[PATCH 2/2 T] mm/mmap.c: expand_downwards: don't require the gap if !vm_prev

[Stefan Bader]

From: Oleg Nesterov <oleg at redhat.com>

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

CVE-2017-1000364

(backported from commit 32e4e6d5cbb0c0e427391635991fe65e17797af8)
Signed-off-by: Stefan Bader <stefan.bader at canonical.com>
---
 mm/mmap.c | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 624ccb2..72f35bc 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2227,7 +2227,6 @@ int expand_downwards(struct vm_area_struct *vma,
 				   unsigned long address)
 {
 	struct vm_area_struct *prev;
-	unsigned long gap_addr;
 	int error;
 
 	address &= PAGE_MASK;
@@ -2236,15 +2235,12 @@ 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. */
-- 
2.7.4