From: Oleg Nesterov <oleg@redhat.com>
Date: Mon, 10 Jul 2017 22:49:54 +0000 (-0700)
Subject: mm/mmap.c: expand_downwards: don't require the gap if !vm_prev
X-Git-Tag: archive/raspbian/4.9.80-2+rpi1~8^2~77
X-Git-Url: https://dgit.raspbian.org/?a=commitdiff_plain;h=db3d1cf8c52a556dd281e87cbc4dba4075db5231;p=linux-4.9.git

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>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Larry Woodman <lwoodman@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name mm-mmap.c-expand_downwards-don-t-require-the-gap-if-.patch
---

diff --git a/mm/mmap.c b/mm/mmap.c
index 82b17dacda1d..66ff511df1b6 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2312,7 +2312,6 @@ 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;
@@ -2321,15 +2320,12 @@ 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. */