Re: [RFC PATCH for 4.18 1/2] rseq: validate rseq_cs fields are < TASK_SIZE

[Linus Torvalds]

On Thu, Jun 28, 2018 at 6:08 PM Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> > On Jun 28, 2018, at 5:18 PM, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> >
> > Make it do
> >
> >        if (rseq_cs->abort_ip != (unsigned long)rseq_cs->abort_ip)
> >                return -EINVAL;
> >
> > at abort time.
>
> You sure?  Because, unless I remember wrong, a 32-bit user program on a 64-bit kernel will actually work at least most of the time even if high bits are set.

Sure.

If you run a 32-bit binary on a 64-bit kernel,. you will have access
to the 0xc0000000 - 0xffffffff area that you wouldn't have had access
to if it ran on a 32-bit kernel.

But exactly *because* you have access to that area, those addresses
are actually valid addresses for the 32-bit case, so they shouldn't be
considered bad. They can't happen on a native 32-bit kerne, but a
32-bit program doesn't even care. If it has user memory mapped in that
area, it should work.

And if it *doesn't* have user memory mapped in that area, then it will
fail when the trying to execute the (non-existent) abort sequence.

After all, depending on configuration, a native 32-bit kernel might
limit user space even more (ie some vendors had a 2G:2G split instead
of the traditional 3G:1G split.

Was that the case you were thinking of, or was it something else?

              Linus