From: Jann Horn <jannh@google.com>
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>
Subject: Re: [PATCH RESEND v2] mm: Optional full ASLR for mmap() and mremap()
Date: Thu, 8 Oct 2020 20:24:10 +0200 [thread overview]
Message-ID: <CAG48ez2bDCEKxUdFUQ2MAkJgKi1zN+h46ypC2zNr0a1fp-FqWQ@mail.gmail.com> (raw)
In-Reply-To: <3413d0c8-17c7-fbae-e5fa-74a918e61239@gmail.com>
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).
prev parent reply other threads:[~2020-10-08 18:24 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-08 16:54 [PATCH RESEND v2] mm: Optional full ASLR for mmap() and mremap() Topi Miettinen
2020-10-08 17:07 ` Matthew Wilcox
2020-10-08 18:11 ` Topi Miettinen
2020-10-08 17:13 ` Jann Horn
2020-10-08 17:23 ` Matthew Wilcox
2020-10-08 17:26 ` Jann Horn
2020-10-08 17:41 ` Matthew Wilcox
2020-10-08 18:10 ` Topi Miettinen
2020-10-08 18:24 ` Jann Horn [this message]
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