Re: [PATCH] mm: larger stack guard gap, between vmas

Michal Hocko
2017-07-04T09:35:44Z

On Tue 04-07-17 10:41:22, Michal Hocko wrote:
> On Mon 03-07-17 17:05:27, Linus Torvalds wrote:
> > On Mon, Jul 3, 2017 at 4:55 PM, Ben Hutchings <ben@decadent.org.uk> wrote:
> > >
> > > Firstly, some Rust programs are crashing on ppc64el with 64 KiB pages.
> > > Apparently Rust maps its own guard page at the lower limit of the stack
> > > (determined using pthread_getattr_np() and pthread_attr_getstack()). I
> > > don't think this ever actually worked for the main thread stack, but it
> > > now also blocks expansion as the default stack size of 8 MiB is smaller
> > > than the stack gap of 16 MiB. Would it make sense to skip over
> > > PROT_NONE mappings when checking whether it's safe to expand?
>
> This is what my workaround for the older patch was doing, actually. We
> have deployed that as a follow up fix on our older code bases. And this
> has fixed verious issues with Java which was doing the similar thing.

Here is a forward port (on top of the current Linus tree) of my earlier
patch. I have dropped a note about java stack trace because this would
most likely be not the case with the Hugh's patch. The problem is the
same in principle though. Note I didn't get to test this properly yet
but it should be pretty much obvious.
---
>From d9f6faccf2c286ed81fbc860c9b0b7fe23ef0836 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@suse.com>
Date: Tue, 4 Jul 2017 11:27:39 +0200
Subject: [PATCH] mm: mm, mmap: do not blow on PROT_NONE MAP_FIXED holes in the
stack

"mm: enlarge stack guard gap" has introduced a regression in some rust
and Java environments which are trying to implement their own stack
guard page. They are punching a new MAP_FIXED mapping inside the
existing stack Vma.

This will confuse expand_{downwards,upwards} into thinking that the stack
expansion would in fact get us too close to an existing non-stack vma
which is a correct behavior wrt. safety. It is a real regression on
the other hand. Let's work around the problem by considering PROT_NONE
mapping as a part of the stack. This is a gros hack but overflowing to
such a mapping would trap anyway an we only can hope that usespace
knows what it is doing and handle it propely.

Fixes: d4d2d35e6ef9 ("mm: larger stack guard gap, between vmas")
Debugged-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Michal Hocko <mhocko@suse.com>
---
mm/mmap.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index f60a8bc2869c..2e996cbf4ff3 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2244,7 +2244,8 @@ int expand_upwards(struct vm_area_struct *vma, unsigned long address)
gap_addr = TASK_SIZE;

next = vma->vm_next;
- if (next && next->vm_start < gap_addr) {
+ if (next && next->vm_start < gap_addr &&
+ (next->vm_flags & (VM_WRITE|VM_READ|VM_EXEC))) {
if (!(next->vm_flags & VM_GROWSUP))
return -ENOMEM;
/* Check that both stack segments have the same anon_vma? */
@@ -2325,7 +2326,8 @@ int expand_downwards(struct vm_area_struct *vma,
/* Enforce stack_guard_gap */
prev = vma->vm_prev;
/* Check that both stack segments have the same anon_vma? */
- if (prev && !(prev->vm_flags & VM_GROWSDOWN)) {
+ if (prev && !(prev->vm_flags & VM_GROWSDOWN) &&
+ (prev->vm_flags & (VM_WRITE|VM_READ|VM_EXEC))) {
if (address - prev->vm_end < stack_guard_gap)
return -ENOMEM;
}
--
2.11.0

--
Michal Hocko
SUSE Labs