Re: [PATCH v2] x86, kaslr: Kernel base can be randomized at 0-1G offset.

[WANG Chao <chaowang@redhat.com>]

On 03/17/14 at 08:56am, Vivek Goyal wrote:
> On Fri, Mar 14, 2014 at 04:20:18PM +0800, WANG Chao wrote:
> > With kASLR enabled (CONFIG_RANDOMIZED_BASE=y), kernel virtual address
> > base is PAGE_OFFSET plus a randomized offset from 0 to 1G.
> > 
> > Current kexec-tools gets kernel vaddr and size from /proc/kcore. It
> > assumes kernel vaddr start/end is within the range [0,512M). If kaslr
> > enabled, kernel vaddr start/end will stay at [0+offset, 512M+offset).

NACK this patch myself.

There are several mistakes I made. I misunderstood some concepts. Then I
realize this kASLR issue is not trivial to fix.

I think if kexec-tools needs to get kernel text mapping from kcore in
kALSR case, the max base offset (CONFIG_RANDOMIZE_MAX_BASE_OFFSET) of
the kernel text area must be exposed to userspace some where. We can use
that value to determine which area is for kernel text mapping and which
is for modules text mapping.

> 
> Hi Chao,
> 
> Documentation/x86/x86_64/mm.txt still says that kernel text mapping area
> is 512MB.
> 
> ffffffff80000000 - ffffffffa0000000 (=512 MB)  kernel text mapping, from
> phys 0
> 
> So has that changed now due to kASLR.

Yes, with kASLR enabled, kernel text mapping is as following

ffffffff80000000 - (ffffffff80000000+CONFIG_RANDOMIZE_BASE_MAX_OFFSET)

That said, if using CONFIG_RANDOMIZE_BASE_MAX_OFFSET=0x40000000 by
default, the kernel text mapping is as following:

ffffffff80000000 - ffffffffc0000000

Thanks
WANG Chao

> 
> Thanks
> Vivek
> 
> > 
> > To adapt kaslr, introduce a new macro X86_64_RANDOMIZED_BASE_MAX_OFFSET
> > to address the max offset and use this macro to filter out the kernel
> > text PT_LOAD from /proc/kcore.
> > 
> > Signed-off-by: WANG Chao <chaowang@redhat.com>
> > ---
> >  kexec/arch/i386/crashdump-x86.c | 3 ++-
> >  kexec/arch/i386/crashdump-x86.h | 3 +++
> >  2 files changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kexec/arch/i386/crashdump-x86.c b/kexec/arch/i386/crashdump-x86.c
> > index cb19e7d..1e6d3a3 100644
> > --- a/kexec/arch/i386/crashdump-x86.c
> > +++ b/kexec/arch/i386/crashdump-x86.c
> > @@ -156,7 +156,8 @@ static int get_kernel_vaddr_and_size(struct kexec_info *UNUSED(info),
> >  
> >  			/* Look for kernel text mapping header. */
> >  			if ((saddr >= X86_64__START_KERNEL_map) &&
> > -			    (eaddr <= X86_64__START_KERNEL_map + X86_64_KERNEL_TEXT_SIZE)) {
> > +			    (saddr <= X86_64__START_KERNEL_map + X86_64_RANDOMIZE_BASE_MAX_OFFSET) &&
> > +			    (eaddr - saddr  < X86_64_KERNEL_TEXT_SIZE)) {
> >  				saddr = _ALIGN_DOWN(saddr, X86_64_KERN_VADDR_ALIGN);
> >  				elf_info->kern_vaddr_start = saddr;
> >  				size = eaddr - saddr;
> > diff --git a/kexec/arch/i386/crashdump-x86.h b/kexec/arch/i386/crashdump-x86.h
> > index e68b626..71a09f8 100644
> > --- a/kexec/arch/i386/crashdump-x86.h
> > +++ b/kexec/arch/i386/crashdump-x86.h
> > @@ -15,6 +15,9 @@ int load_crashdump_segments(struct kexec_info *info, char *mod_cmdline,
> >  #define X86_64_PAGE_OFFSET_PRE_2_6_27	0xffff810000000000ULL
> >  #define X86_64_PAGE_OFFSET		0xffff880000000000ULL
> >  
> > +/* kASLR - Kernel base offset could be randomized up to 1G */
> > +#define X86_64_RANDOMIZE_BASE_MAX_OFFSET	0x40000000
> > +
> >  #define X86_64_MAXMEM        		0x3fffffffffffUL
> >  
> >  /* Kernel text size */
> > -- 
> > 1.8.5.3

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