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Message-ID: <20160926125212.GA19661@dhcp-128-65.nay.redhat.com>
Date: Mon, 26 Sep 2016 20:52:12 +0800
From: Dave Young <dyoung@...hat.com>
To: Thomas Garnier <thgarnie@...gle.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 09/23/16 at 09:33am, Thomas Garnier wrote:
> 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.
Ok, makedumpfile saving /proc/vmcore works fine without this patch, but
gdb will give error when accessing the old memory.
>
> >
> > 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);
For page_offset_base it should only print the error message when
the kernel supports it.
> >> 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;
> >> --
Thanks
Dave
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