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[PATCH 3.16 2/7] mm: fix new crash in unmapped_area_topdown()

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3.16.45-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Hugh Dickins <hughd@google.com>

commit f4cb767d76cf7ee72f97dd76f6cfa6c76a5edc89 upstream.

Trinity gets kernel BUG at mm/mmap.c:1963! in about 3 minutes of
mmap testing.  That's the VM_BUG_ON(gap_end < gap_start) at the
end of unmapped_area_topdown().  Linus points out how MAP_FIXED
(which does not have to respect our stack guard gap intentions)
could result in gap_end below gap_start there.  Fix that, and
the similar case in its alternative, unmapped_area().

Fixes: 1be7107fbe18 ("mm: larger stack guard gap, between vmas")
Reported-by: Dave Jones <davej@codemonkey.org.uk>
Debugged-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
 mm/mmap.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1732,7 +1732,8 @@ check_current:
 		/* Check if current node has a suitable gap */
 		if (gap_start > high_limit)
 			return -ENOMEM;
-		if (gap_end >= low_limit && gap_end - gap_start >= length)
+		if (gap_end >= low_limit &&
+		    gap_end > gap_start && gap_end - gap_start >= length)
 			goto found;
 
 		/* Visit right subtree if it looks promising */
@@ -1835,7 +1836,8 @@ check_current:
 		gap_end = vm_start_gap(vma);
 		if (gap_end < low_limit)
 			return -ENOMEM;
-		if (gap_start <= high_limit && gap_end - gap_start >= length)
+		if (gap_start <= high_limit &&
+		    gap_end > gap_start && gap_end - gap_start >= length)
 			goto found;
 
 		/* Visit left subtree if it looks promising */

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[PATCH 3.16 0/7] 3.16.45-rc1 review Ben Hutchings <ben@decadent.org.uk> - 2017-06-27 13:20 +0200
  [PATCH 3.16 2/7] mm: fix new crash in unmapped_area_topdown() Ben Hutchings <ben@decadent.org.uk> - 2017-06-27 13:20 +0200
  [PATCH 3.16 6/7] rxrpc: Fix several cases where a padded len  isn't checked in ticket decode Ben Hutchings <ben@decadent.org.uk> - 2017-06-27 13:20 +0200
  [PATCH 3.16 3/7] Allow stack to grow up to address space limit Ben Hutchings <ben@decadent.org.uk> - 2017-06-27 13:20 +0200
  Re: [PATCH 3.16 0/7] 3.16.45-rc1 review Guenter Roeck <linux@roeck-us.net> - 2017-06-27 15:40 +0200
    Re: [PATCH 3.16 0/7] 3.16.45-rc1 review Ben Hutchings <ben@decadent.org.uk> - 2017-06-27 19:20 +0200

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