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

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Date:   Tue, 4 Jul 2017 11:35:38 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Ben Hutchings <ben@...adent.org.uk>,
        Hugh Dickins <hughd@...gle.com>, Willy Tarreau <w@....eu>,
        Oleg Nesterov <oleg@...hat.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...hat.com>,
        Larry Woodman <lwoodman@...hat.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Tony Luck <tony.luck@...el.com>,
        "James E.J. Bottomley" <jejb@...isc-linux.org>,
        Helge Diller <deller@....de>,
        James Hogan <james.hogan@...tec.com>,
        Laura Abbott <labbott@...hat.com>, Greg KH <greg@...ah.com>,
        "security@...nel.org" <security@...nel.org>,
        linux-distros@...openwall.org,
        Qualys Security Advisory <qsa@...lys.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: larger stack guard gap, between vmas

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@...adent.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@...e.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@...e.cz>
Signed-off-by: Michal Hocko <mhocko@...e.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