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Message-ID: <CAJcbSZFXDXt1uhG-e8BdNyKXaSnsFeB4gsoMsx8_HxcNgjFaMw@mail.gmail.com>
Date: Fri, 23 Sep 2016 09:33:51 -0700
From: Thomas Garnier <thgarnie@...gle.com>
To: Dave Young <dyoung@...hat.com>
Cc: kexec@...ts.infradead.org, Simon Horman <horms@...ge.net.au>,
Kees Cook <keescook@...omium.org>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v1] kexec/arch/i386: Add support for KASLR memory randomization
On Thu, Sep 22, 2016 at 1:41 AM, Dave Young <dyoung@...hat.com> wrote:
> Hi, Thomas
>
> 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.
>
> What is the user visible behavior without this patch? Could you add more
> in the patch log?
>
> During my testing seems with or without this patch kdump kernel boot
> both fine.
Without this patch, you can't access memory on the generated crash dumps.
>
> My kernel config options is like below, is it enough to test this patch?
> CONFIG_RANDOMIZE_BASE=y
> CONFIG_X86_NEED_RELOCS=y
> CONFIG_PHYSICAL_ALIGN=0x1000000
> CONFIG_RANDOMIZE_MEMORY=y
> CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING=0x0
>
Should be good enough.
>>
>> 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
>>
>> Signed-off-by: Thomas Garnier <thgarnie@...gle.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)
>
> It sounds better to split this to another patch.
>
Why not, that's a very small change though.
>> {
>> 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 */
>> + 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
>>
>>
>> _______________________________________________
>> kexec mailing list
>> kexec@...ts.infradead.org
>> http://lists.infradead.org/mailman/listinfo/kexec
>
> Thanks
> Dave
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