Re: [PATCH v3 08/19] x86, kaslr: Consolidate mem_avoid array filling

[Baoquan He <bhe@redhat.com>]

On 03/07/16 at 03:28pm, Kees Cook wrote:
> On Fri, Mar 4, 2016 at 8:25 AM, Baoquan He <bhe@redhat.com> wrote:
> > From: Yinghai Lu <yinghai@kernel.org>
> >
> > We are going to support kaslr with 64bit above 4G, and new random output
> > could be anywhere. Array mem_avoid is used for kaslr to search new output
> > address. Current code only track range that is after output+output_size.
> > So we need to track all ranges instead of just after output+output_size.
> >
> > Current code has first entry which is extra bytes before input+input_size,
> > and it is according to output_size. Other entries are for initrd, cmdline,
> > and heap/stack for ZO running.
> >
> > At first, let's check the first entry that should be in the mem_avoid array.
> > Now ZO sit end of the buffer always, we can find out where is text and
> > data/bss etc of ZO.
> >
> > Since init_size >= run_size, and input+input_len >= output+output_len,
> > here make several assumptions for better presentation by graph:
> >  - init_size > run_size
> >  - input+input_len > output+output_len
> >  - run_size > output_len
> 
> I would like to see each of these assumptions justified. Why is
> init_size > run_size, etc?
> choose_kernel_location's "output_size" is calculated as max(run_size,
> output_len), so run_size may not be > output_len...

Sure. I will add this case in next post. Thanks a lot.
> 
> >
> > 0   output                       input             input+input_len          output+init_size
> > |     |                            |                       |                       |
> > |-----|-------------------|--------|------------------|----|------------|----------|
> >                           |                           |                 |
> >              output+init_size-ZO_INIT_SIZE    output+output_len    output+run_size
> >
> > [output, output+init_size) is the for decompressing buffer for compressed
> > kernel.
> >
> > [output, output+run_size) is for VO run size.
> > [output, output+output_len) is (VO (vmlinux after objcopy) plus relocs)
> >
> > [output+init_size-ZO_INIT_SIZE, output+init_size) is copied ZO.
> > [input, input+input_len) is copied compressed (VO (vmlinux after objcopy)
> > plus relocs), not the ZO.
> >
> > [input+input_len, output+init_size) is [_text, _end) for ZO. that could be
> > first range in mem_avoid. Now the new first entry already includes heap and
> > stack for ZO running. So no need to put them separately into mem_avoid array.
> >
> > Also [input, input+input_size) need be put in mem_avoid array. It is adjacent
> > to new first entry, so merge them.
> 
> I wonder if this diagram and description should live in a comment with the code.

I think it would be very helpful for people interested in this process.
Do you think it's ok to put it where init_size is calculated in
boot/header.S?  Or other suitable places?
> 
> 
> >
> > At last we need to put boot_params into the mem_avoid too. As with 64bit
> > bootloader could put it anywhere.
> >
> > After these changes, we have all ranges which need be avoided in mem_avoid
> > array.
> >
> > Cc: Kees Cook <keescook@chromium.org>
> > Signed-off-by: Yinghai Lu <yinghai@kernel.org>
> > ---
> > v2->v3:
> >     Adjust the patch log.
> >
> >  arch/x86/boot/compressed/aslr.c | 29 +++++++++++++----------------
> >  1 file changed, 13 insertions(+), 16 deletions(-)
> >
> > diff --git a/arch/x86/boot/compressed/aslr.c b/arch/x86/boot/compressed/aslr.c
> > index 622aa88..b93be03 100644
> > --- a/arch/x86/boot/compressed/aslr.c
> > +++ b/arch/x86/boot/compressed/aslr.c
> > @@ -109,7 +109,7 @@ struct mem_vector {
> >         unsigned long size;
> >  };
> >
> > -#define MEM_AVOID_MAX 5
> > +#define MEM_AVOID_MAX 4
> >  static struct mem_vector mem_avoid[MEM_AVOID_MAX];
> >
> >  static bool mem_contains(struct mem_vector *region, struct mem_vector *item)
> > @@ -135,21 +135,22 @@ static bool mem_overlaps(struct mem_vector *one, struct mem_vector *two)
> >  }
> >
> >  static void mem_avoid_init(unsigned long input, unsigned long input_size,
> > -                          unsigned long output, unsigned long output_size)
> > +                          unsigned long output)
> >  {
> > +       unsigned long init_size = real_mode->hdr.init_size;
> >         u64 initrd_start, initrd_size;
> >         u64 cmd_line, cmd_line_size;
> > -       unsigned long unsafe, unsafe_len;
> >         char *ptr;
> >
> >         /*
> >          * Avoid the region that is unsafe to overlap during
> > -        * decompression (see calculations at top of misc.c).
> > +        * decompression.
> > +        * As we already move ZO (arch/x86/boot/compressed/vmlinux)
> > +        * to the end of buffer, [input+input_size, output+init_size)
> > +        * has [_text, _end) for ZO.
> >          */
> > -       unsafe_len = (output_size >> 12) + 32768 + 18;
> > -       unsafe = (unsigned long)input + input_size - unsafe_len;
> > -       mem_avoid[0].start = unsafe;
> > -       mem_avoid[0].size = unsafe_len;
> > +       mem_avoid[0].start = input;
> > +       mem_avoid[0].size = (output + init_size) - input;
> >
> >         /* Avoid initrd. */
> >         initrd_start  = (u64)real_mode->ext_ramdisk_image << 32;
> > @@ -169,13 +170,9 @@ static void mem_avoid_init(unsigned long input, unsigned long input_size,
> >         mem_avoid[2].start = cmd_line;
> >         mem_avoid[2].size = cmd_line_size;
> >
> > -       /* Avoid heap memory. */
> > -       mem_avoid[3].start = (unsigned long)free_mem_ptr;
> > -       mem_avoid[3].size = BOOT_HEAP_SIZE;
> > -
> > -       /* Avoid stack memory. */
> > -       mem_avoid[4].start = (unsigned long)free_mem_end_ptr;
> > -       mem_avoid[4].size = BOOT_STACK_SIZE;
> > +       /* Avoid params */
> > +       mem_avoid[3].start = (unsigned long)real_mode;
> > +       mem_avoid[3].size = sizeof(*real_mode);
> >  }
> >
> >  /* Does this memory vector overlap a known avoided area? */
> > @@ -319,7 +316,7 @@ unsigned char *choose_kernel_location(unsigned char *input,
> >
> >         /* Record the various known unsafe memory ranges. */
> >         mem_avoid_init((unsigned long)input, input_size,
> > -                      (unsigned long)output, output_size);
> > +                      (unsigned long)output);
> >
> >         /* Walk e820 and find a random address. */
> >         random = find_random_addr(choice, output_size);
> > --
> > 2.5.0
> >
> 
> 
> 
> -- 
> Kees Cook
> Chrome OS & Brillo Security