Re: [PATCH] mm: mm, mmap: do not blow on PROT_NONE MAP_FIXED holes in the stack

[Michal Hocko <mhocko@kernel.org>]

On Wed 05-07-17 20:53:02, Michal Hocko wrote:
> On Wed 05-07-17 11:35:51, Linus Torvalds wrote:
> > On Wed, Jul 5, 2017 at 11:28 AM, Michal Hocko <mhocko@kernel.org> wrote:
> > >
> > > 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
> > 
> > Oh, ok. I think I know which patch from Oleg you're talking about.
> > 
> > Since I do want that patch too, and since I'd hate to cause
> > unnecessary merge conflicts in this area, how about we just plan on
> > letting your original patch (on top of Oleg's) go through Andrew and
> > the -mm tree? I'll get it that way, and it's not like this is
> > timing-critical.
> 
> That would lead to conflicts when backporting to stable trees though
> which is quite annoying as well and arguably slightly more annoying than
> resolving this in mmotm. I can help to rebase Oleg's patch on top of
> mine which is not a stable material.

Here is the rebase of Oleg's patch.
---
>From 61ff0cd972dac218390a5859b89ce386db731d1d Mon Sep 17 00:00:00 2001
From: Oleg Nesterov <oleg@redhat.com>
Date: Fri, 30 Jun 2017 10:19:00 +0200
Subject: [PATCH] mm/mmap.c: expand_downwards: don't require the gap if
 !vm_prev

expand_stack(vma) fails if address < stack_guard_gap even if there is no
vma->vm_prev.  I don't think this makes sense, and we didn't do this
before the recent commit 1be7107fbe18 ("mm: larger stack guard gap,
between vmas").  We do not need a gap in this case, any address is fine as
long as security_mmap_addr() doesn't object.

This also simplifies the code, we know that address >= prev->vm_end and
thus underflow is not possible.

Link: http://lkml.kernel.org/r/20170628175258.GA24881@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Larry Woodman <lwoodman@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Michal Hocko <mhocko@suse.com>
---
 mm/mmap.c | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index ece0f6d3a1b5..f30847405cab 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2316,7 +2316,6 @@ int expand_downwards(struct vm_area_struct *vma,
 {
 	struct mm_struct *mm = vma->vm_mm;
 	struct vm_area_struct *prev;
-	unsigned long gap_addr;
 	int error;
 
 	address &= PAGE_MASK;
@@ -2325,15 +2324,12 @@ int expand_downwards(struct vm_area_struct *vma,
 		return error;
 
 	/* Enforce stack_guard_gap */
-	gap_addr = address - stack_guard_gap;
-	if (gap_addr > address)
-		return -ENOMEM;
 	prev = vma->vm_prev;
-	if (prev && prev->vm_end > gap_addr &&
+	/* Check that both stack segments have the same anon_vma? */
+	if (prev && !(prev->vm_flags & VM_GROWSDOWN) &&
 			(prev->vm_flags & (VM_WRITE|VM_READ|VM_EXEC))) {
-		if (!(prev->vm_flags & VM_GROWSDOWN))
+		if (address - prev->vm_end < stack_guard_gap)
 			return -ENOMEM;
-		/* Check that both stack segments have the same anon_vma? */
 	}
 
 	/* We must make sure the anon_vma is allocated. */
-- 
2.11.0

-- 
Michal Hocko
SUSE Labs