Re: [PATCH v3 16/19] x86, kaslr: Randomize physical and virtual address of kernel separately

[Baoquan He]

On 03/07/16 at 03:51pm, Kees Cook wrote:
> On Fri, Mar 4, 2016 at 8:25 AM, Baoquan He <bhe@xxxxxxxxxx> wrote:
> > On x86_64, in old kaslr implementaion only physical address of kernel
> > loading is randomized. Then calculate the delta of physical address
> > where vmlinux was linked to load and where it is finally loaded. If
> > delta is not equal to 0, namely there's a new physical address where
> > kernel is actually decompressed, relocation handling need be done. Then
> > delta is added to offset of kernel symbol relocation, this makes the
> > address of kernel text mapping move delta long.
> >
> > Here the behavior is changed. Randomize both the physical address
> > where kernel is decompressed and the virtual address where kernel text
> > is mapped. And relocation handling only depends on virtual address
> > randomization. Means if and only if virtual address is randomized to
> > a different value, we add the delta to the offset of kernel relocs.
> >
> > Note that up to now both virtual offset and physical addr randomization
> > cann't exceed CONFIG_RANDOMIZE_BASE_MAX_OFFSET, namely 1G.
> 
> It seems like CONFIG_RANDOMIZE_BASE_MAX_OFFSET should have been
> eliminated when the on-demand page table code was added. Once that was
> added, there's no physical max any more. And virtual randomization
> should have no max at all.
For physically random, yes, CONFIG_RANDOMIZE_BASE_MAX_OFFSET is not
needed anymore. But for virtually random,
CONFIG_RANDOMIZE_BASE_MAX_OFFSET is still mandatory since kernel text
mapping and kernel module mapping share the 2G virtual address space as
follows. Though kaslr increase kernel text mapping from 512M to 1G, it's
still limited, can't exceed CONFIG_RANDOMIZE_BASE_MAX_OFFSET.

[0xffffffff80000000, 0xffffffffffffffff] 

But now as you suggested, I would like to change
CONFIG_RANDOMIZE_BASE_MAX_OFFSET to another name because it's only
valid for virtual randomization. A more specific name is better.

> 
> -Kees
> 
> >
> > Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
> > ---
> >  arch/x86/boot/compressed/aslr.c | 46 +++++++++++++++++++++--------------------
> >  arch/x86/boot/compressed/misc.c | 40 +++++++++++++++++++++--------------
> >  arch/x86/boot/compressed/misc.h | 19 +++++++++--------
> >  3 files changed, 59 insertions(+), 46 deletions(-)
> >
> > diff --git a/arch/x86/boot/compressed/aslr.c b/arch/x86/boot/compressed/aslr.c
> > index 3730a33..75ea931 100644
> > --- a/arch/x86/boot/compressed/aslr.c
> > +++ b/arch/x86/boot/compressed/aslr.c
> > @@ -349,7 +349,7 @@ static void process_e820_entry(struct e820entry *entry,
> >         }
> >  }
> >
> > -static unsigned long find_random_addr(unsigned long minimum,
> > +static unsigned long find_random_phy_addr(unsigned long minimum,
> >                                       unsigned long size)
> >  {
> >         int i;
> > @@ -387,23 +387,24 @@ static unsigned long find_random_virt_offset(unsigned long minimum,
> >         return random * CONFIG_PHYSICAL_ALIGN + minimum;
> >  }
> >
> > -unsigned char *choose_kernel_location(unsigned char *input,
> > -                                     unsigned long input_size,
> > -                                     unsigned char *output,
> > -                                     unsigned long output_size)
> > +void choose_kernel_location(unsigned char *input,
> > +                               unsigned long input_size,
> > +                               unsigned char **output,
> > +                               unsigned long output_size,
> > +                               unsigned char **virt_offset)
> >  {
> > -       unsigned long choice = (unsigned long)output;
> >         unsigned long random;
> > +       *virt_offset = (unsigned char *)LOAD_PHYSICAL_ADDR;
> >
> >  #ifdef CONFIG_HIBERNATION
> >         if (!cmdline_find_option_bool("kaslr")) {
> >                 debug_putstr("KASLR disabled by default...\n");
> > -               goto out;
> > +               return;
> >         }
> >  #else
> >         if (cmdline_find_option_bool("nokaslr")) {
> >                 debug_putstr("KASLR disabled by cmdline...\n");
> > -               goto out;
> > +               return;
> >         }
> >  #endif
> >
> > @@ -411,23 +412,24 @@ 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);
> > +                      (unsigned long)*output);
> >
> >         /* Walk e820 and find a random address. */
> > -       random = find_random_addr(choice, output_size);
> > -       if (!random) {
> > +       random = find_random_phy_addr((unsigned long)*output, output_size);
> > +       if (!random)
> >                 debug_putstr("KASLR could not find suitable E820 region...\n");
> > -               goto out;
> > +       else {
> > +               if ((unsigned long)*output != random) {
> > +                       fill_pagetable(random, output_size);
> > +                       switch_pagetable();
> > +                       *output = (unsigned char *)random;
> > +               }
> >         }
> >
> > -       /* Always enforce the minimum. */
> > -       if (random < choice)
> > -               goto out;
> > -
> > -       choice = random;
> > -
> > -       fill_pagetable(choice, output_size);
> > -       switch_pagetable();
> > -out:
> > -       return (unsigned char *)choice;
> > +       /*
> > +        * Get a random address between LOAD_PHYSICAL_ADDR and
> > +        * CONFIG_RANDOMIZE_BASE_MAX_OFFSET
> > +        */
> > +       random = find_random_virt_offset(LOAD_PHYSICAL_ADDR, output_size);
> > +       *virt_offset = (unsigned char *)random;
> >  }
> > diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
> > index 70445c3..362d508 100644
> > --- a/arch/x86/boot/compressed/misc.c
> > +++ b/arch/x86/boot/compressed/misc.c
> > @@ -251,7 +251,8 @@ void error(char *x)
> >  }
> >
> >  #if CONFIG_X86_NEED_RELOCS
> > -static void handle_relocations(void *output, unsigned long output_len)
> > +static void handle_relocations(void *output, unsigned long output_len,
> > +                              void *virt_offset)
> >  {
> >         int *reloc;
> >         unsigned long delta, map, ptr;
> > @@ -263,11 +264,6 @@ static void handle_relocations(void *output, unsigned long output_len)
> >          * and where it was actually loaded.
> >          */
> >         delta = min_addr - LOAD_PHYSICAL_ADDR;
> > -       if (!delta) {
> > -               debug_putstr("No relocation needed... ");
> > -               return;
> > -       }
> > -       debug_putstr("Performing relocations... ");
> >
> >         /*
> >          * The kernel contains a table of relocation addresses. Those
> > @@ -278,6 +274,22 @@ static void handle_relocations(void *output, unsigned long output_len)
> >          */
> >         map = delta - __START_KERNEL_map;
> >
> > +
> > +
> > +       /*
> > +        * 32-bit always performs relocations. 64-bit relocations are only
> > +        * needed if kASLR has chosen a different starting address offset
> > +        * from __START_KERNEL_map.
> > +        */
> > +       if (IS_ENABLED(CONFIG_X86_64))
> > +               delta = (unsigned long)virt_offset - LOAD_PHYSICAL_ADDR;
> > +
> > +       if (!delta) {
> > +               debug_putstr("No relocation needed... ");
> > +               return;
> > +       }
> > +       debug_putstr("Performing relocations... ");
> > +
> >         /*
> >          * Process relocations: 32 bit relocations first then 64 bit after.
> >          * Three sets of binary relocations are added to the end of the kernel
> > @@ -331,7 +343,8 @@ static void handle_relocations(void *output, unsigned long output_len)
> >  #endif
> >  }
> >  #else
> > -static inline void handle_relocations(void *output, unsigned long output_len)
> > +static inline void handle_relocations(void *output, unsigned long output_len,
> > +                                     void *virt_offset)
> >  { }
> >  #endif
> >
> > @@ -397,6 +410,7 @@ asmlinkage __visible void *decompress_kernel(void *rmode, memptr heap,
> >  {
> >         unsigned long run_size = VO__end - VO__text;
> >         unsigned char *output_orig = output;
> > +       unsigned char *virt_offset;
> >
> >         real_mode = rmode;
> >
> > @@ -434,9 +448,10 @@ asmlinkage __visible void *decompress_kernel(void *rmode, memptr heap,
> >          * the entire decompressed kernel plus relocation table, or the
> >          * entire decompressed kernel plus .bss and .brk sections.
> >          */
> > -       output = choose_kernel_location(input_data, input_len, output,
> > +       choose_kernel_location(input_data, input_len, &output,
> >                                         output_len > run_size ? output_len
> > -                                                             : run_size);
> > +                                                             : run_size,
> > +                                       &virt_offset);
> >
> >         /* Validate memory location choices. */
> >         if ((unsigned long)output & (MIN_KERNEL_ALIGN - 1))
> > @@ -457,12 +472,7 @@ asmlinkage __visible void *decompress_kernel(void *rmode, memptr heap,
> >         __decompress(input_data, input_len, NULL, NULL, output, output_len,
> >                         NULL, error);
> >         parse_elf(output);
> > -       /*
> > -        * 32-bit always performs relocations. 64-bit relocations are only
> > -        * needed if kASLR has chosen a different load address.
> > -        */
> > -       if (!IS_ENABLED(CONFIG_X86_64) || output != output_orig)
> > -               handle_relocations(output, output_len);
> > +       handle_relocations(output, output_len, virt_offset);
> >         debug_putstr("done.\nBooting the kernel.\n");
> >         return output;
> >  }
> > diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
> > index 39d0e9a..cde6aec 100644
> > --- a/arch/x86/boot/compressed/misc.h
> > +++ b/arch/x86/boot/compressed/misc.h
> > @@ -69,20 +69,21 @@ int cmdline_find_option_bool(const char *option);
> >
> >  #if CONFIG_RANDOMIZE_BASE
> >  /* aslr.c */
> > -unsigned char *choose_kernel_location(unsigned char *input,
> > -                                     unsigned long input_size,
> > -                                     unsigned char *output,
> > -                                     unsigned long output_size);
> > +void choose_kernel_location(unsigned char *input,
> > +                               unsigned long input_size,
> > +                               unsigned char **output,
> > +                               unsigned long output_size,
> > +                               unsigned char **virt_offset);
> >  /* cpuflags.c */
> >  bool has_cpuflag(int flag);
> >  #else
> >  static inline
> > -unsigned char *choose_kernel_location(unsigned char *input,
> > -                                     unsigned long input_size,
> > -                                     unsigned char *output,
> > -                                     unsigned long output_size)
> > +void choose_kernel_location(unsigned char *input,
> > +                               unsigned long input_size,
> > +                               unsigned char **output,
> > +                               unsigned long output_size,
> > +                               unsigned char **virt_offset)
> >  {
> > -       return output;
> >  }
> >  #endif
> >
> > --
> > 2.5.0
> >
> 
> 
> 
> -- 
> Kees Cook
> Chrome OS & Brillo Security