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

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Date:   Wed, 19 Apr 2017 08:50:35 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Dave Jiang <dave.jiang@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Dave Young <dyoung@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>, Yinghai Lu <yinghai@...nel.org>,
        Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH 3/4] KASLR: Handle memory limit specified by memmap and
 mem option

On 04/18/17 at 01:36pm, Kees Cook wrote:
> On Mon, Apr 17, 2017 at 6:34 AM, Baoquan He <bhe@...hat.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