mm: fix new crash in unmapped_area_topdown()

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().

Cc: stable@vger.kernel.org
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>

1 files changed, 4 insertions, 2 deletions

diff --git a/mm/mmap.c b/mm/mmap.c
index 8e07976d5e47..290b77d9a01e 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1817,7 +1817,8 @@ 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 */
@@ -1920,7 +1921,8 @@ 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 */