linux-kernel - RE: [RFC][PATCH 0/2] x86/boot/KASLR: Restrict kernel to be randomized in mirror regions if existed

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Date:   Thu, 15 Jun 2017 08:34:57 +0000
From:   "Izumi, Taku" <izumi.taku@...fujitsu.com>
To:     'Baoquan He' <bhe@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "keescook@...omium.org" <keescook@...omium.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "Fan, Chao" <fanc.fnst@...fujitsu.com>,
        "Cao, Jin" <caoj.fnst@...fujitsu.com>,
        "Dou, Liyang" <douly.fnst@...fujitsu.com>
Subject: RE: [RFC][PATCH 0/2] x86/boot/KASLR: Restrict kernel to be
 randomized in mirror regions if existed

Dear Baoquan,

> > Our customer reported that Kernel text may be located on non-mirror
> > region (movable zone) when both address range mirroring feature and
> > KASLR are enabled.

   I know your customer :)

> > The functions of address range mirroring feature are as follows.
> > - The physical memory region whose descriptors in EFI memory map have
> >   EFI_MEMORY_MORE_RELIABLE attribute (bit: 16) are mirrored
> > - The function arranges such mirror region into normal zone and other
> region
> >   into movable zone in order to locate kernel code and data on mirror
> > region
> >
> > So we need restrict kernel to be located inside mirror region if it is
> > existed.
> >
> > The method is very simple. If efi is enabled, just iterate all efi
> > memory map and pick up mirror region to process for adding candidate
> > of slot. If efi disabled or no mirror region existed, still process
> > e820 memory map. This won't bring much efficiency loss, at worst we
> > just go through all efi memory maps and found no mirror.
> >
> > One question:
> > From code, though mirror regions are existed, they are meaningful only
> > if kernelcore=mirror kernel option is specified. Not sure if my
> > understanding is correct.

   Your understanding is almost correct. 
   Only when "kernelcore=mirror" specified, the above procedure works.
   But, if mirrored regions are existed, bootmem allocator tries to 
   allocate from mirrored region independently of "kerenelcore=mirror" option.

   So, IMHO, kernel text is important, so putting it to mirrored (more reliable)
   region is reasonable whether or not "kernelcore=mirror" is specified.

   Anyway thanks for submitting patch.
   We have Address Range Mirroring capable machine, so we'll test your patch.

  Sincerely,
  Taku Izumi

> 
> Since you are the author of kernelcore=mirror related code and expert on
> mirror feature, could you help answer above question?
> 
> Thanks
> Baoquan
> >
> > NOTE:
> > I haven't got a machine with efi mirror region enabled, so only test
> > the
> > e820 map processing case and the case of no mirror region on efi machine.
> > So set this as a RFC patchset, will post formal one after above
> > question is made clear and mirror issue test passed.
> >
> > Baoquan He (2):
> >   x86/boot/KASLR: Adapt process_e820_entry for all kinds of memory map
> >   x86/boot/KASLR: Restrict kernel to be randomized in mirror regions if
> >     existed
> >
> >  arch/x86/boot/compressed/kaslr.c | 129
> > +++++++++++++++++++++++++++++++--------
> >  1 file changed, 104 insertions(+), 25 deletions(-)
> >
> > --
> > 2.5.5
> >