On Wed 05-07-17 10:43:27, Linus Torvalds wrote:
> On Wed, Jul 5, 2017 at 9:56 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> >
> > "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.
>
> Hmm. What version is this patch against? It doesn't seem to match my 4.12 tree.
Dohh, that was on mmotm which has a clean up by Oleg which reorganizes
the code a bit. This is on top of the current master
---
>From fd538009ac373a5f87538786412a3e6191fa6001 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@xxxxxxxx>
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@xxxxxxx>
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
mm/mmap.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/mm/mmap.c b/mm/mmap.c
index a5e3dcd75e79..ece0f6d3a1b5 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? */
@@ -2328,7 +2329,8 @@ int expand_downwards(struct vm_area_struct *vma,
if (gap_addr > address)
return -ENOMEM;
prev = vma->vm_prev;
- if (prev && prev->vm_end > gap_addr) {
+ if (prev && prev->vm_end > gap_addr &&
+ (prev->vm_flags & (VM_WRITE|VM_READ|VM_EXEC))) {
if (!(prev->vm_flags & VM_GROWSDOWN))
return -ENOMEM;
/* Check that both stack segments have the same anon_vma? */
--
2.11.0
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