Re: [PATCH v3 2/3] KASLR: Handle memory limit specified by memmap and mem option

[Baoquan He <bhe@redhat.com>]

On 04/28/17 at 12:39pm, Kees Cook wrote:
> On Wed, Apr 26, 2017 at 3:16 AM, Baoquan He <bhe@redhat.com> wrote:
> > Option mem= will limit the max address a system can use and any memory
> > region above the limit will be removed.
> >
> > Furthermore, memmap=nn[KMG] which has no offset specified has the same
> > behaviour as mem=.
> >
> > KASLR needs to consider this when choosing the random position for
> > decompressing the kernel.
> >
> > This patch implements that.
> >
> > Signed-off-by: Baoquan He <bhe@redhat.com>
> > Acked-by: Kees Cook <keescook@chromium.org>
> > Cc: "H. Peter Anvin" <hpa@zytor.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: x86@kernel.org
> > Cc: Kees Cook <keescook@chromium.org>
> > Cc: Yinghai Lu <yinghai@kernel.org>
> > Cc: Borislav Petkov <bp@suse.de>
> > ---
> >  arch/x86/boot/compressed/kaslr.c | 60 ++++++++++++++++++++++++++++++++--------
> >  1 file changed, 48 insertions(+), 12 deletions(-)
> >
> > diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
> > index 53a06ec..e5eb0c3 100644
> > --- a/arch/x86/boot/compressed/kaslr.c
> > +++ b/arch/x86/boot/compressed/kaslr.c
> > @@ -78,6 +78,10 @@ static bool memmap_too_large;
> >  extern unsigned long get_cmd_line_ptr(void);
> >
> >
> > +/* Store memory limit specified by "mem=nn[KMG]" or "memmap=nn[KMG]" */
> > +unsigned long long mem_limit = ULLONG_MAX;
> > +
> > +
> >  enum mem_avoid_index {
> >         MEM_AVOID_ZO_RANGE = 0,
> >         MEM_AVOID_INITRD,
> > @@ -128,16 +132,22 @@ parse_memmap(char *p, unsigned long long *start, unsigned long long *size)
> >                 return -EINVAL;
> >
> >         switch (*p) {
> > -       case '@':
> > -               /* Skip this region, usable */
> > -               *start = 0;
> > -               *size = 0;
> > -               return 0;
> >         case '#':
> >         case '$':
> >         case '!':
> >                 *start = memparse(p + 1, &p);
> >                 return 0;
> > +       case '@':
> > +               /* memmap=nn@ss specifies usable region, should be skipped */
> > +               *size = 0;
> 
> Please explicitly add a comment like /* Fall through. */ or something
> here, to help future reviewers. :)

Sure, will add. Thanks!

> 
> > +       default:
> > +               /*
> > +                * If w/o offset, only size specified, memmap=nn[KMG] has the
> > +                * same behaviour as mem=nn[KMG]. It limits the max address
> > +                * system can use. Region above the limit should be avoided.
> > +                */
> > +               *start = 0;
> > +               return 0;
> >         }
> >
> >         return -EINVAL;
> > @@ -163,9 +173,14 @@ static void mem_avoid_memmap(char *str)
> >                 if (rc < 0)
> >                         break;
> >                 str = k;
> > -               /* A usable region that should not be skipped */
> > -               if (size == 0)
> > +
> > +               if (start == 0) {
> > +                       /* Store the specified memory limit if size > 0 */
> > +                       if (size > 0)
> > +                               mem_limit = size;
> > +
> >                         continue;
> > +               }
> >
> >                 mem_avoid[MEM_AVOID_MEMMAP_BEGIN + i].start = start;
> >                 mem_avoid[MEM_AVOID_MEMMAP_BEGIN + i].size = size;
> > @@ -189,6 +204,7 @@ static int handle_mem_memmap(void)
> >         size_t len = strlen((char *)args);
> >         char *tmp_cmdline;
> >         char *param, *val;
> > +       u64 mem_size;
> >
> >         tmp_cmdline = malloc(COMMAND_LINE_SIZE);
> >         if (!tmp_cmdline )
> > @@ -211,8 +227,20 @@ static int handle_mem_memmap(void)
> >                         return -1;
> >                 }
> >
> > -               if (!strcmp(param, "memmap"))
> > +               if (!strcmp(param, "memmap")) {
> >                         mem_avoid_memmap(val);
> > +               } else if (!strcmp(param, "mem")) {
> > +                       char *p = val;
> > +
> > +                       if (!strcmp(p, "nopentium"))
> > +                               continue;
> > +                       mem_size = memparse(p, &p);
> > +                       if (mem_size == 0) {
> > +                               free(tmp_cmdline);
> > +                               return -EINVAL;
> > +                       }
> > +                       mem_limit = mem_size;
> > +               }
> >         }
> >
> >         free(tmp_cmdline);
> > @@ -449,7 +477,8 @@ static void process_e820_entry(struct boot_e820_entry *entry,
> >  {
> >         struct mem_vector region, overlap;
> >         struct slot_area slot_area;
> > -       unsigned long start_orig;
> > +       unsigned long start_orig, end;
> > +       struct boot_e820_entry cur_entry;
> >
> >         /* Skip non-RAM entries. */
> >         if (entry->type != E820_TYPE_RAM)
> > @@ -463,8 +492,15 @@ static void process_e820_entry(struct boot_e820_entry *entry,
> >         if (entry->addr + entry->size < minimum)
> >                 return;
> >
> > -       region.start = entry->addr;
> > -       region.size = entry->size;
> > +       /* Ignore entries above memory limit */
> > +       end = min(entry->size + entry->addr, mem_limit);
> > +       if (entry->addr >= end)
> > +               return;
> > +       cur_entry.addr = entry->addr;
> > +       cur_entry.size = end - entry->addr;
> > +
> > +       region.start = cur_entry.addr;
> > +       region.size = cur_entry.size;
> >
> >         /* Give up if slot area array is full. */
> >         while (slot_area_index < MAX_SLOT_AREA) {
> > @@ -478,7 +514,7 @@ static void process_e820_entry(struct boot_e820_entry *entry,
> >                 region.start = ALIGN(region.start, CONFIG_PHYSICAL_ALIGN);
> >
> >                 /* Did we raise the address above this e820 region? */
> > -               if (region.start > entry->addr + entry->size)
> > +               if (region.start > cur_entry.addr + cur_entry.size)
> >                         return;
> >
> >                 /* Reduce size by any delta from the original address. */
> > --
> > 2.5.5
> >
> 
> Otherwise, this looks good to me.
> 
> -Kees
> 
> -- 
> Kees Cook
> Pixel Security