Re: [PATCH 3/4] KASLR: Handle memory limit specified by memmap and mem option

Baoquan He
2017-04-19T00:50:44Z

On 04/18/17 at 01:36pm, Kees Cook wrote:
> On Mon, Apr 17, 2017 at 6:34 AM, Baoquan He <bhe@redhat.com> wrote:
> > @@ -432,7 +455,8 @@ static void process_e820_entry(struct e820entry *entry,
> > {
> > struct mem_vector region, overlap;
> > struct slot_area slot_area;
> > - unsigned long start_orig;
> > + unsigned long start_orig, end;
> > + struct e820entry cur_entry;
> >
> > /* Skip non-RAM entries. */
> > if (entry->type != E820_RAM)
> > @@ -446,8 +470,15 @@ static void process_e820_entry(struct e820entry *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 - 1, mem_limit);
> > + if (entry->addr >= end)
> > + return;
> > + cur_entry.addr = entry->addr;
> > + cur_entry.size = end - entry->addr + 1;
> > +
> > + region.start = cur_entry.addr;
> > + region.size = cur_entry.size;
>
> I find the manipulation of entry->addr +/- 1 confusing; it should just
> be mem_limit that is adjusted:
>
> end = min(entry->size + entry->addr, mem_limit + 1);

Oh, it should be like that. E.g if specify mem=4096M, it means available
memory region are 0~4096M-1, or [0, 4096M). Here mem_limit = 4096M.
Adding 1 could make it wrong.

>
> And maybe to avoid mem_limit being giant by default, maybe have "0" be special?
>
> cur_entry.addr = entry->addr;
> if (mem_limit) {
> unsigned long end = min(entry->size + entry->addr, mem_limit + 1);
> if (entry->addr > end)
> return;
> cur_entry.size = end - entry->addr;
> } else {
> cur_entry.size = entry->size;
> }
>
> or something... and maybe move the whole thing earlier so other tests
> that examine entry->size are checked with the new adjusted value.

Sorry, forget replying to this comment. I am fine with moving it
earlier. In fact I put it here because there are many non-RAM e820
entries below 4G, like ACPI, for them we even don't need check limit
by the help of below check filtering. Maybe move it after below check?

/* Skip non-RAM entries. */
if (entry->type != E820_RAM)
return;

>
> -Kees
>
> >
> > /* Give up if slot area array is full. */
> > while (slot_area_index < MAX_SLOT_AREA) {
> > @@ -461,7 +492,7 @@ static void process_e820_entry(struct e820entry *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
> >
>
>
>
> --
> Kees Cook
> Pixel Security