfo_append_str("PAGE_OFFSET=%lx\n", (unsigned long)value)
>>> +#define VMCOREINFO_VMALLOC_START(value) \
>>> +     vmcoreinfo_append_str("VMALLOC_START=%lx\n", (unsigned long)value)
>>> +#define VMCOREINFO_VMEMMAP_START(value) \
>>> +     vmcoreinfo_append_str("VMEMMAP_START=%lx\n", (unsigned long)value)
>>>
>>>  extern struct kimage *kexec_image;
>>>  extern struct kimage *kexec_crash_image;
>
>
>
> --
> Thomas



-- 
Thomas