Re: Security fix for remapping of page 0 (was [PATCH] Change ZERO_SIZE_PTR to point at unmapped space)

[Eric Paris <eparis@parisplace.org>]

On Wed, Jun 3, 2009 at 11:38 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
>
> On Wed, 3 Jun 2009, Christoph Lameter wrote:
>
>> On Wed, 3 Jun 2009, Linus Torvalds wrote:
>>
>> > The point being that we do need to support mmap at zero. Not necessarily
>> > universally, but it can't be some fixed "we don't allow that".
>>
>> Hmmm... Depend on some capability? CAP_SYS_PTRACE may be something
>> remotely related?
>
> But as mentioned several times, we do have the system-wide setting in
> 'mmap_min_addr' (that then can be overridden by CAP_SYS_RAWIO, so in that
> sense a capability already exists).
>
> It defaults to 64kB in at least the x86 defconfig files, but to 0 in the
> Kconfig defaults. Also, for some reason it has a "depends on SECURITY",
> which means that if you just default to the old-style unix security you'll
> lose it.
>
> So there are several ways to disable it by mistake. I don't know what
> distros do.

Fedora has it on.

As I recall the only need for CONFIG_SECURITY is for the ability to
override the check.

I think I could probably pretty cleanly change it to use
CAP_SYS_RAWIO/SELinux permissions if CONFIG_SECURITY and just allow it
for uid=0 in the non-security case?  Deny it for everyone in the
non-security case and make them change the /proc tunable if they need
it?

-Eric