How does the stack's guard page work on amd64?

[Alan Somers]

On Wed, Mar 31, 2021 at 5:21 AM Konstantin Belousov <kostikbel at gmail.com>
wrote:

> On Tue, Mar 30, 2021 at 08:28:09PM -0600, Alan Somers wrote:
> > On Tue, Mar 30, 2021 at 3:35 AM Konstantin Belousov <kostikbel at gmail.com
> >
> > wrote:
> >
> > > On Mon, Mar 29, 2021 at 11:06:36PM -0600, Alan Somers wrote:
> > > > Rust tries to detect stack overflow and handles it differently than
> other
> > > > segfaults, but it's currently broken on FreeBSD/amd64.  I've got a
> patch
> > > > that fixes the problem, but I would like someone to confirm my
> reasoning.
> > > >
> > > > It seems like FreeBSD's main thread stacks include a guard page at
> the
> > > > bottom.  However, when Rust tries to create its own guard page (by
> > > > re-mmap()ping and mprotect()ing it), it seems like FreeBSD's guard
> page
> > > > automatically moves up into the un-remapped region.  At least,
> that's how
> > > > it behaves, based on the addresses that segfault.  Is that correct?
> > > Show the facts. For instance, procstat -v (and a note which
> > > mapping was established by runtime for the 'guard') would tell the
> whole
> > > story.
> > >
> > > My guess would be that procctl(PROC_STACKGAP_CTL,
> &PROC_STACKGAP_DISABLE)
> > > would be enough.  Cannot tell without specific data.
> > >
> > > >
> > > > For other threads, Rust doesn't try to remap the guard page, it just
> > > relies
> > > > on the guard page created by libthr in _thr_stack_alloc.
> > > >
> > > > Finally, what changed in between FreeBSD 10.3 and 11.4?  Rust's stack
> > > > overflow detection worked in 10.3.
> > > >
> > > > -Alan
> > > > _______________________________________________
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> > >
> >
> > Here is the relevant portion of procstat -v for a test program built with
> > the buggy rustc:
> >   651        0x801554000        0x80155d000 rw-    0   17   3   0 -----
> df
> >   651        0x801600000        0x801e00000 rw-   30   30   1   0 -----
> df
> >   651     0x7fffdfffd000     0x7fffdfffe000 ---    0    0   0   0 -----
> --
> >   651     0x7fffdfffe000     0x7fffdffff000 ---    0    0   0   0 -----
> --
> > <--- What Rustc thinks is the guard page
> >   651     0x7fffdffff000     0x7fffe0000000 ---    0    0   0   0 -----
> --
> > <--- Where did this come from?
> This is the stack grow area, occupied by 'elastic' guard entry.
> It serves two purposes:
> 1. it keeps the space, preventing other non-fixed mappings from selecting
>    the grow area for mapping.
> 2. it prevents stack from growing down to the next mapping below it,
>    preventing issues like StackClash.
>
> See mmap(2) esp. MAP_STACK part of it.
>

I saw that.  And I even saw where libthr uses MAP_STACK when creating new
threads.  However, this program is single-threaded.  Where does the stack
get created for a process's main thread?  I couldn't find that.


>
> >   651     0x7fffe0000000     0x7fffe001e000 rw-   30   30   1   0 ---D-
> df
> >   651     0x7fffe001e000     0x7fffe003e000 rw-   32   32   1   0 ---D-
> df
> >
> > Rustc tries to create that guard page by finding the base address of the
> > stack, reallocating one page, then mprotect()ing it, like this:
> >
> mmap(0x7fffdfffe000,0x1000,0x3<PROT_READ|PROT_WRITE>,0x1012<MAP_PRIVATE|MAP_FIXED|MAP_ANON>,0xffffffff,0)
> > mprotect(0x7fffdfffe000,0x1000,0<PROT_NONE>)
> >
> > If I patch rustc to not attempt to allocate a guard page, then its memory
> > map looks like this.  Notice that 0x7fffdffff000 is now accessible
> It is accessible because stack grown down into this address.
>
> >   662        0x801531000        0x80155b000 rw-    3   17   3   0 -----
> df
> >   662        0x801600000        0x801e00000 rw-   30   30   1   0 -----
> df
> >   662     0x7fffdfffd000     0x7fffdfffe000 ---    0    0   0   0 -----
> --
> >   662     0x7fffdfffe000     0x7fffdffff000 ---    0    0   0   0 -----
> --
> >   662     0x7fffdffff000     0x7fffe001e000 rw-   31   31   1   0 ---D-
> df
> >   662     0x7fffe001e000     0x7fffe003e000 rw-   32   32   1   0 ---D-
> df
> >
> > So the real question is, why does 0x7fffdffff000 become protected when
> > rustc protects 0x7fffdfffe000 ?
> See above.
>
> As I said in earlier response, if you want fully shrinkable stack guard,
> set procctl(PROC_STACKGAP_CTL, &PROC_STACKGAP_DISABLE) during runtime
> initialization.
>
> Or better, do not create custom guard page at all, relying on system guard.
>

That's what my patch does.  But I've only tested it on amd64, and I don't
have access to alternative architectures.  Does every architecture create a
stack guard this way?

-Alan