mm: memblock: fix wrong memmove size in memblock_merge_regions()

The memmove span covers from (next+1) to the end of the array, and the
index of next is (i+1), so the index of (next+1) is (i+2).  So the size
of remaining array elements is (type->cnt - (i + 2)).

Since the remaining elements of the memblock array are move forward by
one element and there is only one additional element caused by this bug.
So there won't be any write overflow here but read overflow.  It may
read one more element out of the array address if the array happens to
be full.  Commonly it doesn't matter at all but if the array happens to
be located at the end a memblock, it may cause a invalid read operation
for the physical address doesn't exist.

There are 2 *happens to be* here, so I think the probability is quite
low, I don't know if any guy is haunted by this bug before.

Mostly I think it's user-invisible.

Signed-off-by: Lin Feng <linfeng@cn.fujitsu.com>
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/memblock.c b/mm/memblock.c
index 625905523c2a1592f539c46f6721723d62376648..88adc8afb6103d6c1ba2eb7e20c1ed226cf2b1d3 100644 (file)
--- a/mm/memblock.c
+++ b/mm/memblock.c
@@ -314,7 +314,8 @@ static void __init_memblock memblock_merge_regions(struct memblock_type *type)
                }
 
                this->size += next->size;
-               memmove(next, next + 1, (type->cnt - (i + 1)) * sizeof(*next));
+               /* move forward from next + 1, index of which is i + 2 */
+               memmove(next, next + 1, (type->cnt - (i + 2)) * sizeof(*next));
                type->cnt--;
        }
 }