[PATCH v1] kexec/arch/i386: Add support for KASLR memory randomization

[Thomas Garnier]

On Wed, Aug 17, 2016 at 9:59 PM, Baoquan He <bhe at redhat.com> wrote:
> On 08/17/16 at 09:47am, Thomas Garnier wrote:
>> Multiple changes were made on KASLR (right now in linux-next). One of
>> them is randomizing the virtual address of the physical mapping, vmalloc
>> and vmemmap memory sections. It breaks kdump ability to read physical
>> memory.
>>
>> This change identifies if KASLR memories randomization is used by
>> checking if the page_offset_base variable exists. It search for the
>> correct PAGE_OFFSET value by looking at the loaded memory section and
>> find the lowest aligned on PUD (the randomization level).
>>
>> Related commits on linux-next:
>>  - 0483e1fa6e09d4948272680f691dccb1edb9677f: Base for randomization
>>  - 021182e52fe01c1f7b126f97fd6ba048dc4234fd: Enable for PAGE_OFFSET
>
> Seems above two commits have been inside Linus's tree, while vmemmap
> not yet.
>
>>
>> Signed-off-by: Thomas Garnier <thgarnie at google.com>
>> ---
>>  kexec/arch/i386/crashdump-x86.c | 29 ++++++++++++++++++++++-------
>>  1 file changed, 22 insertions(+), 7 deletions(-)
>>
>> diff --git a/kexec/arch/i386/crashdump-x86.c b/kexec/arch/i386/crashdump-x86.c
>> index bbc0f35..ab833d4 100644
>> --- a/kexec/arch/i386/crashdump-x86.c
>> +++ b/kexec/arch/i386/crashdump-x86.c
>> @@ -102,11 +102,10 @@ static int get_kernel_paddr(struct kexec_info *UNUSED(info),
>>       return -1;
>>  }
>>
>> -/* Retrieve kernel _stext symbol virtual address from /proc/kallsyms */
>> -static unsigned long long get_kernel_stext_sym(void)
>> +/* Retrieve kernel symbol virtual address from /proc/kallsyms */
>> +static unsigned long long get_kernel_sym(const char *symbol)
>>  {
>>       const char *kallsyms = "/proc/kallsyms";
>> -     const char *stext = "_stext";
>>       char sym[128];
>>       char line[128];
>>       FILE *fp;
>> @@ -122,13 +121,13 @@ static unsigned long long get_kernel_stext_sym(void)
>>       while(fgets(line, sizeof(line), fp) != NULL) {
>>               if (sscanf(line, "%Lx %c %s", &vaddr, &type, sym) != 3)
>>                       continue;
>> -             if (strcmp(sym, stext) == 0) {
>> -                     dbgprintf("kernel symbol %s vaddr = %16llx\n", stext, vaddr);
>> +             if (strcmp(sym, symbol) == 0) {
>> +                     dbgprintf("kernel symbol %s vaddr = %16llx\n", symbol, vaddr);
>>                       return vaddr;
>>               }
>>       }
>>
>> -     fprintf(stderr, "Cannot get kernel %s symbol address\n", stext);
>> +     fprintf(stderr, "Cannot get kernel %s symbol address\n", symbol);
>>       return 0;
>>  }
>>
>> @@ -151,6 +150,8 @@ static int get_kernel_vaddr_and_size(struct kexec_info *UNUSED(info),
>>       off_t size;
>>       uint32_t elf_flags = 0;
>>       uint64_t stext_sym;
>> +     const unsigned long long pud_mask = ~((1 << 30) - 1);
>> +     unsigned long long vaddr, lowest_vaddr = 0;
>>
>>       if (elf_info->machine != EM_X86_64)
>>               return 0;
>> @@ -180,9 +181,23 @@ static int get_kernel_vaddr_and_size(struct kexec_info *UNUSED(info),
>>
>>       end_phdr = &ehdr.e_phdr[ehdr.e_phnum];
>>
>> +     /* Search for the real PAGE_OFFSET when KASLR memory randomization
>> +      * is enabled */
>
> Yeah, this is necessary. That would be great if it can be put into
> get_kernel_page_offset. But then it need parse kcore elf file again,
> seems no better way.
>

I agree.

Simon: Do you have any comments?

>> +     if (get_kernel_sym("page_offset_base") != 0) {
>> +             for(phdr = ehdr.e_phdr; phdr != end_phdr; phdr++) {
>> +                     if (phdr->p_type == PT_LOAD) {
>> +                             vaddr = phdr->p_vaddr & pud_mask;
>> +                             if (lowest_vaddr == 0 || lowest_vaddr > vaddr)
>> +                                     lowest_vaddr = vaddr;
>> +                     }
>> +             }
>> +             if (lowest_vaddr != 0)
>> +                     elf_info->page_offset = lowest_vaddr;
>> +     }
>> +
>>       /* Traverse through the Elf headers and find the region where
>>        * _stext symbol is located in. That's where kernel is mapped */
>> -     stext_sym = get_kernel_stext_sym();
>> +     stext_sym = get_kernel_sym("_stext");
>>       for(phdr = ehdr.e_phdr; stext_sym && phdr != end_phdr; phdr++) {
>>               if (phdr->p_type == PT_LOAD) {
>>                       unsigned long long saddr = phdr->p_vaddr;
>> --
>> 2.8.0.rc3.226.g39d4020
>>
>>
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