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

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Mar 19, 2014 at 03:05:51PM +0800, WANG Chao wrote:
> 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.

How about looking at /proc/kallsyms and look at address of one of the
symbols say _text. And search for the ELF header in kcore which contains
_text address and that's ELF header representing kernel text mapping.

That way you don't have to worry about the value of
CONFIG_RANDOMIZE_MAX_BASE_OFFSET.

> 
> > 
> > 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
> 

Agreed. help text for CONFIG_RANDOMIZE_BASE_MAX_OFFSET says following.

On 64-bit this is limited by how the kernel fixmap page table is
positioned, so this cannot be larger than 1GiB currently.  Without
RANDOMIZE_BASE, there is a 512MiB to 1.5GiB split between kernel
and modules. When RANDOMIZE_BASE_MAX_OFFSET is above 512MiB, the
modules area will shrink to compensate, up to the current maximum
1GiB to 1GiB split. The default is 1GiB.

Thanks
Vivek

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