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

[Konstantin Belousov]

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.

>   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.