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From:   Jann Horn <jannh@google.com>
Date:   Thu, 8 Oct 2020 20:24:10 +0200
Message-ID: <CAG48ez2bDCEKxUdFUQ2MAkJgKi1zN+h46ypC2zNr0a1fp-FqWQ@mail.gmail.com>
Subject: Re: [PATCH RESEND v2] mm: Optional full ASLR for mmap() and mremap()
To:     Topi Miettinen <toiwoton@gmail.com>
Cc:     linux-hardening@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux-MM <linux-mm@kvack.org>,
        kernel list <linux-kernel@vger.kernel.org>
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On Thu, Oct 8, 2020 at 8:10 PM Topi Miettinen <toiwoton@gmail.com> wrote:
> On 8.10.2020 20.13, Jann Horn wrote:
> > On Thu, Oct 8, 2020 at 6:54 PM Topi Miettinen <toiwoton@gmail.com> wrote:
> >> Writing a new value of 3 to /proc/sys/kernel/randomize_va_space
> >> enables full randomization of memory mappings created with mmap(NULL,
> >> ...). With 2, the base of the VMA used for such mappings is random,
> >> but the mappings are created in predictable places within the VMA and
> >> in sequential order. With 3, new VMAs are created to fully randomize
> >> the mappings. Also mremap(..., MREMAP_MAYMOVE) will move the mappings
> >> even if not necessary.
> > [...]
> >> +       if ((flags & MREMAP_MAYMOVE) && randomize_va_space >= 3) {
> >> +               /*
> >> +                * Caller is happy with a different address, so let's
> >> +                * move even if not necessary!
> >> +                */
> >> +               new_addr = arch_mmap_rnd();
> >> +
> >> +               ret = mremap_to(addr, old_len, new_addr, new_len,
> >> +                               &locked, flags, &uf, &uf_unmap_early,
> >> +                               &uf_unmap);
> >> +               goto out;
> >> +       }
> >
> > You just pick a random number as the address, and try to place the
> > mapping there? Won't this fail if e.g. the old address range overlaps
> > with the new one, causing mremap_to() to bail out at "if (addr +
> > old_len > new_addr && new_addr + new_len > addr)"?
>
> Thanks for the review. I think overlap would be OK in this case and the
> check should be skipped.

No, mremap() can't deal with overlap (and trying to add such support
would make mremap() unnecessarily complicated).